- Collect about 750 ml of liquid ammonia in a 1-litre three-necked flask, surrounded by a lagging bath of cork chips and fitted with a sealed stirrer unit, a soda-lime guard-tube and a stopper.
- Weigh out 24g (0.1 mol) of L-cystine, and about 10g (0.48 mol) of sodium cut into small pieces under dry light petroleum.
- Start the stirrer, add about 2g of the sodium followed by L-cystine in small portions until the blue colour has disappeared.
- Repeat this addition sequence until all of the cystine has been added and a permanent blue colour remains.
- Discharge the blue colour by gradually adding powdered ammonium chloride, and then add dropwise 25.3g (23 ml, 0.2 mol) of benzyl chloride.
- Remove the stirrer and lagging bath and allow the ammonia to evaporate overnight.
- Dissolve the residue in 100 ml of cold water and add concentrated hydrochloric acid until the resulting mass is acid to Congo red.
- Heat the mixture gradually to boiling to dissolve the precipitated product and allow to cool.
- Filter off the long needles of S-benzyl-L-cysteine which separate, wash with a little cold water and allow to dry in the air. The yield is 38g (90%), m.p. 214 °C (decomp.).
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