Why is attack by benzene on complexed tert-butyl bromide to give a cyclohexadienyl cation an unlikely mechanistic step?

Benzene is a special molecule due to its aromatic properties. Aromatic compounds like benzene are especially stable, and this stability makes them less reactive compared to other organic molecules. Benzene has a continuous electron cloud above and below its carbon ring, which is due to the delocalized π-electrons. This delocalization contributes to benzene's low reactivity.
Benzene generally participates in substitution reactions rather than addition reactions. Substitution reactions allow benzene to maintain its aromaticity, adding new groups without disrupting the stable electron system. In the context of our exercise, benzene attacking a tert-butyl carbocation would lead to an energetically unfavorable reaction, as it would disrupt its aromaticity.

Aromatic stabilization refers to the significant extra stability of aromatic compounds due to the delocalization of electrons across the ring. This delocalization forms a system of conjugated π-electrons that spans the whole ring.
Here are key aspects of aromatic stabilization:

• Delocalized Electrons: The electrons are shared over multiple atoms, creating a strong and stable bond.
• Resonance Energy: The actual energy is lower than the theoretical energy because of delocalization. This is called resonance energy and is a measure of stability.
• Hückel's Rule: Aromatic compounds follow Hückel's rule, which states that a planar ring molecule will be aromatic if it has (4n + 2) π-electrons, where n is a non-negative integer.
  

In our problem, disrupting this stabilization by forming a cyclohexadienyl cation is highly unfavorable, reinforcing why benzene does not attack the tert-butyl bromide complex.

A carbocation is an ion with a positively charged carbon atom. Carbocations can be primary, secondary, or tertiary, depending on the number of carbon atoms attached to the positively charged carbon.

• Stability Order: Tertiary > Secondary > Primary. Tertiary carbocations are the most stable due to the inductive effect and hyperconjugation from attached alkyl groups.
• Formation: Tertiary carbocations form readily when a leaving group, like bromine in tert-butyl bromide, departs. This leaves behind a positively charged carbon.
• Complexation: When complexed with a catalyst or Lewis acid, the bond between carbon and the leaving group weakens, facilitating carbocation formation.
  

In the exercise, the tert-butyl bromide, especially when complexed, readily forms a tertiary carbocation. However, benzene's attack on this carbocation disrupts its aromatic system, making the step unlikely due to the high-energy barrier involved.

The cyclohexadienyl cation is an intermediate formed during the electrophilic aromatic substitution (EAS) in benzene reactions. This cation results when a substituent temporarily disrupts the aromatic ring.

• Structure and Instability: The cyclohexadienyl cation has a non-aromatic structure with a positive charge, making it less stable than the aromatic benzene ring.
• Energetic Cost: Forming a cyclohexadienyl cation requires breaking the delocalized electron system of benzene, which is energetically costly.
• Role in Substitution: While necessary and transient in EAS, a permanent formation, as in the attack on a tert-butyl carbocation, is highly unfavorable.
  

Thus, the formation of a cyclohexadienyl cation from benzene attacking a tert-butyl carbocation significantly destabilizes benzene's aromatic structure, explaining why this mechanism is unlikely.