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Author: Subject: Preparation of Benzyl Chloride
AvBaeyer
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[*] posted on 3-12-2016 at 21:36
Preparation of Benzyl Chloride


Benzyl chloride is a very useful and reactive alkylating agent in organic synthesis. I recently had a need for this compound in order to make a sample of benzylamine for use as a synthesis intermediate. Recently I posted my results for the preparation of benzylamine using the Delepine reaction in another thread [1]. I thought it might be useful to others that I post my experiences in making benzyl chloride.

Nearly all example methods for the preparation of benzyl chloride involve the chlorination of toluene with elemental chlorine followed by fractional distillation to separate it from its more highly chlorinated congeners. Of course, the immediate downside of this approach in the home laboratory is the generation and handling of chlorine in a safe manner. N-Chloroamides such as N-chlorosuccinimide (NCS) and trichloroisocyanuric acid (TCCA) reacting with toluene have also found use in some cases. In fact, TCCA has been mentioned a number of times in this forum as a possible reagent for the preparation of benzyl chloride from toluene. This latter approach may well be suitable but does require fairly specific conditions to be useful. Finally, any method requiring toluene has the problem that toluene is no longer easily available in many jurisdictions, especially here in California.

Benzyl alcohol, on the other hand, is quite easily obtainable and as I show below is readily converted in high yield to benzyl chloride by simply reacting with aqueous hydrochloric acid. This simple reaction was first published by J. F. Norris in 1907. The reaction as published uses 12M hydrochloric acid, though readily available from various vendors, is not OTC like the 10M acid commonly available in hardware stores. So the question became could OTC 10M hydrochloric acid be used in the Norris procedure with results comparable to the 12M acid. Three different experiments have been run with 10M hydrochloric acid that show that it can work just as well as the 12M acid.

Experiment 1 is a direct repeat of the Norris procedure using reagent grade 12M hydrochloric acid. Experiment 2 was a first attempt at direct replacement of 12M acid by 10M acid. In order to work properly, a small amount of 12M acid needed to be added to the reaction. Experiment 3 was an attempt to “fortify” the 10M acid with sodium chloride and sulfuric acid. Although this experiment worked, the reaction of the sulfuric acid with the hydrochloric acid was quite unpleasant even though everything was at ice bath temperature. Experiment 4 was a result of considering the outcomes of experiments 1-3 and concluding that a greater amount of 10M acid would work fine which is what happened. Descriptions of these experiments follow.

NOTE: It is extremely important to use gloves, eye protection and a lab apron or coat during these experiments. Working in a well ventilated area or out of doors is also necessary. Great care must be exercised in disposal of reaction waste and in washing of contaminated glassware. Benzyl chloride is highly lachrymatory and can cause severe reaction upon contact.

Experiment 1: The Norris Procedure
12M Hydrochloric acid (60 ml, 0.72 mol) is added to benzyl alcohol (20.0 g, 0.185 mol) whereupon one obtains a clear solution. The reaction mixture is slowly heated to 60C (oil bath) during which it becomes cloudy and an oil begins to separate at a bath temperature of about 50C. After about 20 minutes at 60C the reaction, which is now biphasic, is removed from the heating bath and cooled to ambient. The mixture is transferred to a separatory funnel with the aid of hexanes (25 ml, use caution) [3] and the upper layer containing benzyl chloride isolated. The aqueous layer is extracted with hexanes (1x25 ml) and combined with the first organic layer. The combined organic layers are washed (2x25ml water, 1x25ml 5% sodium bicarbonate and 2x25 ml brine) then dried over calcium chloride [3a]. Removal of the hexanes by distillation affords 21.7 g (92%) of a colorless product. Distillation affords 19.4 g (83%, bp 51-52 C @ 5 mm) pure benzyl chloride. A small pot residue remains [4] as well as some hold up in the distillation apparatus [5].

