Sciencemadness Discussion Board

Benzene Extraction from Petrol/Gasoline

len - 17-7-2006 at 18:20

Benzene is a horrible substance. My fat chemical hazards reference book, which admitedly lists most substances as horrible poisons (I think the old adage of crying wolf applies here) goes bezerk under the entry of benzene, with the words carcinogen repeated about ten times, a multitude of chemical societies listed which class it as a carcinogen, all its horrible effects on the body catalogued, its a right-to-know substance, minimum allowable concentration 0.5ppm etc. The entry itself occupies about a page and a half, thats 10 times the volume for most other chemicals. At the end theres a warning that there might not be a minimum allowable exposure to a carcinogen. In Australia its classed an S7 poison together with cyanides etc, which means its purchase is registered and controlled.

After reading all this information I was truely shocked and praying I never come into contact with this horrible stuff.

The next shock was finding out that common old petrol, the stuff my garage (as do most garages) whiffs of when a car is being fixed, the stuff I have seen some people siphon with their mouths, contained up to 5% benzene in 1997 (although in the US and now in the EU legislation is coming up to limit its concentration to 1%). 5%, thats more than acetic acid in vinegar, and if you can get a good whiff of acteic acid with a boiling point of 117C from vinegar, then petrol should give you a good whiff of benzene, which boils at only 80C. Nobody informed me of that. It figures, authorities love to show their overt concern for safety by banning and limiting stuff. This costs them nothing, allows them to demonstrate their high standards, while exercising their real love - power. But when money gets involved, when its a case of quantifying their concern with something of value to them, then they start re-evaluating as to how much it really matters to them. Apparently they are worried about benzene less than it costs money to make petrol benzene-free.


I have found reports on the internet claiming the air at service stations is only 25ppb benzene on average. But that is ambient air, which I know from experienece does not smell of petrol. One gets a good whiff of the later only when standing over a nozzle, and especially when replacing the latter as this normally involves spilling some petrol. So one would expect garage mechanics and perhaps people who do motoring in general to show higher rates of myeloid leukemia than some country folk leading a reclusive life, and in any case, an increase in the incidence of leukemia as the use of petrol increased from the 1930's onwards. Yet I have read reports that no such trend has been observed!

I could find no figures on the internet for benzene in Australian petrol, but with all the overt concern for safety here (we are not allowed to climb ladders alone at work) I thought it would be on a par with the EU. In any case I decided to do my own investigation (with such a sensitive issue I dont know how much one could believe published data anyway).

500ml of petrol were fractionally distilled. The most voluminous fraction was that boiling 40-60C (pentanes etc) 57%. The 60-75C fraction was 8%, while the benzene containing fraction 75-100C, which includes heptane, was only 12ml (2.4%). So far so good. However the 60-75C fraction should also include benzene, since the lower boiling point alkanes, of which petrol mostly consists, lower the boiling point of its mixture with them.

I now looked around for a test to separate the aromatic in my two fractions from the aliphatics. Most of the books I looked at suggested nitration. The benzene ring is stable and nitrates easily in a HNO3/H2SO4 nitrating mixture without cleavage, the alkanes are unreactive, while alkenes oxidize. Although none of the books looked at this as a quantitative problem, I decided that by measuring the nitrobenzene formed, and asuming the yield quoted in nitrobenzene preparations, I should get a good idea of how much benzene was present in my original mixture. One concern was as to the separability of the alkene oxidation products from the nitrobenzene. For this I would need to know what happens when alkenes are oxidised with nitric acid. The textbooks are useless here. They mostly appear to be copies of each other. They all seem so logical and compelete when one reads them without a problem at hand, but when you have a specific problem you see the omissions. I think their authors would regard the inclusion of phenomena that are complex and they can not fully explain as interefering with the aim of their presentation, of a subject they are perfectly in control of, and/or would undermine their authority.

In any case 30 ml of the second fraction were nitrated, proceeding as in Vogel (or in the nitrobenzene thread in this forum, fortunately I did not have fuming HNO3 available, else I would have fallen into the same trap as described there). I noticed during the reaction that as well as an orange oil a green particulate substance which disolved with shaking was formed, and was sure this was oxidation products from other components of petrol. None of the textbooks make mention of it. Yet reading postings on the forum, I see this same phenomenon mentioned starting with, presumably, real benzene. As I said before I see this as a really is a advantage of this forum over so-called professional publications. People here do not mind mentioning things they do not fully understand.

