Sciencemadness Discussion Board

Benzene synthesis

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madscientist - 28-11-2002 at 17:40

Synthesis of benzene seems to be a topic of interest and value, considering that benzene has now been labeled as an "evil scary carcinogen of inevitable death".

There's several routes that come to mind. First, is a Friedel-Crafts reaction that Osmonium suggested: AlCl3 will catalyze the conversion of toluene into benzene and mesitylene. A second route would be decarboxylating benzoic acid by strongly heating it. A third route would be reacting 1,3,5-trichlorocyclohexane with a strong base. And a fourth route, which I am not completely certain would work, would be aldol condensation - involving adding a strong base to acetaldehyde, which should catalyze conversion into benzene.

Any thoughts?

BASF - 28-11-2002 at 18:55

Do you have more information on the decarboxylation of benzoic acid?
Decomposition temp. and similar things?

I think the decarboxylation of benzoic acid would be very attractive for the home chemist due to the widespread use of benzoic acid in the food industry, and therefor it should be easy to get and not that expensive.
I still have some 100-200g of benzoic acid from a drug store.


madscientist - 28-11-2002 at 19:19

I do not know at what temperature benzoic acid decarboxylates. If I remember correctly, salicylic acid decarboxylates at a tad above 200C, so I suppose that benzoic acid would decarboxylate at a temperature somewhere above 200C.

On a side note, is anyone aware of any sources of any cyclohexanes, especially 1,3,5-trichlorocyclohexane?

notagod - 29-11-2002 at 08:27

If I remeber right, it does'nt have to be benzoic acid, the sodium salt will do. Mix it with waterfree sodium carbonate and heat it gently. Benzene will distill over. You can also have some sand mixed in to have a more "smooth" reaction.

sodium benzoate instead of benzoid acid

Rhadon - 29-11-2002 at 09:15

Yes, benzene is an interesting subject.
I think I remember that I saw an experiment in a basic experimental chemistry book which advised to react benzoic acid with sodium hydroxide at first and decomposing the resulting sodium benzoate by heating it.

Madscientist, do you have further information about toluene / AlCl3?

Benzene from tolulene

blazter - 29-11-2002 at 14:13

if i recall correctly megalomania mentioned that he was working on a synthesis route for benzene from tolulene using KMnO4 to oxidize it. Using KMnO4 has the advantage of being fairly easily found OTC or it too can be made from OTC chemicals. I don't think he ever published this route completely but refered to it in a thread on E&W about KMnO4 synthesis.

Polverone - 29-11-2002 at 14:44

He was probably thinking of benzoic acid decarboxylation, then. The KMnO4 would be used to oxidize the toluene to benzoic acid.

madscientist - 2-12-2002 at 15:02

It seems that this reaction would work.

NaC6H5COO + NaOH --(heat)--> Na2CO3 + C6H6

I haven't any more information on preparing benzene from toluene and aluminum trichloride.


Polverone - 2-12-2002 at 17:04

It seems that this reaction would work.

NaC6H5COO + NaOH --(heat)--> Na2CO3 + C6H6

That seems reasonable. Calcium hydroxide might be preferrable, since it won't fuse very easily and is cheaper. One could presumably use other aromatic carboxylic acids, such as phthalic acid. That's not a major improvement, but one could start with the slightly-more-common xylene instead of toluene. Another improvement would be finding an alternative to KMnO4 for the oxidation; its availability and price vary to much to always suggest it.

Marvin - 6-12-2002 at 13:50

Permanganates can always be recycled though, which is a big advantage.

Polverone - 6-12-2002 at 20:53

Recycled? What method did you have in mind? I've seen electrochemical syntheses of permanganates, and ones that use fused oxidizers + MnO2. Neither method is exactly a recycling in my book...

10fingers - 13-12-2002 at 16:27

Potassium permanganate is available as a cleaner for water softeners. I purchase it at a home improvement store for $15.00 per 5 lbs. It seems to be of a reasonably good purity.

Blauw - 4-2-2003 at 12:14

Hi there,

The following might also work to gain benzene. Try to protonate phenol and heat it. Probably the benzene will lose the water. What do you guys think. I never tried it before.


Blind Angel - 7-3-2003 at 07:12

Sorry to dig-up an old post but i found that during search on Furan and Pyrrole Ring and it said (vaguely) that Benzoic Acid can be Decarboxyled by heat and the addition of Calcium Oxide (CaO) wich is Lime....

If the link intereset you:
but it doesn't say much about quantities...

Edit: Url bogus

[Edited on 7-3-2003 by Blind Angel]

Marvin - 25-3-2003 at 12:45

I seem to be bad at replying, sorry Polverone.

MnO2 can be fused with hydroxide, and this will air oxidise to Manganate(VI), acidify and it disproportionates to manganate(VII) and MnO2.

I'm very interested in the production of benzene from acetone, the main step being cyclisation with sulphuric acid to sym trimethylbenzene. Outside of industrial conditions this is only 25% yeild though.

menchaca - 26-3-2003 at 06:33

i´m quite interested too in the production of bencene from acetone, if you know something else please let me know thanks!:)

Nick F - 26-3-2003 at 07:41

Even trimethylbenzene/mesitylene could be useful after some careful nitration to TNTMB. I would expect slightly more power and slightly higher density than trinitronapthalene, which is intermediate in power to DNB and DNT. Not very strong, but would be good with a bit of AN. I'll see what I can find out about making mesitylene from acetone...

Hmm, just remembered something. I think I remember someone saying that nitration of mesitylene is/can be dangerous. In that case oxidising it to 3,5-dimethylbenzoic acid before nitration will help the nitration go nice and calmly. It would probably decarboxylate either during the reaction, or afterwards if you boil it in water, to give TNm-xylene, and from there further oxidations/decarboxylations will lead to TNT then TNB.

madscientist - 26-3-2003 at 13:23

The third nitration of mesitylene is what makes it dangerous - it must not be conducted at elevated temperatures, or an explosion may result.

Marvin - 27-3-2003 at 10:25

Initial report of formation of mesitylene from acetone and sulphuric acid.
KANE, J. prakt. Chem. [l], 15, 131 (1838).

Investigation of maximum yeild (27%)
V. E. TISHCHENKO and L. I. ANTZUS, Zh. prikl. khim. 4, 806 (1931).

Using HCl at 100 atmospheres 36% Yeild,
V. IPATIYEV, B. DOLGOV and J. VOLNOR, Ber. 63, 3072 (1930).

Using HCl, sealed tubes, 145 to 195C up to 47% yeild.
E. SUCHARDA and H. KUCZYNSKI, Roczniki Chem. 14, 1182 (1934)

If anyone has access to a university library, Id be very interested to read these myself.

No prizes for guessing where these refs come from,
It does also state that explosive power of trinitromesitylene is about the same as DNT, but the sensitivity to impact is high, about the same as TNX, probably becuase of the high substitution of the ring.

One of my older organic chemistry books simply states that benzene can be made by distilling 1 part of benzoic acid with 2 parts lime, though its unclear if this is refering to quicklime or slaked lime. I suspect a little extra water in the mixture would not hurt. If the amount of benzoic acid is too high, or if calcium benzoate is used on its own then it forms benzophenone instead (from memory).

From mesitylene the toluene process seems like it would work just as well, alkaline permanganate oxidation (for example) followed by distillation with x6 (or x4 depending on how it scales) of lime.

Nice the edit feature works now.

