Organic Chemistry/Isocyanate

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Organic isocyanates have the formula R-N=C=O where R can be either alkyl or aryl. They can be solids or liquids. Isocyanates are generally toxic and must be handled with great care, especially the more volatile isocyanates where inhalation is a primary route of exposure.

Isocyanates are usually produced by reacting an amine with phosgene (COCl2):

R-NH2 + COCl2 --> HCl + R-NH-(C=O)Cl --> HCl + R-N=C=O

They are very reactive molecules, reacting with nucleophiles such as water, alcohols or amines. The reactions involve attack at the carbon of the isocyanate in a manner similar to that for carboxylic acid derivates such as esters or anhydrides.

Water reacts with isocyantes to give a carbamic acid. This reaction is catalyzed by tertiary amines. Carbamic acids are unstable and decompose to carbon dioxide and an amine, R-NH2

R-N=C=O + H2O --> R-NH-(C=O)-OH --> RNH2 + CO2

Alcohols react with isocyanates to give carbamates which are also known as urethanes. This reaction is catalyzed by tertiary amines or salts of metals such as tin, iron and mercury.

R-N=C=O + R'-OH --> R-NH-(C=O)-O-R'

Primary and secondary amines react with isocyanates to give substituted ureas.

R-N=C=O + R'R"NH --> R-NH-(C=O)-NR'R"

R' and R" can be H, alkyl or aryl

Isocyanates themselves to give dimers and trimers.