Question

The carbocation electrophile in a Friedel-Crafts reaction can be generated in ways other than by reaction of an alkyl chloride with \(\mathrm{AlCl}_{3} .\) For example, reaction of benzene with 2 -methylpropene in the presence of \(\mathrm{H}_{3} \mathrm{PO}_{4}\) yields tert-butylbenzene. Propose a mechanism for this reaction.

Short answer

2-Methylpropene is protonated to form a carbocation, benzene attacks this carbocation, forming tert-butylbenzene after deprotonation.

Step-by-step solution

Formation of the Carbocation

In the presence of the acid catalyst \( \mathrm{H}_{3}\mathrm{PO}_{4} \), 2-methylpropene undergoes protonation. The double bond in 2-methylpropene attacks a proton from the acid, resulting in the formation of a secondary carbocation (tert-butyl cation). This is a key step because the formation of a stable carbocation is crucial for the subsequent electrophilic substitution.

Electrophilic Attack by Benzene

The benzene ring, acting as a nucleophile, attacks the newly formed tert-butyl cation. The electrons from the benzene ring's \( \pi \)-system form a bond with the carbocation, creating an intermediate benzenium ion. This step results in the partial disruption of the aromaticity of the benzene ring, which is temporary.

Deprotonation and Restoration of Aromaticity

To restore aromaticity, the benzenium ion undergoes deprotonation. A hydrogen ion (\( H^+ \)) is removed from the benzene ring adjacent to where the tert-butyl group attached. This removal allows the \( \pi \) electrons to reorganize and restore aromatic stability, resulting in the formation of tert-butylbenzene.

Key concepts

Carbocation Formation

The formation of a carbocation is a pivotal step in the Friedel-Crafts alkylation process. In this reaction, we start with 2-methylpropene that interacts with the acid catalyst, hem{H}_3PO_4. Here, the double bond in 2-methylpropene is eager to grab a proton from the acid. This is known as protonation.

When the double bond attacks the proton, it leads to the breaking of itself and results in the formation of a carbocation. Specifically, it forms a tertiary carbocation (tert-butyl cation) which is very stable. This stability is due to the positive charge being well supported by the surrounding carbon atoms in the tert-butyl group.

Carbocations, especially stable ones, are very reactive and pivotal for the next steps in the Friedel-Crafts alkylation, serving as excellent electrophiles.

Electrophilic Substitution

In the next phase of the Friedel-Crafts alkylation, we have an electrophilic substitution step. Once the tert-butyl carbocation is formed, it becomes an enticing target for a benzene ring,
as benzene acts as a nucleophile.

• The electrons from benzene's aromatic pi system are drawn towards and form a bond with the positive carbocation.
• This results in the formation of a new compound, creating what is known as a benzenium ion.

This part of the reaction temporarily disrupts the aromatic structure of benzene. However, this disturbance sets the stage for the rearrangement of electrons,
which is essential for completing the reaction.

Aromaticity Restoration

The final step in the Friedel-Crafts mechanism involves restoring the aromatic nature of benzene. The temporary disruption caused by the formation of the benzenium ion is resolved through a process called deprotonation.

In deprotonation:

• A hydrogen ion (H+) is shed from the benzene ring.
• This loss allows the electrons to rearrange themselves, reestablishing the aromatic pi system.

When this happens, benzene's original stability and aromaticity are reinstated. The end result after this reorganization is the production of tert-butylbenzene.

This step is crucial, as it transforms a reactive intermediate back into a stable aromatic compound, thus completing the Friedel-Crafts alkylation process.