[Edited on 27-3-2003 by Marvin]

Organikum - 30-3-2003 at 12:12

Benzene by reverse Friedl-Crafts acetylation from toluene with AlCl3 or better AlI3 and a efficient column with very high reflux/distill ratio (suggested 500/1).
Practical: Toss a lot Al foil in a flask add toluene and I2 and distill.
(Rhodium and Osmium merits)

from gasoline by distillation usng a "Podbielniak still" whatever this maybe. Someone a idea?
(Osmium and lugh merits)
here is to add that it is told that aviation fuel for the Cessnas contains much more benzene which is so easier to distill also.

Benzene from benzoic acid.
DE 936,036 (1952)
DE 958,920 (1956)
(I can upload these to the FTP if requested)

The mentioned process of heating benzoic acid with lime/NaOH seems to be problematic was told. At least a copper catalyst is needed (copper/cupric sulfate) and the temperatures needed are very high so there are reasonable problems to catch and condense the produced benzene.

Benzene from acetylene, see the attached file please.
(200°C, Cr/Al catalyst, high yields and purity)

hope this helps

Attachment: acetylene _to_benzene.pdf (354kB)
This file has been downloaded 4741 times

Madog - 3-5-2003 at 06:30

i atualy saw the benzene from acetylene a few years ago in a kiddy chemistry book i have(a thing one with lots of pics). sometimes those little books have some cool reactions

also, i woudl ike to make some benzene, i was thinking about ttakeing some Al and putting it in a flask and heat it as i pump chlorine in that was generated by HCl + KMnO4 ---> through tube or something that has prilled CaCl2 to dry and then reflux the anhydrouse AlCl3 with toluene and distill off the benzene

is this good?

[Edited on 5/4/03 by Madog]

Ramiel - 3-5-2003 at 19:31

The following reaction caught my eye;

7.CO + 6.CH<sub>4</sub> ==(AlCl<sub>3</sub>;)==> C<sub>6</sub>H<sub>6</sub> + C<sub>6</sub>H<sub>5</sub>-CH<sub>3</sub> + 15/2.H<sub>2</sub>O

This is an experimental process for the conversion of synthesis gas and methane to benzene and toluene. Synthesis gas can be made one of two ways;

<html><li>super high pressure controlled oxidation,</li><br>
<li>leading steam over hot carbon in the absence of air</li></html>

I would say the second would be of interest to the amateur chemist. :)

I'm sure that with a little invention and some piping, this could be pulled off.

Zinc + phenol

BASF - 6-5-2003 at 11:08

another possibility:

When the vapours of Phenol are passed over red-hot zinc dust, then reduction takes place, as a result benzene is formed.

C6H5OH + Zn ® C6H6 + Zn0



Organikum - 21-5-2003 at 23:47

I stumbled over a fragment where was mentioned theat benzene can be made very easily from Chloramin-B (B stands for the benzenering).
Formula: C6H5SO2NNaCl
By heating (if a catalyst has to be present wasn´t mentioned, a fragment, as told...)

As Chloramin-B is readily available for water desinfection this could be a very interesting way to benzene so it works.


jimwig - 22-5-2003 at 10:46

I was searching some time back and came up with these sites- the temperature seems to be the stumbling block.

Is this do-able?

and the pressure

Organikum - 22-5-2003 at 20:24

The first reference speaks of 40bar pressure and the multiple injection and control of hydrogen in the reactor.

The second one works with even higher temperatures - if there wouldn´t be the necessity of feeding hydrogen which has to be produced in the needed amounts if one wants to come around the storing hassle - also free from any oxygen - I would see it as possible. A slight variation of the ketene lamp could do it.
More exact parameters would be needed in special what are the nono´s in materials used (unwanted catalytic effects), the temperature range and very important the contact time needed, to calculate the flow.

If anyone brings up an hydrogen generator with constant output of oxygen-free H2 under some pressure - then this might be a way. (for the friends of industrial processes at least ;), the "Knights of the hot tube" :D)

Krypton - 22-5-2003 at 20:55

What kind of ketene lamp do you mean ?
I make my benzene when i throwing with arrows to diketopiperazine
:D !

Organikum - 7-6-2003 at 23:46

Tripopelpamziene decomposition! Thats a favorable way for benzene thats understood - how did you solve the question of collecting it? ;)

And ketene lamp, I thought of the kind which makes ketene. From acetone. Mr. Hurd made one and Mr. Vogel too.
My lamp is more a tube I confess. To be true, it is a tube and no lamp at all.
You got me!

Benzene from chloramine-B

Organikum - 10-6-2003 at 01:57

It works. I had found the information but had forgotten where, lugh at the Hive was so friendly to look it up as he has better library access than I.

All benzylic sulfonic acids get dehydrated by strong mineral acids at elevated temperatures.
Heating chloramine-B with phosphoric acid to about 225°C will yield benzene.
I give it a try tomorrow and report the results. If it works as told it would be the most favorable benzene synthesis up to now here - easy and cheap starting compounds and bearable reaction conditions.


If_6_was_9 - 22-6-2003 at 17:45

Benzene and formaldehyde --> diphenylmethane

madscientist - 22-6-2003 at 19:45

That's not relevant to the topic of this thread. This discussion is on the preparation of benzene, not the reactions of benzene.

vulture - 23-6-2003 at 08:55

Do I smell someone showing off?
You seem to be getting a habit of posting irrelevant stuff.

Theoretic - 16-11-2003 at 08:48

My chemistry book says: 3C2H2 (Ni, 70C) =>C6H6. That's not much, but at least Ni is an active catalyst, so a solution of acetylene in acetic acid or any other high-boiling solvent with a bit of Ni powder or NiO (another book says "certain transition metal compounds", so NiO will work, I think) could be heated to 75C (to prevent evaporation of benzene don't raise any further) and eventually give benzene. I personally advise a sealed vessel to stop C2H2 escaping.
For C2H2 see
For Ni or NiO see

unionised - 16-11-2003 at 13:52

Chloramine B is said to decompose explosively at 185 C.

The decomposition of sodium benzoate to produce pure benzene is famous for making a fool of some famed chemist (Emil Fischer IIRC) because the test for benzene way back when (colour development with isatin and sulphuric acid (again IIRC)) was really a test for thiophene. Thiophene was always present in coal tar derived benzene and gave a blue colour. Pure benzene doesn't react. Imagine very embarassed Chem. prof. at front of lecture....
Anyway, the reaction should be easy. The products you want (benzene and Na2CO3) are about as stable as you can get so it shouldn't matter too much if you overheat it a bit.

pyroscikim - 21-12-2003 at 17:32

"7.CO + 6.CH4 ==(AlCl3)==> C6H6 + C6H5-CH3 + 15/2.H2O " posted by Ramiel

This reaction looks feasible and relatively simple unless pressurising is involved which seems unnecessary.

Methane just comes off the gas tap at pretty high purity, and CO can be generated by heating a mix of excess conc sulphuric acid with oxalic acid (availiable in most hardwares), or anhydrous formic acid, or a metal-formate. With oxalic acid CO and CO2 are both formed in 1/1 ratios

(COOH)2 --> CO + CO2 + H2O

but shouldn't affect the production of benzene i believe.

The aluminium chloride can just be made from anhydrous HCl on aluminium, since this occurs at much lower temp than pure chlorine. HCl is generated by adding hydrochloric acid to concentrated sulphuric acid.

