Question

Show the mechanism of the nucleophilic addition reaction of an alcohol with an isocyanate to yield a urethane.

Short answer

Structure of urethane followed as:

Step-by-step solution

Nucleophilic addition reaction

As the name suggest it is the addition reaction of the nucleophile to the carbonyl compound by converted the double bond to single by delocalisation of electrons the nucleophile like cyanide is used in such reactions.

Reaction of alcohol with isocyanide

Oxygen of alcohol having lone pair lead to the donation of lone pair to the isocyanide carbon and the delocalisation of the pi electron occur to the nitrogen lead to the addition of the ORH group followed by loss of proton ether formed named as urethane as shown below: