Animated Solution for Chemistry - Organic Chemistry: Among the following reaction(s) which gives (give) tert-butyl benzene as the major product is(are)

Imagine you are an architect tasked with attaching a massive, bulky structure—a tert-butyl group—onto a pristine benzene ring. To accomplish this Friedel-Crafts alkylation, you need a highly reactive, electron-deficient building block: the tert-butyl carbocation.

Our mission is to evaluate four different chemical pathways and determine which ones successfully generate this $3^{\circ }$ carbocation electrophile. Let's break them down one by one.

The Elimination Trap

In our first scenario, we mix tert-butyl bromide with sodium ethoxide (${\text{NaOC}}_2{\text{H}}_5$). At first glance, you might hope for a substitution reaction. However, there is a massive catch here.

Sodium ethoxide is a strong base, and tert-butyl bromide is a sterically hindered $3^{\circ }$ alkyl halide. When these two meet, the base doesn't even try to attack the buried carbon. Instead, it rips off a peripheral proton, triggering a rapid E2 elimination.

The result is isobutylene gas. No carbocation is formed, and the benzene ring remains completely unreacted. Option A is a trap!

The Magic of Rearrangement

Next, we react isobutyl chloride with aluminum chloride (${\text{AlCl}}_3$). Aluminum chloride is a classic Lewis acid—it is hungry for electrons. It coordinates with the chlorine atom and pulls it off, leaving behind an isobutyl carbocation.

But wait, this is a $1^{\circ }$ carbocation, which is highly unstable. Nature abhors instability. To find a lower energy state, the molecule undergoes a rapid 1,2-hydride shift. A hydrogen atom, along with its bonding electrons, migrates to the adjacent carbon.

This brilliant maneuver transforms the unstable $1^{\circ }$ cation into the highly stable $3^{\circ }$ tert-butyl cation. Now, the electron-rich benzene ring attacks this electrophile, yielding our target: tert-butylbenzene. Option B is a success!

Direct Protonation

In option C, we treat isobutylene with sulfuric acid (${\text{H}}_2{\text{SO}}_4$). The $\pi$-bond of the alkene is nucleophilic and readily attacks the acidic proton.

According to Markovnikov's rule, the proton adds to the terminal carbon (the one with more hydrogens), leaving the positive charge on the more substituted central carbon.

Just like that, we have directly generated the $3^{\circ }$ tert-butyl cation! The subsequent electrophilic aromatic substitution proceeds smoothly. Option C is correct.

The Lewis Acid Assist

Finally, we look at isobutyl alcohol reacting with boron trifluoride etherate (${\text{BF}}_3\cdot {\text{OEt}}_2$). The hydroxyl group ($-\text{OH}$) is normally a terrible leaving group.

However, ${\text{BF}}_3$ is a strong Lewis acid. It coordinates with the lone pairs on the oxygen, transforming it into an excellent leaving group. As it departs, we briefly form the $1^{\circ }$ isobutyl cation.

Just as we saw in Option B, this unstable intermediate instantly undergoes a 1,2-hydride shift to become the stable $3^{\circ }$ tert-butyl cation. The benzene ring is alkylated, and we achieve our goal once more. Option D is correct.

Options (B), (C), and (D) all utilize different starting materials and reagents, but they all converge on the exact same critical intermediate: the tert-butyl carbocation. This beautiful convergence is what makes organic chemistry so fascinating!