Put the AlCl3 into a ~2cm wide s/steel pipe and with some basic plumbing skills, one should be able to pass the CO and CH4 over the catalyst "bed" in the heated (to 300degC?) s/steel tube, and duct the vapours of benzene into a condenser or just a glass bottle in iced water...

the temperature of the catalyst could easily be adjusted and experimented with, and if this process works, you could get yourself a lifetime's supply of benzene if required... or just some fun with the practical work involved. The tube could also be used to contain other catalysts to make various chemicals like ethene and acetaldehyde...

seems pretty intriguing for the chemists who really want the practical process of synthesise and experimentation more than product

[Edited on 22-12-2003 by pyroscikim]

unionised - 22-12-2003 at 04:31

AlCl3 s a bit volatile, I think you would lose quite a lot of it with the product.


Organikum - 22-12-2003 at 11:16

toluene + Al + Cl2 ->
toluene + AlCl3 -- disproportionation -->
xylene + toluene + benzene

distill off benzene continously and refill toluene to shift the equilibrium to the wanted side.

comments, flames, name me names? ;)

KABOOOM(pyrojustforfun) - 23-12-2003 at 21:06

<b>one phrase!</b>

AlCl<sub>3</sub> catalysed chlorination:D

Organikum - 26-12-2003 at 08:10

one answer:

not as long Al is present - this will react predominantly I would like to believe.

So: excess of Al + Cl2........

Benzene Synthesis from PCBs

Acid Test - 7-1-2004 at 11:55

You could take PCBs and decompose them to
thier simple benzene construction.
Of course you would also have to break the
bond between the two benzene rings also

possibly unfortunate news

Polverone - 7-1-2004 at 16:35

Alkyl migrations can be inter- or intramolecular, depending on the conditions and the R group [...] o-xylene treated with HBr and AlBr3 gave a mixture of o- and m- but no p-xylene, while p-xylene gave p- and m- but no o-xylene, and no trimethyl compounds could be isolated in these experiments (exclusively intramolecular rearrangement). Apparently, methyl groups migrate only intramolecularly, while other groups may follow either path.

That's from March, and the references for the xylene experiments are:
Brown; Jungk J. Am. Chem. Soc. 1955, 77, 5579; Allen; Yats J. Am. Chem. Soc. 1959, 81, 5289.

Unless methyl groups migrate intermolecularly under different conditions or with different reagents, this seems to bode ill for easy benzene from toluene.

reduction of benzen diazonium salts

KABOOOM(pyrojustforfun) - 8-1-2004 at 21:05

in a practical organic chemistry (in farsi) found two (qualitative) tests for benzene diazonium sulfate which yielded benzene:
1. warm the material in alcohol. a vigorous reaction will take place and the solution turns red. after the reaction subsides add some water an oily liquide will come to surface which is benzene mixed with some phenetol
C<sub>6</sub>H<sub>5</sub>N<sub>2</sub>SO<sub>4</sub>H + C<sub>2</sub>H<sub>5</sub>OH <s>&nbsp;&nbsp;&nbsp;></s> C<sub>6</sub>H<sub>6</sub> + N<sub>2</sub> + CH<sub>3</sub>CHO + H<sub>2</sub>SO<sub>4</sub>
C<sub>6</sub>H<sub>5</sub>N<sub>2</sub>SO<sub>4</sub>H + C<sub>2</sub>H<sub>5</sub>OH <s>&nbsp;&nbsp;&nbsp;></s> C<sub>6</sub>H<sub>5</sub>OC<sub>2</sub>H<sub>5</sub> + N<sub>2</sub> + H<sub>2</sub>SO<sub>4</sub>

2. (this one seems quite more interesting) add 1g of the material in some water and chill with ice, alkalify by adding NaOH solution. now prepare an alkaline stannous hydroxide solution (made by dissolving 3-4 g stannous chloride in twice its weight water and some concentrated NaOH solution is added till the precipitate dissolves) when it cools off add the stannous solution to the benzen diazonium sulfate sol, nitrogen will evolve and benzene comes to surface which is easily separated
C<sub>6</sub>H<sub>5</sub>N<sub>2</sub>SO<sub>4</sub>H + Sn(ONa)<sub>2</sub> + H<sub>2</sub>O <s>&nbsp;&nbsp;&nbsp;></s> C<sub>6</sub>H<sub>6</sub> + N<sub>2</sub> + Na<sub>2</sub>SnO<sub>3</sub> + NaOH
Edit: had missed the arrows in last equation

[Edited on 13-1-2004 by KABOOOM(pyrojustforfun)]

well, I was wrong

Polverone - 12-1-2004 at 16:08

R. M. Roberst and D. Shiengthong, J. Am. Chem. Soc. 86:2851 (1964) - heating 0.25 mol of p-xylene with .013 mol of AlCl3 at 30 degrees for 2 hours, with stirring, gave 2% benzene, 2% toluene, 94% xylenes, 2% pseudocumene. Repeating the experiment but at 50 degrees gave 12% benzene, 12% toluene, 70% xylenes, 1% mesitylene, 7% pseudocumene. Orgy's proposed method of "mix alkylbenzene with AlCl3, heat, and watch the benzene come over" seems very workable.

And now, since I think he's interested in various Friedel-Crafts catalysts, here's what Carey and Sundberg have to say:

Very active F-C catalysts:
AlCl3, AlBr3, GaCl3, GaCl2, SbF5, MoCl5

Moderately active:
InCl3, LnBr3, SbCl5, FeCl3, AlCl3-CH3NO2, SbF-CH3NO2

BCl3, SnCl4, TiCl4, TiBr4, FeCl2

No information about iodides, sadly, but it might be easier to just try than to chase down references.

shyam vispute - 19-1-2004 at 21:39

ketene is required continuously for becnch scale please, let me know the details pertaining the equipment used for the generation of ketene
( if possible support me with a neat sketch)

guaguanco - 19-1-2004 at 22:56

Originally posted by madscientist
Synthesis of benzene seems to be a topic of interest and value, considering that benzene has now been labeled as an "evil scary carcinogen of inevitable death".

Looking at the immense efforts people are proposing to create (what would be) small amounts of benzene, I have to ask Why? Does it possess some amazing properties as a solvent that aren't shared with, say, Toluene? It's not particularly interesting as a synthetic intermediate. It doesn't explode.
Other than the thrill of a few milliters of benzene sitting around in a vial, I say save your energy for more interesting things.



chloric1 - 20-1-2004 at 08:47

Well, for starters benzene has the lowest boiling point for aromatic liquids. And sometimes aromatics fill the void when your aliphatic liquids wont dissolve the solute you want dissolved. Secondly one valuable use of benzene is make absolute ethanol using the terniary azetrope (composition: 7.4% Water; 18.5 % Alcohol; & 74.1 % benzene) which boils 64.9 C leaving 200 proof behind. Also, I am reading here about a very simular terniary azetrope with chloroform that boils at 55C! Trouble is you have to use twice as much chloroform as benzene because the chloroform azetrope it only 3.5 % water:( But with calcium chloride treatment of the chloroform distillate) one should be able to reuse the same chloroform over and over again. :o I might head to the liquor store this weekend for Everclear!

Reference: Lange Handbook 10th Edition 1961

[Edited on 1/20/2004 by chloric1]

BromicAcid - 20-1-2004 at 09:09


Does it possess some amazing properties as a solvent that aren't shared with, say, Toluene?

Well benzene holds up much better to oxidizing conditions then toluene or xylene. In addition it is a 'virgin' slate to work with if you're trying to synthesize something. Nitrobenzene, chlorobenzene and others are quite easy to make from the mother benzene but would take in some cases many additional steps to produce from toluene.

Finally, many of the older chemistry books that us home chemists own constantly refer back to benzene as a reference material, they list the solubilities of various substances in it and many of its reactions in detail, also it was one of the solvents of choice back in the day if you want to follow some old reaction to the letter.

not synthetically useful?

