PREPARATIONS 121 (1) Dehydration of the corresponding acid amides. This process usually requires phosphorus pentoxide (correctly termed phosphoric anhydride) as a dehydrating agent. CH 3 CONH 2 — H 1 O -> CH 8 CN Acetamide Acetonitrile (2) By the action of potassium cyanide on the corresponding alkyl halide. CH, I + K CN -* CH 3 CN + KI NOTE.— Silver cyanide in these circumstances gives the isocyanide, CH 3 NC. (3) By the direct addition of hydrogen cyanide to aldehydes and ketones, giving cyanhydrins: CH 3 CHO 4- HCN = CH 3 CH(OH)CN Acetaldehyde cyanhydrin (Lactonitrile) (CHa) 2 CO H- HCN = (CH 8 ),C(OH)CN Acetone cyanhydrin (a-Hydroxy-isobutyronitrile) By this method, the nitrile of an oc-hydroxy acid is necessarily obtained. Aromatic nitriles (or aryl cyanides) can be obtained by methods (i) and (3), but not by method (2). In addition, aromatic nitriles can be prepared by two other methods, (a) from the corresponding diazo compound by Sandmeyer's Reaction (p. 189), (b) by fusing the corresponding sulphonic acid (or its salts)
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