At the separation stage three layers were formed. The upper layer, 17ml, was lighter than water and consists presumably of the alkane/unreacted portion. The middle layer, 13ml was orange, oily, and heavier than water. This must contain the nitrobenzene, and due to its orange rather than yellow colour I would have thought it to contain a good portion of unknown alkene oxidation products, however reading the nitrobenzene thread, I see that a similar result can be obtained with relatively pure benzene. The green stuff was contained in the densest acid layer, and became more apparent when water was added to it.

I washed the nitro layer with water and carbonate several times, and it became opaque orange-brown and emuslified. Disposing of washings down the sink I caught a very definite, pleasant, medicinal-almond odour, I could not distinguish it from benzaldehyde. Reading the same on the forum, I am now confident I have nitrobenzene in good yield.

Now as to the benzene content in Australian winter petrol. Going by the unreacted fraction, if we assume the rest was benzene, we have 13ml out of 30ml in the second fraction. I.e. 43%, while 2nd and 3rd fractions make 10% of petrol. All in all petrol contains 4.3% benzene. Going by the amount of nitrobenzene formed assuming its essentially pure, we have 13ml nitrobenzene, which equates to 11 ml pure benzene initially present. I.e. 36% of the fraction, or 3.6% in petrol. However if we take into account the 85% yield reported in textbooks for the nitration reaction, this becomes 4.3% again!

Overall a very bad picture for Australia, on the fronts of both safety and hypocricy.:mad:

P.S. The final step would be to distil the dryed nitrobenzene, but Im hesitant to do this. The books say it forms explosive hazards whe heated with many other organic substances, while the distilations performed in textbooks would not contain even trace amount of some of the substances present in my case. Furthermore I have no definite idea of what these could be. I also noticed the formation of a yellow percipitate if the washing (containing acid) were allowed to stand overnight (even in the cold). This presumably is dinitrobenzenes, which are explosive when heated. Again this observation was mentioned in the thread, while books say nothing on this. They all talk about a vigorous nitration environment and 100C needed to form dinitro. Has anyone had any experience distilling impure nitrobenzene?

[Edited on 18-7-2006 by len]

Edited title. Chemoleo.

[Edited on 18-7-2006 by chemoleo]

Magpie - 17-7-2006 at 19:06

In response to your question I will quote from two university lab manuals that have nitrobenzene synthesis procedures:

1. copyright 1960: "Distill the nitrobenzene using a small distilling flask and an air-cooled condenser." "Collect the portion boiling at 200-215C, but do not go above this temperature nor distill to dryness because the small quantity of residue contains dinitrobenzene which may decompose violently at high temperature."

2. copyright 1950: "The boiling point rises rapidly, and the product is collected at 205-212C." "It is important not to distill quite to dryness, as the residue (mostly m-dinitrobenzene) may decompose violently if heated much above the boiling point of the nitrobenzene."

I followed the first procedure when I made nitrobenzene from homemade benzene (from Na benzoate).

I hope this helps. Nice work, BTW.

len - 17-7-2006 at 19:31

Thanks for that Magpie. I have seen these very same distillation instructions in my textbooks, but what worries me is that they presume you would be distilling nitrobenzene obtained in the nitration of the relatively pure benzene they describe. The hazardous substances book lists a whole pile of organics with which nitrobenzene forms an explosive hazard, and since none of the publications I consulted revealed a) the relative composition of petrol b) the full range of oxidation products of alkenes with nitric acid, I really
dont know if theres anything mixed with the nitrobenzene that could make distillation dangerous.

Your benzene would of course have been much purer than mine. What sort of temperature is needed to do the decarboxilation you mention? Is there much charring? What sort of yield do you get? I presume if done in glassware it would be ruined since hot NaOH attacks glass. I was thinking of using CaO for the same procedure. Dont know if it would work.

Magpie - 17-7-2006 at 21:20

There is a huge thread here called "Benzene synthesis." It will likely answer all your questions. Use the search function to find it.

I used a large glass test tube, and yes the molten NaOH ruins it after a few runs (cracks). Others have gone to metal containers at a semi-industrial scale. :D

unionised - 20-7-2006 at 06:04

A year or 2 back I had occasion to GC some petrol to measure the benzene content. It was about 1.5%
It's an interesting question about banning benzene from petrol. Yes, it's a carcinogen, but then again, it's not a very good one. Alcohol is also carcinogenic but I can't see anyone banning it (again) in a hurry. People working in labs a few decades ago used to wash up in benzene. Most of them didn't suffer any harm.
Life is full of risks; one reason for benzene in petrol is that it's a useful anti-knock agent. It was put in to replace tetraethyl lead. I wouldn't want to go back to that. The risks from being knocked down by a car are vastly greater than the risk of getting cancer from benzene from fuel. If you really want to reduce the overall risk you need to ban the car rather than benzene.

nitorarenes

chloric1 - 20-7-2006 at 15:14

About 7 years ago I played around with 2 or 3 nitroarenes. I believe I made ortho & para nitrotoluenes and chloronitrobenzene. Just the sweet odors and the literature convinced me of there ominously poisonous nature so I have not messed with them since. I may in the future but...they are scary:o I found a quite satisfactor way of drying the compounds though. I dissolved them in chloroform rapidly stirred the solution with anhydrous calcium chloride. An orange cloudy solution became crystal clear orange or yellow.

As far as distilling nitrobenzene, I assume it can be done safely but you will have to sit down near it and monitor the temperature closely. Be careful!:o

[Edited on 7/20/2006 by chloric1]

len - 20-7-2006 at 17:06

Thanks chloric, thats helpful. Ill follow your procedure with chloroform and report on the results.

As far as sitting next to it closely and monitoring the temperature, I think you might be trying to get rid of me;).

I agree about closely monitoring the temperature but I am not ready to die in the name of establishing that Aussie petrol is full of benzene. In fact devising a way to keep track of the distillation while being a good distance away from it is what is occupying me now.

I have meanwhile found a few worrying reactions in the literature were alkenes (and even some especially reactive alkanes) actually add an NO2
under the influence of nitric acid, rather than being simply oxidised, as my earlier sources have indicated. Aliphatic nitro compunds are explosive. I do not know their solubility in water though. Maybe they have been washed away (the washings were all dark red). Many of the oxidation products should certainly have been washed away as they would have an OH group, the NO2 Im not sure about.

Its interesting to see UK petrol is only 1.5% benzene. We are a technological backwater here unfortunately. We try to cast a good image of ourselves but frequently dont have the technology to follow up on it with actions. I am beginning to agree with unionised that the danger of benzene is overrated. It should not be S7. The human statistics I have mentioned, and what he said do not support the carcinigenicity it is purported to have.

P.S. I have not established the 4.3% benzene in petrol with absolute certainty yet. I believe that would come with distillation of my product and comparison of its boiling and melting points.

And yes, thanks Magpie I did not know how to search this site properly at first and so missed the large benzene synth page. I got one page of results on benzene and thought that was all. I am not entirely to blame though, there were no page numbers at the bottom of the page to indicate more responses to my search were available.

[Edited on 21-7-2006 by len]

[Edited on 21-7-2006 by len]

leu - 20-7-2006 at 17:40

Quote:
Aliphatic nitro compunds are explosive.


The esters of nitronic acids can be quite explosive when distilled to dryness, but aliphatic nitro compounds are merely flammable :) It will take a column of very many theoretical plates to separate benzene from gasoline, a paper on exactly how it's done might be around somewhere :D

len - 20-7-2006 at 19:03

OK, thats good, if aliphatic nitro's are merely flamable, Im
happy to do distillation. Are there any references?

Proper separation of benzene from petrol would require many plates. Please note that I did not claim to have done this. I labeled the reactant merely the 60-105 bp fraction. I clamed a better separation at the nitration stage, since this separates the aromatics from the aliphatic alkanes. This allows me to deduce how much benzene was present in the 60-105 bp fraction of petrol. This I find at 4.3%, a pretty reasonable amount Please note also this is in Australian winter petrol. Other countries have more stringent controls, and I hope they are fulfilled in reality.