Polverone - 20-1-2004 at 12:09

It's not particularly interesting as a synthetic intermediate.

It's a good starting point for nitrobenzene (and therefore aniline), phenol, bromobenzene, allylbenzene, most sorts of benzene with a single side chain...

How would you make aniline starting with toluene, if not through converting toluene to benzene at some point? I realize that these are all simple molecules, and "just buy it" is probably the easiest answer, but not everybody can just buy what they need for experimentation. Xylene and toluene are more readily available in many parts of the world, hence interest in methods of converting them to benzene.

I must say that Organikum's proposed Friedel-Crafts alkylation/dealkylation looks like the simplest method and it could probably provide substantial quantities of benzene starting from toluene or even xylene.

I was wrong

Organikum - 21-1-2004 at 08:00

but not on the Friedel-Crafts but on the AlCl3 in situ using Cl2. KABOOM was right, this would chlorinate the benzene. But It is possible to generate AlCl3 from Al in benzene by using dry HCl-gas instead of Cl2, also a small amount (3%) of AlCl3 is needed to start the reaction - a nice way to produce bigger amounts of AlCl3 btw. ...

The thermal decomposition of chloramine-B seems very interesting too - A quick testtube trial showed that the proposed decomposition takes place and some fumes form which might very well be benzene - I didnt stick my nose into the fumes so thats all I can tell by now. On friday I get some new equipment as the old was all lost including a condensor and then will try it again on a larger scale and condense the fumes - we will see what this is.
The "violent decomposition" is not so violent as I feared it to be and can be rather easily controlled by removing the heat, I just wouldnt try this putside a flask or other vessel as the shit jumps around a little bit, thats it.

If_6_was_9 - 24-1-2004 at 13:25

Maybe the decarboxylation process would work with benzoic acid.

A Preparation Method for Benzophenone

JACS volume 50 page 571 (1928)

A Preparation Method for Benzophenone

It is well known that simple carboxylic acids yield hydrocarbons to a greater or less extent when they are heated with finely divided metals, particularly copper and nickel.

In the case of aliphatic acids the reaction is usually difficult to effect.  Acetic acid, for example, when heated with copper at 390-411°, gives mainly acetone and when nickel is used as the catalyst a considerable portion of the acid is decomposed into complex carbonaceous substances1.  With aromatic acids the decomposition is usually easier but frequently, as with benzoic acid and nickel2, the reaction temperature is so high that the resulting hydrocarbons are largely transformed into other substances.

It has been found that this process of eliminating carbon dioxide from an acid can be used to advantage in the preparation of benzophenone from o-benzoylbenzoic acid.

The yields, 80-90% of the theoretical, exceed the average of those obtained by the Friedel and Crafts synthesis, in which benzene and benzoyl chloride react in the presence of aluminum chloride, by about 10 %.  Furthermore o-benzoylbenzoic acid which is the intermediate in the synthetic anthraquinone process, is manufactured on a large scale and should be obtained without difficulty.  It can also be prepared easily and inexpensively from phthalic anhydride, benzene and aluminum chloride3.  This reaction goes nearly to completion, 95-97%, so that the yield of benzophenone based on the phthalic anhydride is still somewhat better than that obtained when benzoyl chloride and benzene are used, and the cost of the materials is much less. 
     When o-benzoylbenzoic acid is heated alone to about 300° some carbon dioxide is eliminated and benzophenone can be detected among the products.  The yield, however is negligible.  The catalysts which accelerate the reaction most efficiently are metals such as copper, nickel, iron and the o-benzoylbenzoic acid salts of these metals.  It is not necessary that the metals be particularly finely divided as they go into solution in the acid, and the only apparent advantage in having a fine state of subdivision is to increase the rate of solution.  The preferred catalysts are finely divided copper and the copper salt of o-benzoylbenzoic acid.  The later is easily made by adding a copper sulfate solution to a solution of the sodium salt of the acid, washing, filtering and drying the precipitate at 110°.  The velocity of the reaction increases with the amount of catalyst used, but too large a quantity causes inconvenience in getting out the product.   It has been found that a satisfactory ratio is 1-3 g. of copper, or an amount of the copper salt of o-benzoylbenzoic acid containing an equivalent weight of copper, to 100 g. of the acid.  In the presence of these catalysts carbon dioxide elimination begins at about 150° and increases as the temperature rises until 306° is reached, when the reaction mixture begins to boil.  The speed of the reaction below 200° is too slow for practical purposes and above 280° there is decomposition, the formation of anthraquinone and unidentified tarry substances.  The optimum temperature range is about 250-270°.

Preparation Procedure

Three hundred grams of pure o-benzoylbenzoic acid was placed in a distilling flask with a low side tube.  To this was added 20 g. of the copper salt of the acid.  The flask was fitted with a mechanical stirrer arranged through a mercury seal, and a thermometer the bulb of which was immersed in the liquid when the material melted.  The mixture was heated with stirring; carbon dioxide evolution was appreciable at 200°, and it was allowed to escape through the side tube of the flask and to bubble through water, which gave an indication of the progress of the reaction.  The temperature was allowed to rise and was maintained at 260° until carbon dioxide evolution ceased (about four hours).   It is necessary to continue until the carbon dioxide evolution has completely stopped. If a distillation is attempted before this point, a considerable quantity of anthraquinone is formed, which contaminates the product.  The stirrer was taken from the flask, a short air condenser attached to the side tube and the thermometer raised out of the liquid as for distillation.  The crude benzophenone was distilled over until the drops of distillate became dark in color; the weight of this crude product was 209 g., 86.6 % of the theoretical.  One crystallization from 95% alcohol gave pure benzophenone, m. p. 47-48°, in an amount corresponding to 82-84% of the theoretical based on the o-benzoylbenzoic acid.  The crystallization may be replaced by distillation in a vacuum.

1 Sabatier and Senderens, Ann. chim. phys., [8] 4, 467 (1905).
2 Sabatier and Mailhe, Compt. rend., 159, 217 (1914).
3 Cain, “The Manufacture of Intermediate Products for Dyes,” Macmillan and Co., London, 1918, p. 240



[Edited on 24-1-2004 by If_6_was_9]

Nick F - 14-3-2004 at 08:46

Has anyone mentioned organometallic methods? Electrolysis of LiCl in acetone (or pyridine, if you can find it!) to get lithium, dissolve p-DCB in hexane, add Li, add water...
Or you could use a similar method involving grignard formation in ether.

Oooh, but separation may be tricky depending on your solvent choice...

(Sorry if this has been mentioned, I looked through the topic quickly and couldn't find it...)


Turel - 14-3-2004 at 12:31

Won't Li reduce acetone to 2,3-dimethyl-2,3-dihydroxybutane? I know Na and K both do this.

Also, forming lithiorganics are not easy with p-DCB. It is very inert to both lithiation and grignard formation. Lithiation is easier, but from what I have seen, requires the use of a combination of butyllithium and lithium metal in an inert solvent under argon.

Nick F - 15-3-2004 at 15:04

There is a patent which discusses electrolytic production of lithium using solvated lithium compounds, I'm pretty sure it said that acetone could be used (although the lithium is formed as a spongy mess which isn't very convenient).

I had a feeling p-DCB may not be very eager to get metallated, but if it needs BuLi then it's really not worth the effort...

