(also mustard oils or isothiocyanate esters), organic compounds having the general formula R—N=C=S, where R is an aliphatic or aromatic radical. Liquids with a pungent odor, they can be distilled without decomposition, are insoluble in water, are lacrimators, and cause burns upon contact with skin. Table 1 gives the properties of some isothiocyanates.

Many isothiocyanates are found in plants either in the free state or in the form of glucosides—compounds with sugars or other substances. Allyl isothiocyanate is the pungent and odoriferous component of mustard. Isothiocyanates are quite reactive; they readily add to the N=C bond alcohols, phenols, mercap-tans, and other compounds to give thiocarbamic acid derivatives (R—NH—CX=S, where X = OR, OAr, SH, SR, CN, NH₂, and so forth). Addition of carboxylic and thiocarboxylic acids is accompanied by liberation of COS and CS₂, respectively, with the formation of acid amides (R—NH—COR’)- Isothiocyanates are hydrolyzed on heating (particularly readily in the presence of acids or alkalies) and are reduced by nascent hydrogen to amines (RNH₂); they undergo halogenation with the formation of aminocarbonyl dihalides (R—N=CX₂) and react with mercuric oxide, forming isocyanates (R—N=C=O). Isothiocyanates are obtained by thermal isomerization of thiocyanates (R—S—C=N), by decomposition of the derivatives of dithio-carbamic acid or of thiourea, and by other methods.

Many isothiocyanates have bactericidal, fungicidal, and insecticidal properties. Some of them are used, for example, in the production of synthetic fibers.