Carbonylation of isobutylene

The addition of carbon monoxide to isobutylene under high pressures and in the presence of an acid produces a carbon monoxide-olefin complex, an acyl carbocation. Hydrolysis of the complex at lower pressures yields neopentanoic acid:

Neopentanoic acid (trimethylacetic acid) is an intermediate and an esterifying agent used when a stable neo structure is needed.