Benzene in Australia is hard to purchase since its an S7 poison, and now a fairly rare chemical. The 4.3% content in petrol makes this an economic source for it (and the Australian position concerning benzene fairly hypocritical) I believe, depending on what you want to do. To isolate it further from the 60-105C fraction, of which it forms 43%, is quite an arduous task, as you point out. The other compounds its mixed with in that fraction all have very similar boiling points. But if you need it for nitration - and this is often the next step from benzene to other aromatics - then that separation need not be done. Most of the other components in the mixture do not get nitrated.

My position regarding our authorities is either

1) Benzene is a dangerous poison, put your money where your mouth is and make petrol benzene free

2) Its not so bad - a not very good carcinogen - then remove the S7 rating and all the hoopla

Magpie - 20-7-2006 at 20:51

len if you are especially worried about the alkenes I was thinking it should be possible to destroy them them by shaking the gasoline with either KMnO4 in water (Baeyer test for unsaturation) or bromine in water. I believe these tests leave benzene intact.

not_important - 21-7-2006 at 20:14

Think I would have gone for a 72-92 range for a first benzene cut. After that the KMNO4 oxidation magpie suggests, whash with Na2CO3 to pull out organic acids acids, dry, and redistil using a narrower cut for the benzene fraction.

After that you might try Br2 + Fe to make bromobenzene, instead of nitrating. Or you could steam distill the nitro layer, salt out the nitrobenzene - even extract using your pet. ether cut, and distill for the final isolation.

You might have picked up a little toluene in your benzene cut, especially if there was any water in the gas. Dry the gasoline before the first distillation. Hmmm ... you could apply a more vigorous KMNO4 oxidation to the crude nitrobenzene to convert any nitro-alkylbenzenes to benzoic acids, remove with a alkaline wash.

len - 21-7-2006 at 21:00

Very interesting Magpie and 'not_important', thank you very much.

I dont know why I did not think of the KMnO4, indeed all the alkenes should be converted to acids and washed out. The KMnO4 is aqueous of course, so I am not quite sure what sort of conditions are needed to get a good conversion of C5-C8 alkenes. There would be a problem using Br, I think its reasonably soluble in organics amd might be hard to get rid of, plus I think the nitro is more on the path to other aromatics if synthesis is what you are after. Br's plus is that its safer to distill, minus that I'd need a fume hood. Id like to try it anyway as a check on my results.

'Not important', I did not know water reduces separability of benzene and toluene. I dont believe it forms an azeotrope with either. Is that because its boiling point lies between them (I havent heard of this), or is there another reason?

As for the cut, I agree with you in theory, but in practice, almost nothing came over between 85-100C anyway, which surprised me. As the last distilate came over at 83-84 degrees the bulb temperature jumped almost immediately to 106C (and toluene). The lower range of my cut was actually 65C and not 60C as I wrote, and there was some instability in the vigreux column I used in the separation. I could only vouch for the temperature in a five degree range at best. So as far as that part is concerned I effectively did what you suggested.

About the steam distillation. That would be useful if there are dissolved non-volatiles in the nitrobenzene. The volatiles would all come along for the ride and I might end up with what I started with. Are other nitro products from C5-C8 nitration equaly volatile with steam? What about the alkene oxidation products? I have so far found very little information on the various branched compounds.


[Edited on 22-7-2006 by len]

not_important - 21-7-2006 at 22:10

Thanks, len.

benzene 91.1% : water 8.9% bp 69.3
toluene 80.4% : water 19.6% bp 84.1

Benzene was, and toluene is, used to force reactions through the azeotropic removale of water.

If you used Br2, you'd need an aq. bisulfite or thiosulfate wash to remove excess. Just that ArBr is less worrysome than ArNO2 to distill.

Steam distallation might decompose non-aromation nitro compuonds, and leave behind some of any dinitro aromatics. All the alkanes + alkenes would come on over as well, which is why you'd need to dry and second distill.

Yeah, fractionating columns can be a hassle; vigreux usually isn't a problem but usually isn't always.

len - 21-7-2006 at 23:28

Thanks not_important, you obviously know quite a lot about it, pretty impressive to comunicate with someone like that.

Meanwhile I have done the reaction with KMnO4, so can answer some of my own questions, and the degree of unsaturation in petroleum ether (I used the low bp fraction as I had none of the benzene fraction left).

