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Synthesis of Benzyl Chloride


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Photo-Catalyzed Chlorination of Toluene [1]

Method 1. Fit a 600-ml. three-necked flask with a thermometer, a reflux condenser and a gas inlet tube, extending almost to the bottom of the flask. Connect the top of the condenser through a calcium chloride (or cotton wool) guard tube to two wash bottles containing 10 percent. sodium hydroxide solution: the long lead-in tubes in the wash bottles should be just above the surface of the alkali solution in order to avoid "sucking back." Place 100 g. (115.5 ml) of dry toluene and a few chips of porous plate in the flask. Boil the toluene gently and pass in a stream of chlorine from a cylinder (or from a gas generator) *Interpose an empty wash bottle between the flask and the gas source* until the thermometer registers 157-158 (1). The reaction time may be considerably shortened by exposing the mixture to bright sunlight or to a small mercury-vapour lamp or any other UV lamp; if neither of these is practicable, support a ordinary 200-watt lamp a few inches from the flask.

Transfer the reaction mixture to a Claisen flask and distil under atmospheric pressure until the temperature reaches 135-140 (2). Distill the residue under diminished pressure and collect the benzyl chloride at 64-69C at 12 mmHg. The latter upon redistillation boils largely at 63-65C at 12 mmHg. The yield of benzyl chloride is about 100 g.

  1. An alternative method of determining the completion of the reaction is to weigh the flask and toluene, and to stop the passage of chlorine when the increase in weight is 37 g.
  2. The benzyl chloride may also be isolated by distillation under atmospheric pressure. The material boiling between 175C and 185C is collected and redistilled, the final product is collected at 178-182C (pure benzyl chloride has bp 179C). The resulting benzyl chloride is, however, of lower purity unless a fractionating column is used.

Peroxide-Catalyzed Chlorination of Toluene [1]

In a 500ml round-bottomed flask, fitted with an efficient reflux condenser, place 92g (106 ml) of toluene, 68 g (41 ml) of redistilled sulfuryl chloride and 1 g of dibenzoyl peroxide. Reflux gently, when a vigorous reaction takes place: the reaction is complete in 30 minutes. Isolate the benzyl chloride as described in the method above. The yield is 60 g.

Preparation of Dibenzoyl peroxide [1]

Immerse a 600 ml. beaker, containing 50 ml. of "40-volume" hydrogen peroxide (100-volume is 30%) and equipped with a mechanical stirrer, in an ice bath. Support two dropping funnels, containing respectively 30 ml. of 1N sodium hydroxide solution and 30 g (25 ml) of redistilled benzoyl chloride, with their stems inside the beaker. Add the two reagents alternately a few drops at a time, taking care that the temperature does not rise above 5-8C and that the solution is maintained faintly alkaline throughout. When all the reagents have been added, stir the solution for a further half an hour; by this time the odour of the benzoyl chloride should have disappeared. Filter off the flocculent precipitate at the pump, wash it with a little cold water, and dry upon filter paper. The yield of dibenzoyl peroxide is 12 g. It may be purified by dissolving in chloroform at *room temperature* and adding twice the volume of methyl alcohol. It should not be recrystallized from hot chloroform as serious explosion may result. The compound melts at 106C with decomposition. Like all organic peroxides, dibenzoyl peroxide should be handled with care.

Chloromethylation of Benzene [1]

Into a 1-litre three-necked flask, equipped with a reflux (double surface) condenser, a mechanical stirrer (preferably of the Hershberg type, and a gas lead-in tube extending to near the bottom of the flask, place 200 g (227 ml) of dry benzene, 20g of paraformaldehyde (40% Formalin may also be used; the proportions are then 200g of benzene, 38g of 40% formalin and 50g of pulverized zinc chloride.) and 20 g of finely powdered anhydrous zinc chloride. Support the flask on a water bath so arranged that the level of the water in it is about the same height as the reaction. Heat the bath to 60C and pass in (through an intervening empty wash bottle) a rapid stream of hydrogen chloride until no more gas is absorbed (about 20 minutes): allow to cool. Transfer the reaction mixture to a separatory funnel, wash it successively with two 50 ml. portions of cold water, two 50 ml. portions of saturated sodium bicarbonate solution (It is essential to remove all the zinc salts in the washing process, otherwise the product largely resinifies during the distillation.) and finally with 20 ml. of water. Dry with anhydrous calcium chloride or magnesium sulfate, and distill under normal pressure from a Claisen flask with fractionating side arm until the temperature rises to 100-110C. After cooling somewhat, distill under reduced pressure and collect the benzyl chloride at 63-65C at 12 mmHg. The yield is 70 g. Some (about 4 g.) p-xylylene dichloride, m.p. 100, and a small amount of diphenylmethane are present in the residue in the flask.

Ca(OCl)2 Chlorination of Toluene [2]

Toluene and dry Calcium Hypochlorite (bleaching powder) are heated together to 105C in the abscence of other reagents. This avoids by-product formation. If equal amtounts are used, volume-wise, there is a high conversion. If more bleaching powder is used, the conversion is more robust, but contaminants such as benzal chloride and benzotrichloride are formed [...]

Chlorination of benzyl alcohol with conc. HCl [3]

2g benzyl alcohol and 6g concentrated hydrochloric acid were mixed and slowly heated. at 60C the mixture separated into two layers. the yield of benzyl chloride was 70% of the theoretical. When the alcohol and a large excess of the acid were mixed, the reaction took place at the room temperature after a few minutes, and a theoretical yield of the chloride separated. Benzyl alcohol dissolves sparingly in hydrochloric acid of the specific gravity 1.12. when the solution is warmed, benzyl chloride is formed. Benzyl bromide and benzyl iodide were prepared in the same way from the corresponding acids.

References

[1] Vogel, Practical Organic Chemistry, 3rd Ed.
[2] U.S. Pat.#1,280,612; CA 12, 2573 (1918)
[3] J F Norris, American Chemical Journal 38: 627-642 (1907)