Sauron
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Alternative Benzyl Chloride Preps (for those who don't like Chlorine and some who Do)
Len1 has splendidly demonstrated the direct side chain chlorination of toluene with Cl2 under UV to produce first benzyl chloride, then benzal
dichloride and finally benzotrichloride. See his Illustrated Practical Guide.
As some people do not much like making Cl2 I thought I would mention some alternatives that do not require it, and one that produces it in nascent
form.
1. Chloromethylation of benzene. The good old Blanc chloromethylation employs formaldehyde (either as formalin solution, as paraformaldehyde or even
as hexamine), ZnCl2 (anhydrous), and HCl gas. The reaction is rapid and gives high yields. You can find this one in Vogel. Look, Mom! no chlorine.
Fuson also describes the procedure in Organic Reaction v.1 p.63 and following. (forum library.)
If the generation of dry HCl is deemed inconvenient you caninstead replace it and formaldehyde with chloromethyl methyl ether. Now, before a chorus
of nay-sayers starts up, the assymetrical ether is not a big problem. The old fashioned prep was tedious and resulted in contamination with the
hazardous carcinogen sy,-chloromethyl ether. But there is now a very convenient prep which totally avoids this contamination. Hexanoyl chloride is
used to react with dimethoxymethane. The resulting methyl hexanate is easily seperated from the chloromethyl methyl ether and absolutely no
bis(chloromethyl) ether is formed. JOC, 59, 6499-6500. Everything is oin first page. Procedures for chloromethylation using chloromethyl methyl ether
are given in Fuson, op.cit.
2. Treatment of benzyl alcohol with TCT (cyanuric chloride). In the simplest variation TCT is added to excess benzyl alcohol, stirred and heated at
about 10-20 C below the reflux temperature. This produces moderate yields.
Specifically for benzyl alcohol to benzyl chloride, 3 mols alcohol were refluxed for 1.5 hours with 0.5 mol CC (aka TCT) and a 71% recovered yield was
obtained. Sandler, JOC 35, 3966-3967 (1970). Workup is by filtration then fractionation in vacuo. Benzyl alcohol boils at 205 C; benzyl chloride at
179 C. An air condenser ought to be used for reflux. Anything water cooled is laible to crack.
In an improved version, TCT is reacted with dry DMF to ptoduce a solid 1:2 adduct (Gold's reagent) which is taken up in DCM and reacted with benzyl
alcohol at room temperature. The reaction is quantitative and completes in 15-30 minutes stirring. Chemrox advises that Gold's reagent is
lachrymatory and so this ought to be done only in a good hood.
See de Luca et al, Organic Letters 4 553-555 (2002)
3. Toluene is side-chain chlorinated with sulfuryl chloride using dibenzoyl peroxide as radical initiator. AIBN can also be used; the peroxide is
easier to make (see Vogel.). Excess SO2Cl2 will give benzaldichloride, but the process cannot be pushed to benzotrichloride. Kharasch and Brown
described this in JACS 61 2142 in a paper I am awaiting, and will attach here as soon as I receive it.
Downsides
Reaction 1 involves benzene.
Reaction 2 requires TCT
Reaction 3 needs SO2Cl2 if you can get it. If not you can make your own from SO2 and Cl2. The gases need to be dry, and are percolated through a
cooled fixed bed (vertical column) of granulated activated carbon. The SO2Cl2 forms and trickles out as a vile corrosive liquid. On a lab scale it is
easy to make 500 ml in a work day. See Example 2 in US patent attached below. Using a 13mm ID x 300 mm long jacked bed (i.e., a Liebig condenser,
Hempel distilling column or a chromatography column) flow rates of 0.01 mol/min for Cl2 and SO3 gave quantitative conversions. That amounts to 1 mol
SO2Cl2 per 100 minutes, a little more than 1.5 hours. The MW of SO2Cl2 being c.135 g IIRC that's not bad. Nor is it a problem to generate and dry 0.6
mols of Cl2 and SO2 per hour. Those are very modest requirements. Do this for 12 hours and you have about a Kg of sulfuryl chloride.
