Some time ago, I made some benzyl chloride by the halogenation of toluene (link). The process was on the whole quite unpleasant, very long and gave a miserable yield. It is the primary industrial method for producing benzyl chloride. I deciding to try the more common laboratory method, which is much better suited to small scale operations. Benzyl chloride is the only chlorinated organic compound produced, so the product is generally more pure, in addition the procedure is relatively short.
As a warning, benzyl chloride is highly toxic, lacrymatory, corrosive and potentially carcinogenic. This procedure should only be attempted by an experienced chemist with a good respirator in a well ventilated area. All contact should be avoided.
To a 1000ml flat-bottom boiling flask, I added 575ml of 33% hydrochloric acid. I then slowly added 78ml of benzyl alcohol while swirling the flask slightly. Once addition of the alcohol was complete, I attached a cold water circulated condenser to the flask, with one end of a tube sealed to the end of the condenser. The other end of the tube was suspended just above some sodium hydroxide solution in a separate vessel. This serves as a gas scrubber to neutralize the hydrogen chloride gas that escapes the system. Anyway, I then refluxed the mixture for 10 minutes. Even before the reflux began, an upper layer of clear liquid had separated from the rest of the liquid in the flask. Throughout reflux, the reaction flask was frequently swirled until the layers merged. After reflux, I allowed everything to cool to near room temperature with the help of an icebath. During this time the layers had completely separated out into a cloudy lower aqueous phase and a clear organic upper phase. The organic phase should be mostly benzyl chloride with some dissolved benzyl alcohol. Anyway, I then poured as much of the mixture into a 250ml separatory funnel as could fit and drained off the lower aqueous layer. The rest of the mixture was added to the separatory funnel in portions, draining off the lower aqueous waste teach time until nothing but the entirety of the organic benzyl chloride layer was left in the funnel. The benzyl chloride was then washed with two 50ml portions of saturated sodium bicarbonate solution in succession, stoppering, shaking, and venting the separatory funnel each time. Note that for these washes, benzyl chloride forms the bottom layer. I then washed the benzyl chloride one last time with 100ml of saturated sodium chloride solution for which, the benzyl chloride formed the upper layer. After draining off the aqueous layer and discarding it, I drained off the benzyl chloride into a 200ml conical flask and dried it over anhydrous calcium chloride.
I then poured the benzyl chloride into a 500ml round-bottom flask and setup for simple distillation. The fraction boiling at 170-182 C was collected. This should be relatively pure benzyl chloride. I then dried the product again over anhydrous calcium chloride and transferred it to an amber glass bottle containing 3A molecular sieves for storage. I got 50ml of benzyl chloride which works out to a respectable yield of 58%. The product was also quite pure with a density of 1.06/cm3!
The reaction is a nucleophilic substitution, which unusually can proceed via SN1 and SN2 mechanisms. Substitution with primary alcohols (of which benzyl alcohol is one) almost exclusively proceed via SN2. This is because the carbocation intermediate involved in SN1 is much less stable for primary alkyls than it is for secondary or tertiary. However there is another factor that can come into play called resonance. Resonance allows the positive charge of carbocations to be slightly spread out across the whole molecule, and thus increase the stability. The aromatic ring in benzyl alcohol creates resonance and so the carbocation that would form in an SN1 reaction is more stable. This is why substitution with benzyl alcohol progresses via both SN1 and SN2.
SN1
HCl <==> H (+) + Cl (-)
C6H5CH2OH + H (+) ==> C6H5CH2O(+)H2
C6H5CH2O(+)H2 ==> C6H5C(+)H2 + H2O
C6H5C(+)H2 + Cl (-) ==> C6H5CH2Cl
SN2
HCl <==> H(+) + Cl (-)
C6H5CH2OH + Cl(-) ==> C6H5C(+)H2(OH)Cl
C6H5C(+)H2(OH)Cl ==> C6H5CH2Cl + OH (-)
OH(-) + H (+) ==> H2O
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