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Author: Subject: benzyl alcohol oxidation
Magpie
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[*] posted on 24-4-2008 at 20:27


Len:

I agree that it is nice to be able to buy BnOH, while hoping that it is not being too heavily watched. However, I don't think anyone should take this for granted, especially in the US. It is good to have other routes, especially those using more OTC reagents.
Quote:

The only thing I can sugget is that the relative solubility of the alcohol/aldehyde in water is used. The reaction takes place in aqueous medium - thats were the persulphate is. The benzaldehyde separates from this into a separate layer - and so is essentially removed from equilibrium. If one vigorously mixes such a solution instead of letting it separate that should rapidly oxidize the benzaldehyde and send the yield well down. However if one has an excess of the alcohol, so not all of it dissolves stirring is precisely what one has to do to promote the reaction. The result is - this is my hypothesis, that the yield is entirely based on the solubility differential 5gms:0.5gms per 100ml H2O of benzyl alcohol/benzaldehyde. In this case it is indeed impossible to increase the yield as its limited by the amount of water needed to dissolve the small amount of alcohol, i.e. size of vessel is 20 times greater than max yield volume.


I think that this is a very good theory. Something I never would have suspected going into my experiments. The proof will be when I try persulfate using n-butanol as reactant. I hope to do this fairly soon.

I trust my thermometer to within a degree or two even though it is a cheap one. What I don't really trust is my vacuum guage. It is a cheap one of the hardware store variety.

[Edited on 24-4-2008 by Magpie]
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[*] posted on 25-4-2008 at 05:43


Beautiful experimental work Magpie!
A measurement of the boiling point could give you a very rough estimate on how free of benzyl alcohol is your product. I'm quite sure it must be relatively pure since BnOH steam distill much less efficiently when compared to PhCHO.
Out of interests in seeing how much, if any, benzoic acid forms, you might want to extract it from the reaction mixture rests after the steam distillation (though you probably already discarded it, but just a thought if you would repeat the experiment).

Quote:
Originally posted by len1
Magpie, in your experiment 3 you have already done whatb Ive suggested - sorry I didnt notice that before. What I dont understand is why dont we have

PhCHO + S2O8-- + H2O -> PhCOOH + 2HSO4-

given we have

PhCH3O + S2O8-- -> PhCHO + 2HSO4-

its not like theres a lack of water, and its a well known fact that aldehydes are much more prone to oxidation than the corresponding alcohols. Normal methods for preparation of aldehydes, eg formaldehyde, acetaldehyde, rely on the volatility of the aldehyde to remove it as soon as its formed, so no appreciable concentration builds up on the LHS of the first reaction. But that apparently is not whats happening here.

Fortunately, not all reagents oxidize aldehydes faster than they oxidize alcohols under proper conditions. Those reagents that proceeds trough the oxidation of geminal diols formed by nucleophilic addition of H2O on the carbonyl group will therefore normally not be able to oxidize aldehydes under anhydrous conditions (CrO3 and its complexes are among these). Actually, I think that under anhydrous conditions only radical oxidations can result in the oxidation of aldehydes to carboxylic acids. Other reagents will tend to oxidize alcohols faster than aldehydes even in aqueous conditions (HOCl is one such example; here I once gave a possible explanation on why).

Removing the aldehyde product as it forms is also an effective strategy. The removal of the aldehyde product by extraction might also be efficient to some degree, but the unfortunate problem for such a strategy is in that the partition of compounds in a biphasic system is a function of concentration. For example, let's assume the partition coefficient of the aldehyde would be a magnitude or so lower than for the alcohol. At the beginning of the reaction this difference would work toward the desired end. But toward the end, after the majority of the alcohol would be consumed, the concentration of the aldehyde product in the organic phase would increase so much as to be partitioned in the aqueous phase in greater concentration than the alcohol (resulting in the oxidation of the aldehyde inevitably prevailing over the oxidation of alcohol). Apart the consequential lowering of the yield, such strategy would also be limited to those reagent systems whose ratio of RCHO->RCOOH vs. RCH2OH->RCHO kinetics is at least somewhat less than the ratio of partitioning coefficients of RCH2OH vs. RCHO.