Re: Synthesis of benzene

JohnWW - 3-8-2004 at 01:25

Most (technical-grade, at least) benzene is from fractionation in petroleum refining, possibly with some dehydrogenation, although it is very difficult to obtain free of thiophene, which has similar physical and chemical properties. In fact, thiophene was originally discovered as a contaminant in petroleum-derived benzene some decades ago, when a reaction of benzene (with thiophene impurity) from petroleum was found not to occur in benzene made synthetically.

It is, however, probably still cheaper to obtain it this way than from some synthetic method, of which the best for large-scale industrial production is undoubtedly the trimerization of acetylene under suitable temperature and pressure and catalytic conditions. Acetylene is obtained from heating crushed lime with a surplus of carbon (e.g. as coke from coal) which produces calcium carbide, CaC2, in which the carbon is present as the acetylide anion, and this is hydrolysed by water to produce acetylene.

As a means of producing benzene for large-scale use as an industrial solvent, or for synthesis of aromatic compounds e.g. as reagents or plastic polymers, it is still more expensive than that derived from petroleum, but this will eventually change.

John W.


Organikum - 3-8-2004 at 01:52

Disproportionation of toluene or xylene using the Radzivanovskii* in-situ AlCl3 catalyst works and yields about 30% benzene from toluene.

*This catalyst is usually prepared by venting dry HCl gas into benzene/Al. Here the Al can be added to the toluene directly, foil works. HCl is vented in for several hours then the benzene is distilled off continously.

IPN - 4-8-2004 at 10:39

I gave a try to the NaC6H5COO + NaOH reaction and it was a success :)

From 50g NaC6H5COO and 14g NaOH I got ~24ml of orange liquid with a sharp smell and a boiling point of ~82C. It probably contains impurities so I will redistill it tomorrow. Theoretical yeild would have been 26ml.

It seems that this method is quite productive, and the best, very simple :)

vulture - 4-8-2004 at 12:51

Sharp smell? In my experience benzene has a rather pleasant smell, very similar to toluene.

Organikum - 5-8-2004 at 03:01

Benzene is definitively a colorless liquid with a pleasant aromatic smell - being the mother of all aromatics the smell of benzene is THE aromatic smell per definition.

I thought the sodium benzoate to benzene synthesis would require CaO and a copper-salt catalyst, but I may be wrong here.

vulture - 5-8-2004 at 03:52

on a side note; my recent oxidation of toluene with persulfate yielded a yellow/orange layer too. Maybe this is another oxidation/polymerisation product?

Organikum - 5-8-2004 at 09:27

...dunno, vulture. No recent experiences with persulfate oxidations and memory...

The Radzivankovskii catalyst is somehow tricky it seems. It takes a long time until the reaction HCl/Al kicks in. The addition of AlCl3 or SnCl4 so avail should help. Other salts will be tried soon.
Amalgamation of a small amount of the Al works well (no poisonous Hg-salts required, just add some Hg to a piece Al-foil...).

Alternativly some AlCl3 may be prepared with Al in DCM or perchlorethylene and chlorine. This can be used to kickstart the HCl/Al reaction....

No problem, I prefer Cl2 over HCl-gas anytime :D

unionised - 5-8-2004 at 12:41

Mixtures of Al and chlorinated solvents can decompose violently. I bet you can get a detonation if you add Cl2 and are unlucky.

The deffinitive aromatics are from the resins benzoin, tolu, and styrax.
Benzene stinks, but it's not "sharp". my guess is an impurity the colour hints at that too.

[Edited on 5-8-2004 by unionised]

IPN - 7-8-2004 at 05:52

Well there certainly are some impurities as I evaporated a small sample of the distillate and it left small amount of orange highly staining precipitate. I would have redistilled the benzene today but I forgot make some ice. :(

I also noted that the bottom of my 250ml rb flask was severely dissolved, probably because of the NaOH :(
Next time I test this I will use that CaO and copper-salt catalyst as Organikum said.

[Edited on 7.8.2004 by IPN]

Organikum - 7-8-2004 at 07:14

Maybe certain chlorinated solvents form explosive mixtures with Al under certain conditions. But I guess if chlorine is present in excess decomposition is unlikely to happen.
I had never problems using DCM or perchlorethylene and the method is not mine but from some russian article.

But of course - and thats truely my fault that I didnt mention this - chlorination of Al in chlorinated solvents has to be done IN THE COLD - cooling is to be applied. Also here: adding a small piece of amalgamated Al, a piece of foil 2x2cm dipped into mercury for five minutes suffices, will get the reaction going smoothly and no elevated temperatures are needed anyways.

Sorry its easy to forget things which got understood for oneself.

Organikum - 7-8-2004 at 07:20

Another point:
Toluene and chlorinated solvents have to be dried before use - mostly.
I have one manufacturer I know the solvents are dry but another - the products even higher priced but coming in nice brown chem-glass-bottles - has water in all products as I found out. To late I want to add.

CaCl2 for two days and shaking from time to time does the trick nicely. Coffee-filter filtration is sufficient, some small amounts of CaCl2 dont have negative influence here, perhaps it even helps, NaCl2 (dry) is told to do so.

IPN - 8-8-2004 at 04:27

At last I got it redistilled and as a result I have 22,6ml of water clear benzene (or so I think :P) which boils at ~81C and smells much nicer than the unpurified stuff :)

Finally I can make some aniline. :D

Btw. when I made benzene in the first place I noticed that the temperature control is quite critical, too hot and you get very dark orange oil, too cold and you get nothing.

ordenblitz - 15-8-2004 at 11:25

After reading this thread I decided to try the distillation of a benzoate to benzene.
In a rb flask, in a heating mantle, I placed 15 gms of sodium benzoate.
After about 20 minutes or so of heating, through the condenser, began to arrive a slightly viscous, yellow-orange liquid with no appreciable smell. It was about the thickness of motor oil and I don’t really know what it was but it certainly didn’t remind me of benzene. Yield was about 4ml until the residue in the flask was fully carbonized.

Marvin - 15-8-2004 at 11:59

Maybe from not understanding the method.

C6H5COONa -> C6H6 + whatever,
Whats wrong with this picture? Missing hydrogen.

It would be very interesting to know what you have made. Maybe biphenyl, maybe benzophenone.

The idea of the pyrolysis with Ca(OH)2 or excess NaOH is that the H provides the missing hydrogen as the CO2 is removed.

Basic calcium benzoate decomposes,
Ca.C6H5COO.OH => C6H6 + CaCO3.

The reaction with sodium hydroxide doubtless proceeds in a very similar manner.

JohnWW - 15-8-2004 at 12:55

I have an idea that, if there was any water present, the oily yellow-orange liquid, instead of the expected benzene, obtained from pyrolysis and distillation of sodium benzoate, may be a (possibly impure) solution of benzoin in benzene or other organic liquid, with NaOH being left behind:

2 C6H5COONa + 2H2O -> C6H5-CO-CHOH-C6H5 + 2NaOH

Its usual method of pure synthesis is by the self-condensation of benzaldehyde in the presence of KCN, however.

To produce C6H6 from C6H5COONa, in any case, requires a source of additional H in a reducing environment, with other possible left-behind products being Na2CO3 and HCOONa if there is cleavage from the C6 ring. In the absence of a source of additional H, probable products would instead be substances like biphenyl, C6H5-C6H5, or diphenyl ether, C6H5-O-C6H5, or benzophenone, (C6H5)2C=O, or diketobenzoin, C6H5-CO-CO-C6H5, or benzoic anhydride, C6H5-CO-O-CO-C6H5, depending on the temperature conditions and any other substances present. The last two do not involve any cleavage of the C6H5-C bond. In any case, pyrolysis of compounds like benzoates seldom results in any one pure product.

