Isocyanates

esters of isocyanic acid, R—N=C=O, where R is an acyclic, aromatic, alkyl aromatic, or heterocyclic radical. Isocyanates are colorless or pale orange liquids or crystalline solids (see Table 1).

Depending on the number of isocyanate groups in the molecule (one, two, three, or more), isocyanates are divided into monoisocyanates, diisocyanates, triisocyanates, and so on. Isocyanates are very reactive; they react readily with compounds containing mobile hydrogen atoms. With ammonia or amines, monoisocyanates yield urea derivatives (1), and with alcohols they yield substituted urethanes (2):

(1) RNCO + NH₃ → RNHCONH₂

(2) RNCO + R′OH → RNHCOOR′

Isocyanates dimerize and trimerize, yielding, for example, isocyanurates:

With diols or diamines, diisocyanates yield the corresponding poly urethanes or poly ureas:

nHOROH + nOCNR’NCO→ [—OROCONHR’NHCO—]n

The main industrial method of preparing isocyanates is phos-genation of primary amines or their hydrochlorides in the liquid phase or, in the case of low-boiling amines, in the vapor phase:

RNH₂ + COCl₂ → RNCO + 2HCl

The liquid-phase process is run in inert solvents, such as chlorinated hydrocarbons, ethers, and esters, and the isocyanates are isolated by fractional distillation.

Isocyanates are widely used in industry for the production of urethane rubbers, polyurethanes, glues, paints and varnishes, and herbicides. Isocyanates in which R is lead, silicon, boron, or phosphorus are also made.