Therefore, I would rather search for the answer to this question in the reaction kinetics of all possible pathways. After all, in organic chemistry, it is the evaluation of all equilibrium species and the comparison of activation energies of all possible transition states that gives an answer as to what the products of a particular reaction under particular conditions are. I would therefore not be too fast in drawing conclusion based on phase transfer only.


PS: A few notes about formulas and abbreviations used in this thread:

"BnO" is a wrong abbreviation since such a molecule can only exist as a radical and it is not identical to benzaldehyde anyway.

PhCH3O is an ambivalent formula for BnOH as it can stand for anisole as well. Using rational formulas is more customary in organic chemistry, while using empirical or partly empirical/partly rational formulas is particularly discouraged exactly to avoid ambivalence.

BzOH stands for benzoic acid.

BzCl2 is not a compound and besides is not benzal chloride as obviously intended

BzO would be the benzoic radical PhCOO*, also not what was intended.

So to reiterate the international organic abbreviation conventions: Ph = phenyl ; Bn = PhCH<sub>2</sub>- = benzyl ; Bz = PhCO- = benzoyl

Let's take care with these standards or else we well have the forum suffering from an abbreviation induced disorder (nothing new, it happens all the time).

[Edited on 25/4/2008 by Nicodem]




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len1
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[*] posted on 25-4-2008 at 06:16


Quote:
BzCl2 is not a compound and besides is not benzal chloride as obviously intended


I know all that - and whats more coudlnt give a hoot and a half. We are not writing the Collins dictionary here - just look around. I havent much time, I understood what Magpie was saying. Thats all that matters. Ill leave the rest to you. Dont know what repels me more, pedantics or obfuscation.

[Edited on 25-4-2008 by len1]
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[*] posted on 25-4-2008 at 07:10


I thought i would give a link of the other benzyl alcohol thread here, in which H2O2 and TCCA/bleach are used in various conditions, to get a more global vision.

Maybe we could just merge this to one big benzyl alcohol thread? it seems to spring up enough interest to be sticky? Just a proposition.

Benzylic and secondary alcohols oxidations using various oxidants
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[*] posted on 25-4-2008 at 09:15


I remember wanting to oxidise a benzylic alcohol to a ketone, and found this neat reference, but then I winded up using another route towards my product.

Attachment: Aerobic photocatalytic oxidation of activated benzylic and allylic alcohols to carbonyl compounds catalyzed by molecular (146kB)
This file has been downloaded 1036 times

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Magpie
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[*] posted on 25-4-2008 at 10:23


Thanks Nicodem. Nicodem says:
Quote:

Out of interests in seeing how much, if any, benzoic acid forms, you might want to extract it from the reaction mixture rests after the steam distillation (though you probably already discarded it, but just a thought if you would repeat the experiment).


When I was distilling the benzaldehyde at reduced pressure I stopped when the temperature dropped off. What was left in the pot was a few mLs of a yellow translucent liquid. After cooling it turned to a milky-yellow slush. I saw no crystals (needles) of benzoic acid. I suppose it could have contained a little MgSO4 from imperfect decanting. And yes, this residual is long gone.
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MagicJigPipe
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[*] posted on 25-4-2008 at 13:58


I don't see why someone would discard benzoic acid. It's not like it's extremely cheap or common.

An amusing anecdote: When I was trying to acquire benzoic acid from pharmacies about half of them told me it was "prescription only".

Also, why is BnOH watched/unavailable? Because it can be made into phenylacetonitrile in a semi-roundabout way? It wouldn't surprise me. I was just wondering why.




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[*] posted on 25-4-2008 at 14:17


Is Benzaldehyde and BnOH watched in the US only or in other countries? Where I live fortunately they and a lot of other chemicals are free to buy (if you find someone who will sell them which is very difficult).

And a little OT, but could benzoyl peroxide be made from benzaldehyde? If yes what would be the easiest route?