With some SO2Cl2 in hand you can get fancier and set up a stirred loop reactor as described in same patent. SO2 and Cl2 are dissolved in SO2Cl2 as
solvent and circulated from a stirred flask through the cooled reaction tube using peristaltic pumps and tubing resistant to SO2Cl2 (PTFE, Viton).
Precise control of flow and ratio of the gases is not critical as an excess of either is readily dissolved. At any point, the flow of one gas can be
stopped so that the other gas can be allowed to react with the excess present until only SO2Cl2 is present. Excess Cl2 is easy to spot because of the
yellow-green color of the product. This can be followed colorimetrically using a UV-Vis spectrometer with an inline flow cell. By the oddest
coincidence I happen to have a Sequoia-Turner 340 spec. Isn't it a funny old world?
Anyway it is not necessary to get so fancy.
Sartori described prep of same substance catalyzed by camphor in THE WAR GASES (forum library). Take your pick.
[Edited on 24-8-2008 by Sauron]
Attachment: US5498400.pdf (441kB)
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Klute
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Nice review, thanks.
But I don't understand why use CC with BnOH while conc. HCl does the trick in high yields?
Your statement about chloromethyl methyl ether not being as dangerous as the symetricla ether is pretty intriguing. Could other acyl chlorides than
hexanoyl be used ion the preparation, or is it the minium number of carbons on the chain?
\"You can battle with a demon, you can embrace a demon; what the hell can you do with a fucking spiritual computer?\"
-Alice Parr
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Sauron
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The authors of the paper on prep of clean chloromethyl methyl ether experimented with a wide variety of acyl chlorides after another worker reported
low yields with acetyl chloride and dimethoxymethane. Hexanoyl (caproyl) chloride was selected based on low cost and ease of seperation from the
ether. Read the JOC paper.
I like TCT because I can get it easily and cheaply. I know others elsewhere often can't. AFAIK there's nothing wrong with HCl aq chlorination of
benzyl alcohol. I do know that the US Army when they wanted a shitload of benzyl benzoate for insect repellant (WWII) elected to prep BnCl by
chloromethylation of benzene rather than any chlorination of benzyl alcohol. Maybe it was economics.
In some situations such as the xylene side chain perchlorinations I am looking at, UV catalyzed direct chlorination is the only sensible method.
Chloromethylation of toluene gets a nearly 1:1 ortho/para mixture of xylyl chlorides that was impossible to seperate without a spinning band colmn.
Chlorinating the mixture gets you pentachloro-o-xylene and hexachloro-p-xylene, easily seerable. But the PCOX is pretty useless so a whole lot of
chlorine went to waste. No. Starting with pure p-xylene is better. (Or m-xylene.) But not the ortho isomer.
Here's the paper on peroxide-initiated SO2Cl2 chlorination of hydrocarbons.
It states that toluene reacts almost quantitatively in the dark to afford benzyl chloride in 15 minutes. Use of excess SO2Cl2 gets benzaldichloride
but benzotrichloride cannot be prepared by this method.
[Edited on 25-8-2008 by Sauron]
Attachment: ja01877a048.pdf (946kB)
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Sauron
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And here is a 1946 report from Ind.Eng.Chem. authored by Chemical Corps personnel (US Ar,y) describing bench scale and pilot scale chloromethylation
of benzene to produce benzyl chloride for use in manufacture of benzyl benzoate mosquito repellant.
[Edited on 25-8-2008 by Sauron]
Attachment: ie50437a013.pdf (1.1MB)
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Ephoton
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Mood: trying to figure out why I need a dark room retreat when I live in a forest of wattle.
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interesting topic. good to see you take a brake from the politics and join the other basic wana BEE's
concrete stripper must be no oder, maturic acid.
leave in glass container for a week and benzylchloride will sep to the bottom.
I dedicate this post to the letter A and a true beliver CHROMIC.
a side note
roughly $125 au for 5 Kg of benzyl chloride 
I would say that most major tactical desisions like that would
be determined by what is immediatly available. IE they had
the chems readily at hand for the chloromethylation.
thank you for the chloromethylation refs
they are much more interesting than making benzyl chloride.
[Edited on 3-9-2008 by Ephoton]
e3500 console login: root
bash-2.05#
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