Q16-30E - Organic Chemistry (Mcmurry)

Question

The carbocation electrophile in a Friedel–Crafts reaction can be generated by an alternate means than reaction of an alkyl chloride with. For example, reaction of benzene with 2-methylpropene in the presence of ${\mathbf{H}}_{\mathbf{3}}\mathit{P}{\mathit{O}}_{\mathbf{4}}$ yields tert-butylbenzene. Propose a mechanism for this reaction.

Step-by-Step Solution

Answer

Step 1 : Friedel craft alkylation

It is the process of adding alkyl by replacing hydrogen in the aromatic ring. It is the electrophilic aromatic substitution reaction of the arene with the alkyl chloride.

Step 2: Reaction of phosphoric acid with 2methyl propene

When 2 methyl propene reacts with phosphoric acid, tert butyl carbonate is formed which acts as an electrophile in Friedel craft alkylation to yield tert-butyl benzene.

Step 3: Reaction of tert-butyl carbocation with benzene

When tert butyl carbonation reacts with benzene, one of the unsaturation bonds is broken and tert butyl is added to the benzene ring. Then, in the presence of base, deprotonation of hydrogen occurs and benzene regains its aromaticity and tert butyl benzene is formed as shown below.

Formation of trimethylbenzene