I shook 10 ml of the 40-60bp fraction in the cold with dropwise addition of KMnO4 (1.4gms in 30 ml H2O acidified with 0.6ml H2SO4). It reacts with unsaturateds in petroleum ether in the cold but requires vigorous shaking to mix the layers, a brown percipitate of MnO2 was formed, which surprised me, I though in acidic solution Mn went all the way to Mn2+. Just the fumes of petroleum ether are enough to turn, the tip of an inserted pipette with KMnO4 solution brown. Its a bit hard to tell the end point of the titration, because a fine layer of MnO2 percipitate sticks to the walls of the flask. But when the liquid splashed against fresh walls by the shaking no longer showed a purple tinge, I knew all KMnO4 reacted and added more.

I was able to add 1.2 gms KMnO4 this way, which (assuming all alkenes react) gives the degree of unsaturation of the petroleum fraction as 14%. If the same is true of the benzene fraction, this is quite abit higher than I hoped :(. More concerned now about an explosion during distilation.

I hope everyone wishing to try this realises that to distill petrol one does no use a bunsen burner. A heating mantle and quickfit flask/condenser are required.:o

[Edited on 22-7-2006 by len]

[Edited on 22-7-2006 by len]

not_important - 22-7-2006 at 09:25

len - you have too little acid to take that fully to K2SO4 + MnSO4. Even twice as much weight of H2SO4 to KMnO4 will result in MnO2, roughly 1cc 98% H2SO4 per gram KMnO4; the molar ration for full reaction is 3 H2SO4 : 2 KMnO4. But then it will oxidise alcohols and such as well.

Also any alcohols are likely to consume KMnO4 too, and they will end up in the low cut - azeotropes again.

I'm a bit surprised at the apparent amount of alkenes; might be interesting to titrate a ml with KMnO4 and another with bromine water. Also, as another project, to oxidize 50 to 100 ml of raw petrol, and try to figure out the oxidation products - ketones and corboxylic acids of what chain length.

Sounds as if you may need to restart the test. Measure out the sample, KMnO4 to take out alkene, and then dry the petrol, then fractionate and nitrate.

To distill petrol? Just pour it into the bucket drop the big funnel over the top, and stack a few logs on the fire underneath.

And isn't Astralian winter blend about the same as they use in Canada during the summer? ;-)

len - 22-7-2006 at 22:12

Sorry, you are right not_important I did not do the stoichiometry. 1mol H+ per 1mol MnO4- is not enough to give Mn2+. However I think my degree of unsaturation is still right (at least it cant be less than the already fairly high figure) since the MnO4- was used up, and I assumed it all went to MnO2. The Mn couldnt reduce to a higher oxidation state than MnO2.

I think you are right, with the amount of alkenes there I have to repeat my experiment. Before that though, Ill distill the ArNO2 (with suitable preliminaries) since I cant thrown it away after all the work Ive done. It'll take some time, come to think of it, since I dont want to expose the indoors to toxic (carcinogenic?) nitrobenzene fumes and an explosion hazard, and I can't distill it outside lest the neighbours think this is on direct assignment from bin Laden.;)

Your proposal to analyse the oxidation products sound interesting - but to separate the individual products Ill need and MS and GC wont I. Iff there are N unsaturateds in petrol, then after oxidation I should get something like 3N products right?

I can just see the petrol distillation on a good fireplace. You wont need many logs to keep warm that night! Mind you some of the things I read on this site made me think that someone might just try something like that. And then there will be trouble. I wish our winters were like Canadian summers. Weve had a few -5C at night lately!

[Edited on 23-7-2006 by len]

unionised - 23-7-2006 at 00:41

All of these oxidations and manipulations will expose you to a lot of benzene and its unpleasant nitro derivatives. You have a nasty chance of blowing yourself up with the distillation too. What's the point exactly?

not_important - 23-7-2006 at 00:43

Most certainly salvage the current batch of products. You've put some effort into this, might as well hve something out of it, and a bit of oil of mirbane is nice to have around.

Most places I've read suggest that the alkenes are going to be C7 or smaller, meaning the largest product after acid-permanganate will be C6; any 1,2 alkenes will lose their terminal C as CO2. I'd be surprised if you ended up with more than 4 or 5 carboxylic acids and ... 5 ketones. While a GC would make the task easier and a MS- -would bereal nice, i'd be it could be done without either. If there are as high of percentage of alkenes as you quick test suggests, along with the degree of aromatics you seem to have found, petrol might be a useful source of reagents.