John W.

Marvin - 15-8-2004 at 21:55

Seems to be a few extra oxygens on the left of that equation John. I'm not convinced you could sell me a pyrolysis reaction that leaves sodium hydroxide behind anyway.

After furthur consideration, and dispite the lack of calcium or any other divalent cation in the system, I'd put my money on benzophenone as the primary distillate.

Many Methods

Turel - 15-8-2004 at 22:30

There are several methods used to decarboxylate and decarbonylate aryl compounds.

Heating aromatic aldehydes with H2SO4 yields the arene + carbon monoxide.

Benzoic acid heated with copper and quinoline yields benzene. You can even heat benzoic acid with concentrated H2SO4 and end up with decent yields of benzene.

Other noncatalytic methods involve redox disproportionation of calcium formate benzoate half salts.

I can post mechanisms for most of these style of reactions if the desire arises.


Marvin - 16-8-2004 at 01:06

The mixed calcium salt of benzoic acid and formic acid is the classic (allthough not very high yeilding) method for benzaldehyde.

It would be nice to see some yeilds for these processes, but I havn't seen any.

Polverone - 16-8-2004 at 10:20

The reaction pathways for the decomposition of salts of aromatic carboxylic acids was investigated by TG-MS. The calcium, potassium, and sodium benzoates undergo major weight losses at 509 ± 2 °C. Calcium benzoate undergo a free radical decomposition reaction to form benzophenone, benzene, and calcium carbonate as the major products. Sodium and potassium benzoate undergoes an uncatalyzed Henkel Reaction to form dicarboxylic acids and aromatic hydrocarbons and a free radical reaction to form coupled aromatic products and char.

The entire <A HREF="">article</A> makes interesting reading.

thefips - 21-8-2004 at 12:04

I read,that benzene can be made by distillation of phenol and Zn-powder.Has someone more information about it?
Phenol can be made by heating salicylic acid and I think it is a good starting material.

JohnWW - 21-8-2004 at 13:13

Originally posted by Marvin
Seems to be a few extra oxygens on the left of that equation John. I'm not convinced you could sell me a pyrolysis reaction that leaves sodium hydroxide behind anyway.

After furthur consideration, and dispite the lack of calcium or any other divalent cation in the system, I'd put my money on benzophenone as the primary distillate.

I have checked the equation I gave for balance, and it seems to be correct.

John W.

JohnWW - 21-8-2004 at 13:15

Originally posted by thefips
I read,that benzene can be made by distillation of phenol and Zn-powder.Has someone more information about it?
Phenol can be made by heating salicylic acid and I think it is a good starting material.

That method would be thermodynamically favored; any electropositive metal would probably do it

John W..

Marvin - 21-8-2004 at 22:27

Ive found this myself but I dont know if its just something that apears in books as a pinch of this, pinch of that reaction to demonstrate the relationship between phenol and benzene, or if its a process that can be done in decent yeild large scale.

Phenol can be made from oil of wintergreen, but its a lot of steps from that to benzene even if this works.

Quoteing entire posts and adding one line is not nice to read and one post per person replied to isnt good netequette either. Please quote as little as required for your answer to make sense and put all replies in the same post.

2 C6H5COONa + 2H2O -> C6H5-CO-CHOH-C6H5 + 2NaOH

Left hand side, 6 oxygens, right hand side, 4.

More electropositive metals than zinc may also be thermodyanically favoured for the reduction of phenol, but you also favour other reactions like liberation of hydrogen and formation of a salt. In organic chemistry if the references say one specific metal every time, there is usually a reason.

Organikum - 22-8-2004 at 01:58

IPN, what temperature do you recommend as being not to cold (no benzene) and not to hot (oil)?

I am right if I assume that you want to say the mixture shall be heated slowly just to point decomposition starts? How hot is this?

More NaOH might help, I heard an excess of base is favorable, adding CuSO4 might be worth a try too.

If this works out fine, say clean benzene after two redistillations over a Vigreux in or near to the claimed yields, I admit this being the best method for benzene up to now, beating my toluene disproportionation by far.

It would be a real charm. :D

JohnWW - 22-8-2004 at 20:22

Re my post of 16th August: I have had another look at the possibility of hydrolysis of sodium benzoate and subsequent coupling of radicals by way of boiling in aqueous solution, to produce benzoin:

2 C6H5COONa + 2H2O -> C6H5-CO-CHOH-C6H5 + 2NaOH + O2

However, because it involves partial reduction of two carboxyl groups to a C-C bond (via a free-radical mechanism) and a C-H bond. while leaving one C=O and one C-O bond unchanged, with removal of oxygen, such a reaction would be thermodynamically possible only in the presence of a strong reducing agent which takes up the O2 removed. At the same time, this reducing agent would have to be one that is not dissolved by NaOH, which rules out Zn or Al and all other amphoteric metals, and dies not react too rapidly with water, which rules out nearly all more electropositive metals. The only common metal that therefore just might be usable as a reductant is Mg, as a powder:

2 C6H5COONa + 2H2O + 2Mg -> C6H5-CO-CHOH-C6H5 + 2NaOH + 2MgO

Other possibilities are use of some non-metallic reductant which is not affected by H2O or NaOH, and is stable in boiling aqeous solutions, although I cannot think of any offhand.

Even then, competing reactions may result in formation of other compounds like benzaldehyde, benzyl alcohol, toluene, benzodiketone, 1,2- diphenylethane, and, in the presence of gaseous oxygen, benzoyl peroxide.

Of course, the usual method of synthesizing benzoin is by the self-condensation of benzaldehyde, involving an oxidation rather than a reduction, in the presence of KCN - but the use of KCN is very hazardous, so alternatives would be desirable if they culd be found.

John W.

Organikum - 22-8-2004 at 23:34

JohnWW: After my experience the theoretical approach to determine reactions is mostly futile - these stupid molecules just dont behave as they are thought to ;)

Therefor in Chemistry - in organic chemistry at least - reactions are first done and afterwards a nice explanaition WHY it did yield certain products in certain amounts is produced. In the article written afterwards this order is changed of course - looks much better. :mad:

This limits the scope of your theoretical considerations as you will agree - not that I dont see them as interesting.


IPN - 23-8-2004 at 09:47


I am right if I assume that you want to say the mixture shall be heated slowly just to point decomposition starts?

You assume correctly. I didn't have the possibility to monitor the temperature of the decomposing mixture, but it can be easily seen that too strong heating will start to produce some sort of thick orange vapour which will condense in to that oil. Then again when you heat it slowly, it will first melt nicely and then start to decompose, this will take time but the benzene will be purer. One thing to keep in mind is the intimacy of the mixture. The reagents should be ground to a very fine powder (preferably in a ball mill for few hours). This will reduce the erosion of the flask and speed up the reaction.

Basically just heat slowly with a small flame and you'll be fine. :)

ordenblitz - 23-8-2004 at 11:54

Second attempt using 7 gm sodium hydroxide and 25 gm Sodium Benzoate roughly crushed together in a mortar and pestle.
The mix placed in a 500 ml rb flask, in mantle equipped with a condenser. Heat was applied in a slow manner for 30 minutes before a slightly orange/yellow low viscosity condensate began to collect. The process continued for 35 more minutes until no more was collected.
The product was then redistilled rather quickly, easilly separating the orange solid from the desired material.
The final result was roughly 8 ml of, by all apperances, is a fairly pure product.

No visable damage to the flask was noted even though the reactants were not milled to a fine state as was suggested.

FrankRizzo - 23-8-2004 at 15:40

Would heating a mixture of NaC6H5COO and H2SO4 produce a yield?