Experiment 2: 10M Hydrochloric acid with added 12M Hydrochloric acid.
10M Hydrochloric acid (60 ml, 0.60 mol) is added to benzyl alcohol (20.0 g, 0.185 mol) which gives incomplete solution. Heating was commenced but incomplete dissolution was still present at 40C. At this point, 12M acid was added until dissolution was complete which required about 6 ml of acid. When the bath temperature reached about 50C, there was immediate separation of an oil as in Experiment 1. From this point on, the same procedures as in Experiment 1 were followed. Distillation afforded 15.08 g of benzyl chloride (64%, bp 52-54 C @ 4 mm). The pot residue tested positive for both chloride and alcohol [4].

Experiment 3: 10M Hydrochloric acid with sodium chloride and sulfuric acid.
A mixture of 10M Hydrochloric acid (60 ml, 0.60 mol) and powdered sodium chloride (7.0 g, 0.12 mol) was well chilled in an ice bath. Concentrated sulfuric acid (ca 18M, 6 ml, ca 1.0 mol) was added very cautiously with a dropper while stirring. This caused a vigorous reaction and considerable ejection of HCl fumes. After warming to ambient temperature, benzyl alcohol (20 g, 0.185 mol) was added which appeared to completely dissolve [6]. From this point, the conditions of Experiment 1 were followed. Distillation afforded 20.38 g of benzyl chloride (87%, bp 77-79 @ 25 mm) [7]. The pot residue was not tested.

Experiment 4: 10M Hydrochloric acid alone.
Benzyl alcohol (21.98 g, 0.203 mol) was added to 10M hydrochloric acid (100 ml, 1.0 mol) to give a clear solution. The reaction was slowly heated to 65 C. At 60 C there was an immediate cloudiness and separation of an oil. The reaction was maintained at 65 C for 1.5 hr then allowed to cool to room temperature. Work up was as in Experiment 1. Distillation afforded 23.24 g of benzyl chloride (90%, bp 69-72 C @19 mm). The pot residue was not tested.

The last three samples of distilled benzyl chloride gave negative tests with chromic acid reagent indicating no benzyl alcohol within the limits of sensitivity of the test [8].

Conclusion

OTC 10M hydrochloric acid works remarkably well for converting benzyl alcohol to benzyl chloride. More acid and a longer reaction time relative to the original Norris procedure using 12M hydrochloric acid may be the key variables for success.


[1] http://www.sciencemadness.org/talk/viewthread.php?tid=70913#...
[2] JF Norris, American Chemical Journal 1907, v38, 647-642.
[3] I use a distillation cut of Coleman fuel with bp 60-75 C.
[3a] About 12 g of calcium chloride was used for the drying. The hexane solutions stood over the drying agent for 18 - 24 hrs. The calcium chloride was washed with 20-30 ml of hexanes in the filtration process. It was noted that the calcium chloride became quite "brittle" during the drying process.
[4] The pot residue tested positive for chloride (ethanolic silver nitrate) and negative for alcohol (dilute chromic acid). It is most likely that the major part of the residue is dibenzyl ether.
[5] All distillations were done using standard 14/20 glassware.
[6] Considerable solids were present in the reaction, presumably sodium chloride.
[7] At this point I switched to a recirculating aspirator pump as use of my vacuum pump did not seem like such a good idea.
[8] Shriner, Fuson and Curtin, “The Systematic Identification of Organic Compounds,” 1964 edition, page 125.

AvB


[Edited on 4-12-2016 by AvBaeyer]

[Edited on 4-12-2016 by AvBaeyer]

[Edited on 4-12-2016 by AvBaeyer]
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DJF90
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[*] posted on 4-12-2016 at 02:49


Nice work Av. One comment from me: Did you consider the use of calcium chloride or zinc chloride as catalysts for the reaction? Not that there is anything wrong with your optimised procedure (except theres a typo in the quantity of acid - 100 mL is 1 mol).
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[*] posted on 4-12-2016 at 08:48


This is very valuable information. Benzyl chloride is a reagent of many applications, and all but impossible to purchase. These procedures make it readily available.



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[*] posted on 4-12-2016 at 09:01


Generally, it is nothing new, see for example this:
http://www.lambdasyn.org/synfiles/benzylchlorid.htm
However, exeptionally high yields are reported here.
I would suggest to check real purity of prepared BnCl by its hydrolysis and titration with NaOH and/or to check refractive index before and after 24 hours treatment with fresh CaCl2 (it removes BnOH).




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[*] posted on 4-12-2016 at 09:49


This is really good work AvB. I actually bought toluene specifically to make benzyl chloride with, and was not looking forward to the attempt in the least. I heard about the alcohol method and so far simply haven't got around to trying it. My home chemistry is on hiatus currently. This practical description is very valuable to me.

The improved yields relative to the lambdasyn description may be due to the hexane extraction.
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[*] posted on 4-12-2016 at 10:03


Thank you AvB for this valuable contribution. I know how tedious and time consuming research can be. I'm currently in that situation myself, but without your success.



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[*] posted on 4-12-2016 at 11:51


Superb quality post AvBaeyer.



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AvBaeyer
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[*] posted on 4-12-2016 at 15:00


Responses to comments

DJF90: Thank you for the typo sighting. It is corrected. I did not consider the use of either calcium chloride or zinc chloride in this reaction.

kmno4: I am making no claims to “new” chemistry. After all, the original procedure is over 100 years old. I am simply making fellow chemists aware that benzyl chloride is easily available. Moreover, I am unaware of the lambdasyn website. Thank you for pointing this out.

I have today checked the purity of my products as best I can using qualitative analysis. Negative results were seen with chromic acid reagent which rules out benzyl alcohol contamination though I do not know the lowest level of detection. The test was that described in Shriner, Fuson and Curtin, “The Systematic Identification of Organic Compounds,” 1964 edition, page 125. I have also added a note in the edited procedure giving more detail about the calcium chloride drying of the hexane extracts.

careysub, Marvin, Magpie and aga: Thank you for your comments- much appreciated.

AvB
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[*] posted on 4-12-2016 at 15:18


Quote:
OTC 10M hydrochloric acid works remarkably well for converting benzyl alcohol to benzyl chloride. More acid and a longer reaction time relative to the original Norris procedure using 12M hydrochloric acid may be the key variables for success.


In this particular molecule, there's no potential for eliminations or rearrangements. I don't see any potential for side reactions. So let it reflux, forget about it, and check on it after a few days. Nothing to lose!

This is a good write up. Were you monitoring each of these experiments using the chromic acid test? If so, it would be interesting if you could repeat this with some qualitative kinetics measurement. i.e.: how long until the first negative test?

I have an editorial critique: Could you make a tabular summary of your data?

Something like:

Experiment / benzyl alcohol (mmol) / Acid conc. / Acid mmol / Additives / Yield
1 / 185 / 12 M / 720 / none / 83%
2 ....
3....
4.....

You decide which parameters are important. But doing so will be helpful to readers.




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[*] posted on 4-12-2016 at 16:33


I recently ran a similar experiment with a substituted benzyl alcohol, and while I used 12 N HCl, I think most of these will go to a good degree with, 10N, it just shifts he equilibrium slightly to the left, so you either need more acid, or the yield is lower, or you can also use lower conc, and then do two steps, react the first time, work it up, then use a fresh portion of acid to further drive the reaction to the right. That would allow you to convert part of the alcohol in the first reaction, then extract the partially pure product with hexanes, then re-react the material with another portion of acid.

I found that for my compound I got about 95% conversion the first time, and retreating it with HCl again, go it purer, to about 98%. In my case the distillation would have been less practical, due to a higher MW. But sometimes doing two small amounts of acid can get you to the same level of purity that one treatment with 10x as much would do in one reaction.
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[*] posted on 5-12-2016 at 00:03


Great work. We did this using 36% hydrochloric acid and with anhydrous zinc chloride added (Lucas' reagent). But you are getting just as good yields if not better after distillation.

The problem with the zinc chloride method is that the exotherm created as the zinc chloride dissolves in the acid causes a lot of highly corrosive HCl fumes to come off. Slow addition with cooling is needed.

Distillation is necessary for a pure product - the top layer of oily liquid which separates during the reaction does seem to contain appreciable dissolved benzyl alcohol.




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