Question

Write a mechanism for the formation of tert-butylbenzene from benzene and tert-butyl alcohol in the presence of phosphoric acid.

Short answer

Answer: The mechanism involves four key steps: (1) Protonation of tert-butyl alcohol by phosphoric acid, (2) Formation of a tert-butyl carbocation, (3) Electrophilic attack of the carbocation on the benzene ring, and (4) Restoration of aromaticity through deprotonation by a phosphate ion.

Step-by-step solution

Protonation of tert-butyl alcohol

In the presence of phosphoric acid, the oxygen atom of tert-butyl alcohol will donate a lone pair of electrons to one of the acidic hydrogen atoms in phosphoric acid, forming a bond and creating its conjugate base. This process will result in the protonation of tert-butyl alcohol, forming a positively charged oxonium ion and a negatively charged phosphate ion.

Formation of tert-butyl carbocation

The positively charged oxonium ion will undergo a process called solvolysis, where a water molecule will leave the complex, resulting in the formation of a tert-butyl carbocation. This highly reactive species will be the electrophile that will react with benzene in the next step.

Electrophilic attack on benzene

The delocalized electrons within the benzene ring will donate a pair of electrons to the positively charged tert-butyl carbocation, creating a bond between the benzene ring and the tert-butyl group. This process will result in the formation of a resonance-stabilized sigma-complex, where the benzene ring now has a positive charge.

Aromaticity restoration

In this final step, a base (in this case, the phosphate ion generated in Step 1) will abstract a proton from the carbon atom of the benzene ring that is now bonded to the tert-butyl group. Simultaneously, the electrons from the C-H bond will be used to restore the double bond within the benzene ring, thereby regaining its aromaticity. The product formed is tert-butylbenzene. The overall mechanism for the formation of tert-butylbenzene from benzene and tert-butyl alcohol in the presence of phosphoric acid involves protonation of the alcohol, the generation of a carbocation electrophile, electrophilic attack on the benzene ring, followed by aromaticity restoration through deprotonation.