Question

A urethane is a molecule in which a carbonyl group is part of an ester and an amide (it is an amide in one direction and an ester in the other direction). Propose a mechanism for the reaction of an isocyanate with an alcohol to form a urethane.

Short answer

Question: Describe in brief the mechanism for the reaction of an isocyanate with an alcohol to form a urethane. Answer: The mechanism involves the nucleophilic addition of the alcohol to the isocyanate, forming an O-alkylated isocyanate intermediate. A proton transfer then occurs, rearranging the molecule to form the final urethane product with a carbonyl group as part of an ester (R'-O-C) and an amide (N-C=O).

Step-by-step solution

Identify the reactants

We have an isocyanate (R-N=C=O) and an alcohol (R'-OH) as reactants.

Nucleophilic attack of alcohol to isocyanate

The oxygen atom in the alcohol, being nucleophilic and having a lone pair of electrons, attacks the electrophilic carbon atom in the isocyanate, forming a bond. This also results in the movement of the pi electrons in the double bond (C=O) to the oxygen atom, giving it a negative charge.

Formation of intermediate

The intermediate formed from step 2 is an O-alkylated isocyanate, in which the alcohol is now attached to the carbon atom in the isocyanate but has an extra negative charge on the oxygen (R-N=C(O-)R').

Proton transfer

The oxygen with the negative charge will abstract a proton (H+) from the adjacent nitrogen atom of the isocyanate, resulting in a proton transfer and rearrangement of the molecule.

Formation of urethane

After the proton transfer in step 4, we obtain the final urethane product with a carbonyl group as part of an ester (R'-O-C) and an amide (N-C=O). The final urethane molecule will have the structure R-NH-(C=O)-O-R'.