2NaC6H5COO + H2SO4 -->Na2SO4 + 2C6H6 + CO2 ??

I'm worried about damaging expensive glassware with NaOH..

[Edited on 23-8-2004 by FrankRizzo]

[Edited on 23-8-2004 by FrankRizzo]

JohnWW - 23-8-2004 at 17:31

I have done some more digging n textbooks as regards possible ways to decarboxylate carboxylic acids including benzoic acid and benzoates. Carboxylic anions do not decarboxylate readily; but carboxyl radicals, RCOO•, do, by losing CO2 and leaving an organic radical:

RCOO• -> R• + CO2

Such carboxyl radicals can be generated by:

(1) the Kolbe electrolysis, in which an alkali metal carboxylate is electrolysed. The anion loses an electron at the anode to produce the carboxyl radical, which loses CO2 to produce an organic radical as above, and two organic radicals then usually combine to produce a dimer:
R• + R• -> R-R
In the case of benzoates, this would result in biphenyl, (C6H5)2. Catalytic hydrogenation of the C-C single bond between the rings (which is longer and weaker than normal C-C bonds) under mild conditions (insufficient to hydrogenate the C6 rings) would probably result in benzene, although I cannot find a reference to the hydrogenation of biphenyl.

(2) In the Hunsdiecker reaction, an Ag (not sodium) carboxylate is heated with Br2 in CC4, resulting in the first instance AgBr and Br• and RCOO• free radicals, and then the carboxylate radical decarboxylates to R•, after which the R• and Br• combine to form an organic bromide. The net reaction is:
RCOOAg + Br2 -> RBr + CO2 + AgBr
An attempt to remove the Br from the bromobenzene produced, using an electropositive metal to reduce it, would probably result in biphenyl again; or, in the case of Mg in ether or THF being used, a Grignard reagent, C6H5MgBr. This Grignard reagent would react, when added to water, to produce benzene:
C6H5MgBr + H2O -> C6H6 + MgBrOH

John W.

Organikum - 31-8-2004 at 23:27

Ok. Now we will see..... ;)

JohnWW - 1-9-2004 at 14:04

Large quantities of sodium benzoate are produced as a food preservative, and to some extent as an antiseptic. How much did that 25 kg bag cost?

John W.

Organikum - 1-9-2004 at 15:42

It is an universal antioxidant and used in large quantities for passivation of metals.

About 30 Euro. incl. VAT and delivery. EU/Jap food grade.

ordenblitz - 3-9-2004 at 16:28

I did some experimenting today with different hydroxides in combination with sodium benzoate for the production of benzene. Sodium, potassium and calcium were tested the results as follows.

7 gm. CaOH + 25 gm. C7H5NaO2 = 6ml C6H6
7 gm. KOH + 25 gm. C7H5NaO2 = 6.8ml C6H6
7 gm. NaOH + 25 gm. C7H5NaO2 = 7ml C6H6

Working with small amounts and the typical losses in the distillation these numbers are so close I call it pretty much a tie. The CaOH was the easiest to prepare since it is already a fine powder. However it has a higher melting temperature and did take a bit longer to go to completion.

I have included pics of the residue from re-distillation. It is rather pleasant smelling somewhat reminiscent of naphthalene and soap perfume.

Benzounknown2.JPG - 12kB

Esplosivo - 3-9-2004 at 22:08


7 gm. CaOH + 25 gm. C7H5NaO2 = 6ml C6H6
7 gm. KOH + 25 gm. C7H5NaO2 = 6.8ml C6H6
7 gm. NaOH + 25 gm. C7H5NaO2 = 7ml C6H6

Quite good yields I see. Working out on these yields organikum could be swimming in benzene soon :P Kidding apart, could you try a mixture of bases instead? I mean in literature a mixture of CaOH/NaOH is commonly used for decarboxylation, this being known as sodalime. The fact is that most propably it sort of lowers the melting point of the CaOH which is difficult to melt. Maybe a better yield could be obtained.

Organikum - 4-9-2004 at 01:12

If I did the calculations right, this says about molar 40-50% yields.

25g benzoate being about 0,17 mol
7ml benzene being about 0,08 mol

(correct me please if I got it wrong - it was a quicky...:D )

Wasnt the classical method starting from calcium-benzoate and CaO ?

S.C. Wack - 4-9-2004 at 01:57

Yes, 3 or 4 gallica refs say 3 parts(wt) CaO to 1 of benzoic acid. Yet all the later textbooks and practical lab books that I've seen (5 or 6) say NaOH.

Nice purchase.

Might as well paste these though they don't say much. From Noyes' Organic Chemistry for the Laboratory:
82. Preparation of a Hydrocarbon by Distillation of
a Salt of an Acid with Soda-Lime.-Benzene, C6H6.
Literature.--Mitscherlich : Ann. Chem. (Liebig), 9, 39 : Mar-
ignac: Ibid, 42, 217; Wohler: Ibid, 51, 146; Berthelot: Ann.
Chim. Phys. [4], 9, 469; Hofmann ; Ber. d. chem. Ges., 4, 163;
Baeyer : Ibid, 12, 1311 ; V. Meyer : Ibid, 16, 1465.
20 grams benzoic acid.
40 grams soda-lime.
Mix 20 grams of benzoic acid with 40 grams of soda
lime by grinding together in a mortar. Put the mixture
in a small flask, connect with a condenser, and distil
over the free flame. Separate the benzene from the
water, dry it with calcium chloride, and distil. If per-
fectly dry benzene is desired, distil it a second time over
metallic sodium. Yield 8 to 9 grams.
Benzene solidifies at a low temperature, and melts at
5.42°: It boils at 80.36 ° .
This method of preparation is no longer practically
used, but it was of very great importance in the early
study of the aromatic hydrocarbons, and illutstrates a
method very general in its application.

From Norris' Experimental Organic Chemistry:
169. Preparation of Benzene from Benzoic Acid (Section 440).
--In a 6-inch evaporating dish place 10 grams of sodium
hydroxide and 25 cc. of water. Heat over a free flame and stir
until the sodium hydroxide dissolves; then stir in gradually
12 grams of benzoic acid. Evaporate to dryness over a free flame,
which is kept constantly in motion; this will take about 10
minutes. Grind the mixture of sodium benzoate and sodium
hydroxide in a mortar, and transfer it to an 8-inch test-tube.
Clamp the test-tube in an inclined position so that the mouth
of the tube is slightly lower than the other end; this will prevent
any water given off during the heating from running back into
the tube and cracking it. Connect the tube with a condenser
and receiver, and heat with a free flame kept constantly moving,
until no more liquid distils over. Measure the volume of the
benzene and calculate the number of grams and the percentage
yield obtained. Separate the benzene from the water, dry it
with calcium chloride, and distil, noting the temperature.
Benzene melts at. 5.5 °, boils at 79.6 °, and has the density
0.878 20C/4C. The yield should be about 6 grams.
NOTE.--By converting the benzoic acid into sodium benzoate in the pres-
ence of an excess of sodium hydroxide, an intimate mixture of the two sub-
stances is obtained. In this condition the compounds enter into reaction
more readily and at a lower temperature than does a mixture of the acid
and soda-lime; such mixtures are commonly used in the preparation of
hydrocarbons from acids.
The benzene prepared in this way contains a small amount of biphenyl,
which may be isolated from the residue left after the distillation of the hydro-
carbon. The residue on crystallization from alcohol yields crystals
of biphenyl, which after two sublimations melt at 71C.