[Edited on 25-4-2008 by Zinc]




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MagicJigPipe
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[*] posted on 25-4-2008 at 16:30


No, it's not unavailable in the US (I don't think it is "watched" either). I was speaking of the people in other countries (Austrailia) who said it was watched and/or unavailable.

It's easy to get in the US.




"There must be no barriers to freedom of inquiry ... There is no place for dogma in science. The scientist is free, and must be free to ask any question, to doubt any assertion, to seek for any evidence, to correct any errors. ... We know that the only way to avoid error is to detect it and that the only way to detect it is to be free to inquire. And we know that as long as men are free to ask what they must, free to say what they think, free to think what they will, freedom can never be lost, and science can never regress." -J. Robert Oppenheimer
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len1
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[*] posted on 25-4-2008 at 17:35


I dont know if we have watched chemicals. We have restricted chemicals - ones you can possess, but cant buy unless youre incorporated. I believe BzOH is restricted because benzyl chloride is, the latter being easily manufactured from the former

BzOH + HCl -> BzCl + H2O

Though I have shown them now how easy it is to make evil BzCl from BzH - which is as abundant as water - and in much better yield. If they want to change that they better refurbish all their refineries - then we'll really see if they are serious.

My advice to LE is learn some chemistry, and start going after the real crooks instead of the periodic table.


[Edited on 26-4-2008 by len1]
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[*] posted on 26-4-2008 at 09:45


What do you mean by much better yield? Benzyl alcohol with HCl should give a higher yield than any chlorination of toluene could hope to do.



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[*] posted on 26-4-2008 at 11:24


From my old experiments:
1) ØCH2OH can be detected in ØCHO via adduct with NaHSO3 (Na2S2O5 in solid state). Adduct is odorless, alcohol has weak but characteristic smell. ØCHO can be easily recovered from its adduct with the use of Na2CO3 and weithed (as has been said earlier)
2) Detecting ØCOOH can be done just with washing aldehyde with saturated NaHCO3 and precipitating acid with HCl/H2SO4.
Solubility of ØCOOH in ØCHO is high (better than 20g/100g).
3) ØCHO (ØCH2OH too) can be distilled at normal pressure without any complications. The purer ØCHO, the more inert to O2, but the best protection is tight closing bottle.

BTW. In my coutry benzyl alcohol costs ~20$/liter and is OTC, with benzaldehyde are problems, but not so big ;)

[Edited on 26-4-2008 by kmno4]
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[*] posted on 26-4-2008 at 14:48


These calculations are OK. but final result is a random number, belonging to chosen values of parameters. Besides I cannot find

PhCO-H <=> Ph-CO<sup>*</sup> + H<sup>*</sup> . . . . . . . . . . . . . . . . . BDE<sub>2</sub> = 87 kcal/mol <--- this value in RSC paper. On what page shall I search it ?
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[*] posted on 26-4-2008 at 15:00


Ups, I wanted to edit my post and I deleted it by mistake. I used the wrong value of BDE<sub>1</sub> which is even lower (79 instead of 81 used before "editing"). I got confused with the authors describing a previous estimation was 81 kcal/mol, while their calculated value was ~ 79 kcal/mol. The BDE<sub>2</sub> value is on page 179 at the end of the text column (the same value is also referenced in Table 1 of the JOC paper). I agree that the result is a random number, now even 4 times greater due to the corrected BDE<sub>1</sub>. Thanks for checking out the calculations.

--- the original post that should be above kmno4's reply above, edited with the new BDE<sub>1</sub> --- :(

Being interested in why the oxidation of BnOH is faster than the oxidation of PhCHO with the method Magpie presented, I went to hunt down the information required for a plausible explanation.
The reaction is quite certainly a radical oxidation given the conditions used (aqueous Na2S2O8 at >50°C in close to neutral conditions). Therefore the rate limiting step in the oxidation of either BnOH or PhCHO is the abstraction of the benzylic hydrogen. The bond dissociation energy (BDE; the energy required to abstract the H atom) commonly linearly correlates to the activation energy (E<sub>a</sub>;) of the two oxidation reactions. The E<sub>a</sub> is an important factor in the reaction rate constant equation (the famous Arrhenius equation). Most other parameters in the rate constant and reaction rate equations are either identical (T), average to nearly the same value (concentrations of the two substrates over time), or are assumed to be of a similar order (like the correlation coefficient between BDE and E<sub>a</sub> and the A factor in the Arrhenius equation).