By the friend in Vancouver I talked a few weeks ago, it was running 3 to 5 C at night, but that was just past Mid-Summer so I'd expect it warmer now.

Suspected of being on assignment from Osama. Ah, the bygone innocent days of standing outside shining the laser up into the falling snow, firing up the carbon arc furnace in the back yard during the same snow, floating bits of sodium in the runoff from rain, turning off all the streetlamps with a lofted flash bomb, going down to the tips to blow up old furniture and snog...(sigh)

len - 23-7-2006 at 01:01

unionised - the point is to find out exactly how much benzene Im already being exposed to daily anyway.

not_important, I did see some bubbling, and was wondering what the hell that was. Thanks for that. Yes our Australian variety of petrol seems not as boring as I had imagined. And I havent investigated the higher bp fractions yet. Mind you its probably coming out of our antiquated refineries. In a more technologically advanced country petrol might turn out to be a lot more boring.

not_important - 24-7-2006 at 00:49

len - Australian petrol, like Canadian, has MMT in it if I've been told right. Another reason to hit the raw stuff with KMnO4 before starting on fractionating.

Your comments on the local refineries may have something to do with the apparent high amount of unsaturatedness. Alkenes are a cheap way to boost octane, within limits, so they may have increased the percentage of alkenes.

len - 24-7-2006 at 03:54

Sorry if this is a dumb question, what is MMT? I couldnt find it anywhere on the web.

Theres been a racket in Australian petrol recently, the petrol companies were getting around the 49c/l govt. excise by mixing in hydrocarbons from paint thiners (toluene) and the like, which had no tax. So we now have the excise on all hydrocarbons. Wouldnt be surprised if the alkenes are a new scheme

not_important - 24-7-2006 at 06:41

Not dumb, it's new to Aus (if they haven't dropped it) but the Canadians have been muttering over it for years

MMT :

http://en.wikipedia.org/wiki/Methylcyclopentadienyl_manganes...

http://www.globalleadnet.org/advocacy/initiatives/mmtfactshe...

http://www.sciencemag.org/cgi/content/summary/300/5621/926


I'm not fond of getting stuff with CO ligands in it on my person, a nice oxidising acid wash might be just the thing.

unionised - 24-7-2006 at 09:30

Dear God! will people never learn?
They argued for years about taking the lead out of petrol because it was a known neurotoxin, but they said it was"indispensible". I'm not saying that any fuel is going to be entirely devoid of toxicity but why did they think this was a good idea? Sure, benzene isn't nice stuff, but at least it burns to CO2 and water (ideally). Finely divided MnOx isn't a good idea.

Ironicly, alcohol has a relatively low toxicity (Any arguments and I will drink 100ml of it mixed in a pint of cola while inviting people to try the same with other solvents) but is responsible for an awful lot of poisonings.

len - 24-7-2006 at 15:08

Good God, the darn thing looks like nickel carbonyl! with a methyl pentadiene ring replacing one of the CO, and a manganese instead of a nickel central atom. (I guess thats why its called manganese tricarbonyl ..)

Its description reads:
MMT is a viscous yellow liquid with an herbaceous odor

Couldnt find its bp or whether it decomposes, but my 120-150C bp fraction is light yellow and I was wondering why. Even if it does have a high bp I feel it is likely to come over earlier, say with the xylenes. I dont know how much of it is added and what would be the test for it. Its use hasnt attracted any attention in Australia at all, I guess the mass populace is largely ignorant here, and the petroleum companies like to keep it that way.

Meanwhile I have done one of the preliminaries, and nitrated the petroleum ether, which earlier gave 14% alkenes. The products showed only two layers: aqueous and non-aqueous. No middle ring. There was plenty of frothing. I guess the lower bp alkenes are oxidised and do not add NO2. The results are acids/ketones, which are water soluble, and CO2. This is good. I hope same holds for the 65-100C alkenes.

not_important - 24-7-2006 at 21:19

If the acid strength and temperature are right you maybe can oxidise tertiary hydrogens. I assume the frothing was obvious NOx ?

Ah, the other thing that might do it - oxygenates in the gas such as ethanol?

