Question

The carbocation electrophile in a Friedel-Crafts reaction can be generated by an alternate means than reaction of an alkyl chloride with 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

The carbocation is generated by protonation of 2-methylpropene by \( \mathrm{H}_{3} \mathrm{PO}_{4} \), followed by electrophilic attack on benzene.

Step-by-step solution

Formation of the Carbocation

The first step in this reaction involves the generation of a carbocation. Here, the 2-methylpropene reacts with \( \mathrm{H}_{3} \mathrm{PO}_{4} \) to form a stable tert-butyl cation. The protonation of the double bond in 2-methylpropene by the \( \mathrm{H}^{+} \) from \( \mathrm{H}_{3} \mathrm{PO}_{4} \) leads to the formation of the tert-butyl carbocation and water as a byproduct.

Electrophilic Attack on Benzene

Once the tert-butyl carbocation is formed, it acts as an electrophile and attacks the electron-rich benzene ring. This occurs through the formation of a \( \sigma \)-complex (also known as an arenium ion), where the tert-butyl group temporarily binds to one of the carbons on the benzene ring. This step disrupts the aromaticity of the benzene.

Restoration of Aromaticity

In this step, the temporary loss of aromaticity is resolved by the deprotonation of the arenium ion. A base (often the solvent or \( \mathrm{H}_{2} \mathrm{O} \) in the reaction mixture) removes a proton from the carbon that the carbocation initially bonded to, restoring the double bond and the aromatic nature of the benzene ring, resulting in tert-butylbenzene.

Key concepts

Carbocation Formation

In a Friedel-Crafts reaction, the creation of a strong electrophile is essential for the progression of the reaction. This is where carbocation formation comes into play. In the provided exercise, 2-methylpropene reacts with \(\mathrm{H}_3\mathrm{PO}_4\) to generate a tert-butyl carbocation.
The process occurs through protonation, which means adding a proton. The double bond in 2-methylpropene is attacked by a proton from \(\mathrm{H}^{+}\) derived from the acid, leading to the formation of the carbocation.

• This adds a positive charge, making the carbon extremely reactive.
• Ultimately, water is also produced as a byproduct during this step.

The generated tert-butyl cation is a stable carbocation due to hyperconjugation and the inductive effect of neighboring methyl groups.

Electrophilic Aromatic Substitution

Once the carbocation is formed, it acts as a powerful electrophile. In electrophilic aromatic substitution, the benzene ring, full of electron density, is the target.
Let's break this down:

• The tert-butyl carbocation attacks the benzene, targeting a particular carbon on the ring.
• This results in the formation of a σ-complex, or arenium ion, where the tert-butyl group is temporarily attached to the benzene.
• This interaction disrupts the delocalized π-electron system of benzene, affecting its aromatic stability.

This step is crucial as it initiates the transformation of the substrate into the desired aromatic product.

Aromaticity Restoration

The temporary loss of aromaticity during electrophilic attack is, thankfully, reversible in the Friedel-Crafts reaction. Aromaticity restoration is the process of regaining the stable aromatic state of benzene.
Here's how it happens:

• The arenium ion loses a proton, typically abstracted by a base present in the reaction, such as water.
• This deprotonation allows the electrons to return to the benzene ring, restoring the double bonds.
• As a result, the full aromatic, lexicon becomes reinstated, giving us tert-butylbenzene.

Such transformations highlight the remarkable balance between chemical reactivity and stability characteristic of aromatic systems.