In short, the BDE of the two rate limiting processes are:

PhCH(OH)-H <=> Ph-CH(OH)<sup>*</sup> + H<sup>*</sup> . . . . . . . . BDE<sub>1</sub> = 79 kcal/mol (J. Org. Chem., 70 (2005) 9521-9528)

PhCO-H <=> Ph-CO<sup>*</sup> + H<sup>*</sup> . . . . . . . . . . . . . . . . . BDE<sub>2</sub> = 87 kcal/mol (J. Chem. Soc., Perkin Trans. 1 (1986) 173-182)

So the corresponding Arrhenius equations can give us a very rough estimate of the rate constants ratio between the two substrate oxidation reactions:

Rate constant of BnOH>PhCHO oxidation: k<sub>1</sub> = A<sub>1</sub>*e^(-E<sub>a1</sub>/(RT)) , E<sub>a1</sub> = x*BDE<sub>1</sub>
Rate constant of PhCHO>BzOH oxidation: k<sub>2</sub> = A<sub>2</sub>*e^(-E<sub>a2</sub>/(RT)) , E<sub>a2</sub> = y*BDE<sub>2</sub>
Approximations: x ≈ y , A<sub>1</sub> ≈ A<sub>2</sub> (possibly very rough approximations on factors that have a great influence on the outcome!)

Therefore, after solving the above equations we get the ratio of the two rate constants:
ln(k<sub>1</sub>/k<sub>2</sub>;) = x*(BDE<sub>2</sub>-BDE<sub>1</sub>;)/(RT)
ln(k<sub>1</sub>/k<sub>2</sub>;) = x*(33495 J/mol)/( 2769 J/mol) = x*12.1 (taking T = 333 K or 60°C)

Assuming x = 0.5 as a realistic value we get:
k<sub>1</sub>/k<sub>2</sub> ≈ e^(6.05) ≈ 423

Since I'm pretty much dumb in regard to mathematics, I probably made some terrible mistake somewhere in the calculations, but anyway they are not particularly important since the information that BDE<sub>2</sub> > BDE<sub>1</sub> is already enough to tell us that the oxidation of BnOH is faster than PhCHO in this particular system (even without using a bunch of approximations). Yet, taking those approximations, the calculation indicates the oxidation of BnOH might even be a few hundred times faster than the oxidation of PhCHO (assuming the same order of reaction).




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len1
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[*] posted on 26-4-2008 at 16:45


Quote:
Originally posted by Fleaker
What do you mean by much better yield? Benzyl alcohol with HCl should give a higher yield than any chlorination of toluene could hope to do.


Why do you feel so? I have the results for the second reaction - 75% without even trying to optimize for BnCl - if you use a two-step process you can get that to 90%. My experience hydrolizing BnCl shows its an equilibrium two phase process - the yields for such things are not normally very high. Have you gotten a yield?


Rate constant calculations are interesting - I rather tend to trust my suggestion more at this stage because

1) Aldehydes, with BzO no exception, are known to be much easier to oxidize. The rational would have to be that hydrogen abstraction is only relevant to persuphate type oxidation (where its only an approximation - these things depend heavly on reaction conditions) to explain why this type of calculation gives results in accord with a presumed rate difference in persulphate oxidation but not with molecular oxygen. Its too easy to say the latter proceeds by a different mechanism - that would be selectively using facts, such things prove nothing.

2) The solubility difference we know will favour alcohol oxidation without any presumption of things that are hard to prove.

What are the corresponding energies when Ph is replaced by CH3 ? It would add credibility to this theory if they went the other way.

2) can of course be tested directly - stir the contents vigorously for 1hr and see what it does to the yield. The incomete conversion of BzOH in this experiment suggests that is a relevant test.