MMT - the following sez 10 to 40 ppm in fuel, bp 231-something, photochemical short-lived. It could co-distill, dunno.

http://www.epa.gov/chemrtk/mthmntri/c14889tp.pdf


Interesting project you have, keep the updates coming.

len - 13-8-2006 at 01:19

I tried to steam distill C6H5NO2. Its a real b...., and was hard to get moving. The first thing that came over was a yellow oil, lighter than water, also smelling of almonds. I have no idea what that is, but would love to know. I know it was present in the original mixture, and was not formed during the steam distill.

The heavy stuff came over with vigorous boiling, but was unfortunately still orange. This not having had the effect I desired I decided to start again with fractional distill of petrol this time attacking with KMnO4 prior to nitration to remove branched aromatics, alkenes and any other non-benzene stuff that might be in there.

So far I have surpising results in that the sub 90C fraction turned out to be 400/750 = 53%, as opposed to 68% last time (last time almost nothing came over in the 90C-105C range, this time the flow here was substantial). The vigreux is really unstable when fractioning close boiling point mixtures with many components such as petrol. This affects the results substantially. I can maintain the temperature in the boiling flask below to an accuracy of 10C only. Most of the time nothing is coming over, and the bulb temperature is sub 50C. When the mixture below reaches a temperature at which a component present in substantial proportion is at bp at the top of the vigreux it comes over in a rush and the bulb temperature rises rapidly. A small percentage component does not have the energy to penetrate the vigreux on its own. I might need a better T controller before I proceed

[Edited on 13-8-2006 by len]

not_important - 13-8-2006 at 19:54

You try putting insulation around the fractionating column? It's also tough unless there's a substantial amount of fraction in relation to the column 'size', otherwise acting as you describe. The insulation makes it easier to keep the column at the boiling point of the currrent fraction. Sometimes a 'hat' over the upper section of the flask helps as well.

There are series of test that could be run to get an idea what is in each product group. Or maybe you're thinking of building an FTIR and a NMR 8-)

The oder of the lighter layer makes me wonder if you got some co-distillation (besides water)

len - 18-9-2006 at 02:22

I finally got round to titrating the benzene fraction with KMnO4 to remove the alkenes and substituted aromatics. The idea being to separate the benzene from the residual alkanes by nitration. I first did the procedure with pure toluene. Here are the results.

Toluene does not reduce KMnO4 to Mn2+ under any conditions. Even in almost 30% acid it yields a dark sediment of MnO2. I suggest that the authors of a book stating 'clarification of solution' did not do this experiment.

This oxidation is extremely messy. It takes a lot of KMnO4, 3.2gms per ml of toluene. In addition it is not very soluble, 5gms/100gm H2O, the reaction, although it starts at room temperature, completes only at the boiling point of toluene after 10mins, and it produces a sticky precipitate of MnO2 which covers any clean flask surfaces obscuring observation of the reaction. This makes it difficult to tell when all the MnO4 has been used up despite its intense colour. All glassware, has to be subsequently cleaned in conc HCL to remove the MnO2. To put this in perspective, to oxidise 40ml of toluene and thus make about 50gms of benzoic acid 130gms of KMnO4 are required, dissolved in 3l of water! The whole thing needs to be boiled for 20mins.

Doing the oxidation on my benzene fraction took all day. I did not have a 3l flask, and so had to add the KMnO4 in stages. In the end I was simpling adding KMnO4 in powder form to replace that consumed. I added 106gms to 40ml of the second fraction, and this still did not bring about an end point. There was an unpleasant smell produced, somewhat like molten plastic, I even had to check I had no heating rubber anywhere. I read that benzoic acid smells like maple syrop so I presume the smell was from lower carboxilic acids, resulting from oxidation of alkenes. I stopped the eperiment at the end of the day. After about 10 additions of KMnO4 and heating and cooling cycles there would have hardly been any benzene left - despite my use of a reflux followed by air condenser.

I presume most of my fraction was toluene even though it was collected below 100C, and I have to remove my earlier claim to making nitrobenzene - and therefore proving the quantity of benzene in Australian petrol. I most likely made the almost identical nitrotoluene - seeing toluene can come over before 100C. I looked up the vapour pressure of benzene, and its already half an atmosphere at 55C. This means that if the petrol cotains a fraction x of it, it will be in a ratio of about x/3 in the vapour of petroleum ether at its boling point, ie it will be evaporating 1/3 as fast as the petroleum ether. Seeing the ether forms about 50% of petrol, while benzene is expected in the range 1-5%, there will be hardly any benzene left by the time all the ether had evaporated in a simple distillation. In a setup with N theoretical plates I can expect the x/3 will change roughly to x/3^n. The vigreux collumn does not correspond to theoretical plates, and I have difficulty in estimating how many it is equivalent to. Probably 2-3, which still means all the benzene would have left the the pet ether. I hence have to modify my procedure, and repeat the experiment.