[Edited on 4-9-2004 by S.C. Wack]

Organikum - 4-9-2004 at 02:36

Thanks, S.C.Wack!
Very helpful!

I did some quick calculations again and something up.

-Sodalime is a mixture of CaO and 5-20% NaOH. It contains usually 6-18% water.
This speaks the Merck. The Merck doesnt tell why the CaO doesnt react with the water present, is it bound by the NaOH so strongly that thus is prevented?

- The calculations with the second procedure posted by S.C.Wack showed that there a far bigger amount of NaOH was used than ordenblitz had in his experiments.
It might be favorable to double numbers on the NaOH to use.

Say 14g NaOH to 25g sodium benzoate.

He talks of about whopping 75% molar yields, this Norris guy, wow, promising, very promising...... :)

Intimate mixing will help. I will yank the benzoate together with the hydroxide through my trusty secondhand coffeegrinder model "socialistic realism" and will hope this suffices.

JohnWW - 4-9-2004 at 21:28

S.C. Wack,
Where did you get those two textbooks you gave as references? Can they be downloaded anywhere?

John W.

Marvin - 4-9-2004 at 21:53

The Merck is being cryptic.

Sodalime is made slaking quicklime with a concentrated solution of sodium hydroxide, so most of the water does hydrolyse the quicklime.

I understand its particually good for gas phase reactions because sodium hydroxide itself would melt at a (fairly) low temperature presenting a low surface area. Sodalime keeps a high surface area even at high temperatures but otherwise behaves much like sodium hydroxide.

I look forward to hearing about this decarboxylation being done large scale.

Organikum - 5-9-2004 at 00:52

Some observations whilst trying a 100g benzoate batch:

Tried 100g benzoate intimately mixed with 60g sodium hydroxide in a 500ml flat bottom flask placed in a sandbath. (actually a saltbath, NaCl, but makes no difference)

Upon slow heating benzene started to come over, perfectly as it seemed at 80°C.
But not very much.
Then temperature rised and yellow stuff came over at about 90°C.
The content of the flask seemed to be not molten at all?
It was found that the lower part of the mixture has molten but the upper part formed a kind of cover not falling down to the bottom. Some agitation with a strong wire brought this down, some more benzene came over, but the amount is to small to be called a satisfying result.
The first fluffy then sticky consistence of the benzoate/hydroxide mixture seems to cause problems.

I will try again using an Erlenmeyer which seems to have a better suited shape for this kind of reaction.
If this doesnt help much I will mix some NaCl in or on top, maybe this provides enough weight to get the mixture down.

Other ideas are welcome.

Organikum - 5-9-2004 at 14:17

New run:

- 1000ml Erlenmeyer, 100g benzoate, 60g sodium hydroxide, about 50g tablesalt.

Hydroxide and some salt where grinded together in the coffee mill and admixed with the benzoate by the ol´ "inflated plasticbag" (tm) method, whereby some ungrinded salt was added.
Into the Erlenmeyer, put in a sandbath isolated with glasswool and heated up.
Clear distillate comes over at about 85°C - regarding the known misreading in this constellation I can say thats pretty ok.

Tomorrow I post quantitative results on this, after washes and redistillation, but I am optimistic on this.
Also tomorrow I will try something else which is thought to solve the problem of clogging and heatdistribution. No, not sand mixed by, as I am not sure if thats a good idea with molten NaOH.
What? Surprise, surprise.
And pictures so it works :D


Organikum - 5-9-2004 at 16:46

This is a real killer of glassware.
Dont try this with expensive labglass! I did and every try cost me one flask up to now.:(

Not during the reaction but afterwards whilst cooling down/cleansing.
The fused mass clings so badly to the glass that the different thermal expansion breaks it whilst cooling down. A thickwallet roundbottom might survive it, but its not me to try it.
I will have to do an expedition to the used-glass container tomorrow to get me suited disposable glassware.
This for sure can be done in a beerbottle, but I will get me some more longnecked bottles.
I running short on grounded glassware now.
Damn, damn, damn.

Sounds like you could use a guy that's handy with a glasstorch.

Hermes_Trismegistus - 5-9-2004 at 17:30

;)Cuz ground joints are only about 7 bucks apeice and/or sticking new bottom blanks on old joints isn't exactly rocket science if you have a pair of crossfire burners (or so I have been reading)

Someone had suggested a method for plating borosilicate with nickel in a thread long ago in a faraway land. Would that help protect your glassware?

What about manipulating a few peices of gold foil into position, covering the bottom and sides of your flask with a stirring rod and rubber policeman? Would that help your flasks survive? Would the gold foil be inert to the reagents at elevated temperatures?

ordenblitz - 5-9-2004 at 18:20

I've done all my testing in a 500ml pyrex single neck rb flask in a heating mantle. It seems that I have been using far higher temperatures as well.
I don't see anything happening usually until 150c + especially when using CaOH. Cleaning really isn't a problem either, a little carbonaceous material that comes right out with a hot water/micron soap- slosh.
I wonder what is going on.

BromicAcid - 5-9-2004 at 19:27

Reminds me of decomposing lead nitrate to form nitrogen dioxide. The lead monoxide forms readily attacks the glass so you take aluminum foil and cram it into the bottom of the flask, spreading it out with a stirring rod and then you add your nitrate, the lead monoxide thus formed does not get to the glass to attack it.

Although aluminum is not going to cut working with hydroxide, maybe go to an arts and crafts store and pick up some gold leaf or silver leaf, might be a little thin but then again it might work. Of course my vote goes to nickel foil ;)

S.C. Wack - 5-9-2004 at 20:47

Would slowly reducing the heat help? I am mostly through distilling now, following Norris, except using 24/40 glassware and a 50 ml RBF. So I'll find out.

I'm using a blowtorch. Quite a bit of heat and time seems necessary, no surprise. The smell is aromatic and there is no color. So far. The yield looks to be as advertised. My benzoic acid was sold as "lab grade" with no other comments.

(EDIT: Turning up the heat slightly only gave dark vapor that colored the distillate dark yellow as I was typing the rest of this post, with no extra benzene yield.)

A metal bath or tube furnace would probably be best for this. Lots of room for experimentation to get this to work on a large scale.

The rest of this post is off-topic, in reply to JohnWW's post above.
JohnWW, the scans were done myself, and I would upload them if I had scanned the whole books. But I only scanned perhaps 50 pages of each. They were from the library. The William A Noyes (1897) book was literally falling apart due to the horrible original binding. It could not have been scanned any more than it was without having to pay for it.

The James F Norris book (1933 ed.) has been checked out since I posted, and isn't due back for 3 months. Interesting since I was the only one to check it out in the past several years. Perhaps it will soon be seen on the FTP?

I didn't want to scan the whole books because they are so similar in syntheses to the ones already on the FTP - and to many others that also are not - and I will be scanning a similar 550 page organic book soon. I've done enough scanning lately (other partial scans, Inorganic Syntheses, Inorganic Preparations, Inorganic Laboratory Preparations, Glassblowing for Laboratory Technicians, Techniques of Glass Manipulation) so I am going to be very picky about what I scan. I was going to scan a classic, but then someone said that he was going to soon. I hope he rescans the first attempt, too. By odd coincidence, those 2 books are also not due back for 3 months.

Everyone else, there are (the computer says, this seems low) 4546 chemistry books and journals in the main local university library, one of thousands of such libraries across the world. And there is always So many books, so few scanners. There are enough theoretical books uploaded - practical books are so much more useful. Perhaps you could be doing more for your online community?

[Edited on 6-9-2004 by S.C. Wack]

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