[Edited on 27-4-2008 by len1]
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[*] posted on 26-4-2008 at 17:54


I think we speak of two different reactions. I was talking of reacting benzyl alcohol with HX to give benzyl halide, a smooth reaction which I have done and works exceptionally well, as compared to chlorination of toluene, which works less well and is not as simple.

As for benzyl alcohol and persulfate oxidation to benzaldehyde, I make no claim to its superiority over your method as I've never done either, having had no need as benzaldehyde is purchasable for me. My friend and home chemistry lab ''partner'' has done the reaction and has mentioned its usefulness with the benzylic carbon (I don't recall him mentioning it for other alcohols). His yields were high; I don't recall the exact number, but it was certainly above 80% based on benzyl alcohol. He mentioned that the formed benzaldehyde was free of the alcohol, and that any alcohol that had not been made into the aldehyde was instead oxidised further to benzoic acid. Based on his testimony, we should not assume that this oxidising agent is selective only to the aldehyde, but as you said, depends upon the reaction conditions.

Len, your route is the cheaper of the two (toluene is twice as cheap as benzyl alcohol, about 7 USD/L versus 11/L for the alcohol) but it also comes with the nuisance of benzyl chloride. Not the worst reagent to deal with but for me, it is irritating and bothersome. It is also quite probably a longer synthesis with more material required. I think favourability of which reaction to use for benzaldehyde depends on your locale.




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len1
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[*] posted on 26-4-2008 at 20:08


Fleaker, I was talking about the reaction

BnCl + H2O -> BnOH + HCl

the reverse of the reaction you used to make benzyl chloride. I found its equilibrium constant not to be particulalry favourable to either side, and so expected the same to be true for the reverse rection by reciprocity. I dont have any experience with the reverse reaction, because BnOH is restricted here. I have to make it from the chloride rather than vice versa. Even if I could buy it it would be through speciality outlets, where nothing is cheaper than about $40/L. Toluene can be bought from hardware stores and is in a completely different price category. What sort of outlets sell BnOH in the states? I would also be interested in the yield of the above reaction. At best I think it would equal the toluene, it certainly can not be mucg better, which makes a mockery of the restrictions.

I dont of course lay claim to any method of making things, and am always on the look out for interesting reactions. Nonetheless I think the chlorination of toluene is a great way to make BnO even if I had BnOH and even at the cheap price you mention. Thats because persulphate at $10 per 2 mole is I think very expensive, Cl2 in the form of HCl and TCCA is much cheaper here being a mass consumer ingredient.

[Edited on 27-4-2008 by len1]
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[*] posted on 26-4-2008 at 20:39


That is a valid point about the molecular weight of the persulfate--one does not get much oxidant on a molar basis.

I don't know what the Keq is for the reaction of PhCH2OH and HCl but I do know that the reaction is practically quantitative in its yield of benzyl chloride. It is a quick and efficient reaction. The acid is the solvent for the alcohol and is in great excess. Everything should be stirred well for several hours (that is probably too long). The dense lower layer is collected and separated with a funnel to remove any of the acid solution. It is then dried with a suitable drying agent and is ready to use for many purposes. Although not necessarily required, a careful distillation of the product could not hurt if one were to plan on making a Grignard reagent. The only reason benzyl chloride should be made in the laboratory is for the simple reason of the cost of shipping as it is considered a hazardous material.


In the U.S. benzyl alcohol is widely available for making bacteriostatic water, and also for use in paint stripping amongst other uses.




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[*] posted on 26-4-2008 at 22:03


Quote:
Originally posted by Nicodem
In short, the BDE of the two rate limiting processes are:
PhCH(OH)-H <=> Ph-CH(OH)<sup>*</sup> + H<sup>*</sup> . . . . . . . . BDE<sub>1</sub> = 79 kcal/mol (J. Org. Chem., 70 (2005) 9521-9528)

PhCO-H <=> Ph-CO<sup>*</sup> + H<sup>*</sup> . . . . . . . . . . . . . . . . . BDE<sub>2</sub> = 87 kcal/mol (J. Chem. Soc., Perkin Trans. 1 (1986) 173-182)