1) Collect the whole low boling point fraction, below the bp of benzene, 40-80C.

2) Titrate a small portion of this with permanganate to find amount required for complete fraction. To the latter add about 500ml of water, sufficient acid to take it to MnO2 (I think H2SO4 can be counted diprotic in this regard), and slightly over the calculated amount of KMnO4.

3) Boil with reflux and shacking for 30mins. Decolorise the
solutions with NaHSO3. Only benzene and alkanes can now be left in the organic layer

4) Separate the organic layer and nitrate to separate the alkanes from benzene.

Does anyone have any comments on the new procedure

len1 - 1-3-2007 at 21:02

Well I have now been able to complete the first two steps above, the results are surprising.

Under slow distilation of petrol using a vigreaux there is a very wide regions of temperatures in which nothing comes over, between 75-109 degrees. This allows easy separation into a low and high boiling point mix. From the bp of the later it is obvious it contains toluene. Any benzene present will be contained in the low bp mix. Checking its vapour pressure, which is already 1/2 atmosphere at 50 degrees, it is obvious it will all be carried away by the low boiling point fractions well before its own boiling point is reached. 500ml of petrol generated 190ml of the low bp mixture.

This was then titrated with KMnO4 as stipulated above, and this is the surprise. Several titrations showed that 3.6gms KMnO4 are fully reduced by the low bp mixture to MnO2. Assuming this is due to alkenes with a single double bond, we can take dimethyl-butene as a representative with a suitable boiling point. This is then converted into a 5-carbon carboxylic acid and carbon dioxide if the double bond is terminal, i.e. a gain of 10 electrons per mole. The KMnO4 releases 3 electrons per mole, so 3.6 grams oxidize 3.6/158 * 3/10 moles of the alkene = 0.0068 mole, thats 0.56 gram.

In any case the low bp fraction is highly unsaturated which is a real surprise. This means the nitration products obtained in the first few experiments were all aliphatic nitrates. Referring to the TNT book on this site, these are all quite explosive. Just as well I resisted the temptation at purification by distillation.

The benzene content can not be established by this method due to the high level of unsaturation. Attempts at chromatography will be next. Dont know if IR can help me get a quantitative result with such low concentrations?

[Edited on 17-4-2007 by len1]

not_important - 1-3-2007 at 21:14

Welcome back

What if you steam distilled the mix after oxidation, 'titrating' that near the end to avoid overshoot, and leaving it alkaline so there are no free carboxylic acids. What comes over with the water should be any alkanes, benzene if any, and ketones if they were formed.

I think that some alkanes with tertiary hydrogens do get oxidised by alkaline KMnO4, in acid solutions that's a for sure; you didn't specify the temperature you used, alkane oxidation would be unlikely in the cold. If you can make some bromine, you might try checking double bonds that way as well; make a known concentration dilute solution of Br2 in an inert solvent, titrate in dim light.

[Edited on 2-3-2007 by not_important]

len1 - 1-3-2007 at 22:15

Hello again Not Important, I see you have been busy.

Its possible what you say, I think steam distillation would be a way to separate the remnant hydrocarbons from the large amount of MnO2 they cling to. For 20ml of starting hydrocarbon the result is about 160gms MnO2 in 1 l of water in which the non-oxidised product (less than 5ml) is immersed in. Im not sure if I will recover much from this.

Your really interesting point is that alkanes also get oxidized by KMnO4. I did not know that. I have always been told that KMnO4 does not attack alkanes. Checking the internet I see that is not quite so, although rates and temperatures are not quoted. Could the entire reaction I witnessed be with alkanes? I dont think so. Although proving it will take me on another side tour. The reaction conditions were 100-120 and acidic, below that there is no reaction, or its very slow. I thought it KMnO4 attacks 3 alkanes it might also attack benzene, but could find no references to this.