CYANOGEN COMPOUNDS 163 



Alkyl Thiocyanates and Alkyl Isothiocyanates. 



Allyl isothiocyanate occurs in combination in the glucoside, sinigrin, of 

 mustard seed. The allyl thiocyanate is formed on contact with water, the 

 glucoside being decomposed by the enzyme, myrosin. To the allyl thio- 

 cyanate is due the pungent smell and taste of mustard. 



Alkyl thiocyanates are prepared by the action of alkyl halides upon 

 potassium thiocyanate : 



KSCN + C 2 H 6 I = C 2 H 8 SCN -f KI. 



Alkyl isothiocyanates, or mustard oils, are obtained by heating alkyl 

 thiocyanates, isomeric change occurring : 



C a H 6 SCN -> C 2 H B NCS. 



They are also prepared by the action of primary amines on carbon bisul- 

 phide in alcoholic or ethereal solution, and then heating the aqueous solution 

 of the thiocarbamate so formed with mercury chloride or ferric chloride : 



/NH . C 2 H 5 

 CS + 2C 2 H 5 .NH 2 = CS( 



\SH.NH 2 .C 2 H 5 

 /NH . C 2 H B 

 CS/ = H 2 S + NH 2 . C a H 5 + C 2 H 5 NCS. 



\SH.NH 2 .C 2 H 5 



The alkyl isothiocyanate distils over with steam. 



The alkyl thiocyanates are oily liquids, insoluble in water, possessing a 

 garlic-like smell. 



The alkyl isothiocyanates are pungent smelling liquids, the odour of which 

 provokes tears. They are generally called mustard oils on account of the 

 occurrence of allyl isothiocyanate in mustard. They boil at a lower tempera- 

 ture than the isomeric alkyl thiocyanates and are almost insoluble in water. 



Constitution of Alkyl Thiocyanates and Isothiocyanates. 



In the thiocyanates the alkyl group is joined to the sulphur atom ; in the 

 isothiocyanates it is attached to the nitrogen atom as is shown by the following 

 reactions : 



(1) Thiocyanates on reduction give the primary amine and a mercaptan : 



C 2 H 5 SCN + 3 H 2 = C 2 H 6 SH + CH 3 NH 2 . 



(2) Thiocyanates on oxidation give a sulphonic acid : 



C 2 H 6 SCN + O -> C 2 H 5 S0 3 H. 



(3) Thiocyanates on treatment with alcoholic potash give alcohol and 

 potassium thiocyanate : 



C 2 H 5 SCN + KOH = KSCN + C 2 H 5 OH. 



These reactions point to the attachment of the alkyl group to the sulphur 

 atom. 



(1) Isothiocyanates on heating with hydrochloric acid yield amines : 



C 2 H 5 NCS + 2H 2 O = C 2 H 5 NH 2 + CO 2 + H 2 S. 



(2) Isothiocyanates on reduction yield a primary amine and thioform- 

 aldehyde : 



C 2 H 8 NCS + 2H 2 = C 2 H 6 NH 2 + HCSH. 



(3) Isothiocyanates are converted into isocyanates by boiling their solu- 

 tion in alcohol with mercuric oxide or chloride : 



C 2 H 5 NCS + HgO = C 2 H B NCO + HgS. 



The alkyl group is thus attached to the nitrogen atom, 



6 * 



