HALOGEN AND CYANOGEN SUBSTITUTION PRODUCTS

Vinyl Chloride.—As ethylene or ethene contains doubly linked carbons only, substitution of halogen will result in a compound of the second class given above. 

Allyl Chloride.—The halogen substitution products of propene and the higher hydrocarbons of the ethene series, when the substitution is in a carbon group not doubly linked, are of importance in the synthesis of derivatives in the same way as are the alkyl halides. 3-Chlor propene or propenyl chloride, CH2 = CH—CH2CI, is known also as allyl chloride, the radical (CH2 = CH—CH2—) being known as allyl. 

Allyl Cyanide, Iso-thio-cyanate, etc.—From allyl chloride or the iodide there may be prepared by the customary reactions allyl cyanide and other of the cyanogen compounds. With potassium cyanide allyl iodide yields allyl cyanide. The reaction, however, instead of yielding a cyanide of the expected constitution is accompanied by a shifting of the double bond to the second carbon so that the cyanide has a constitution unlike that of the iodide from which it is made. 

This is proven by the fact that allyl cyanide on hydrolysis yields crotonic acid in which the double bond is at carbon-2 (p. 173). Of the other cyanogen derivatives of propene the following are known though the position of the double bond is not established in all cases. 

Oil of Mustard.—Only the last compound named is important, viz., allyl iso-thio-cyanate, CH2 = CH—CH2NCS. Strange as it may seem, from statements made in connection with the cyanates and iso-cyanates of the saturated series, this compound is made by treating allyl-iodide, not with silver thio-cyanate, but with potassium thio-cyanate. As, however, the tautomeric iso-compounds are made from the cyanates and thio-cyanates by heat, the conversion of the first formed thio-cyanate into the iso-thio-cyanate can readily be understood. 

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