Please download and post here those two articles, Nicodem.
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[*] posted on 27-4-2008 at 02:49


well i live in australia and benzyl alcohol is not restricted here
200 litre drums if you need . benzaldehyde is EUD.
Anyway i don't see why you would want to go to benzyl chloride from toluene when it is a about 100 times easier from benzyl alcohol ! Of course unless you cannot get it.
And if you could get benzyl alcohol it would be much beter to make benzaldehyde by reacting with Mn02 i have done this before and it doesn't get much easier than that.;)
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[*] posted on 27-4-2008 at 04:41


My statement that its restricted isnt just me saying so - like most things I do I supported it with a quote from a supplier. Can you support your claim?
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[*] posted on 27-4-2008 at 04:44


Quote:
Originally posted by Fleaker
I don't know what the Keq is for the reaction of PhCH2OH and HCl but I do know that the reaction is practically quantitative in its yield of benzyl chloride. It is a quick and efficient reaction. The acid is the solvent for the alcohol and is in great excess. Everything should be stirred well for several hours (that is probably too long). The dense lower layer is collected and separated with a funnel to remove any of the acid solution. It is then dried with a suitable drying agent and is ready to use for many purposes. Although not necessarily required, a careful distillation of the product could not hurt if one were to plan on making a Grignard reagent. The only reason benzyl chloride should be made in the laboratory is for the simple reason of the cost of shipping as it is considered a hazardous material.

I see at once that you have never done this. It is the same case as drying ethanol with MgSO4 - virtual experiment with virtual effects.
BTW, crude benzyl chloride must be perfectly dry (istead of wet, heh) and without traces of acid - in another case you will get 40% yield of distillation and flask full of "polybenzyl".
BTW 2 I still cannot find BDE2 = 87 kcal/mol in given paper. I am attaching page 179 - the longest read page in my life ;) ***I have found it, at last. It was classical case of blindness :D

[Edited on 27-4-2008 by kmno4]

[Edited on 27-4-2008 by kmno4]
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[*] posted on 27-4-2008 at 05:20


I can support my claim because i have a 200 litre drum not that i need to make benzyl chloride or benzaldehyde thow.
I have a friend that also has twelve 2 litre bottles of reagent grade benzaldehyde.

isn't that nice

regards azo :D
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[*] posted on 27-4-2008 at 05:41


As everyone knows chemistry is 90% possession, 10% being able to pour it from a can.
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[*] posted on 27-4-2008 at 06:21


Quote:
Originally posted by kmno4
Quote:
Originally posted by Fleaker
I don't know what the Keq is for the reaction of PhCH2OH and HCl but I do know that the reaction is practically quantitative in its yield of benzyl chloride. It is a quick and efficient reaction. The acid is the solvent for the alcohol and is in great excess. Everything should be stirred well for several hours (that is probably too long). The dense lower layer is collected and separated with a funnel to remove any of the acid solution. It is then dried with a suitable drying agent and is ready to use for many purposes. Although not necessarily required, a careful distillation of the product could not hurt if one were to plan on making a Grignard reagent. The only reason benzyl chloride should be made in the laboratory is for the simple reason of the cost of shipping as it is considered a hazardous material.

I see at once that you have never done this. It is the same case as drying ethanol with MgSO4 - virtual experiment with virtual effects.
BTW, crude benzyl chloride must be perfectly wet and without traces of acid - in another case you will get 40% yield of distillation and flask full of "polybenzyl".



He means perfectly dry and acid-free to prevent the side reaction. If it were perfectly wet it would be acidic :-) wouldn't it? However, he is still correct that I was wrong to suggest a distillation without careful purification. I have never distilled benzyl chloride, or benzyl bromide nor did I claim to have done so. I made it from the alcohol and excess of the concentrated acid and separated and used it without any further purification. That I have done.

My suggestion

Quote:
Originally posted by FleakerAlthough not necessarily required, a careful distillation of the product could not hurt if one were to plan on making a Grignard reagent.

is wrong unless you have done what kmno4 has said to do.




Neither flask nor beaker.


"Kid, you don't even know just what you don't know. "
--The Dark Lord Sauron
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