Synthesis of Tert Butylbenzene via Electrophilic Aromatic Substitution Reactions

To determine which reactions yield $\textit{tert}$-butylbenzene as the major product, we analyze the mechanism of each option:

1. Option (A): When $\textit{tert}$-butyl bromide, $(CH_3)_3C-Br$, is treated with a strong base like sodium ethoxide ($NaOC_2H_5$), an $E2$ elimination reaction predominates over substitution. The major product formed is 2-methylpropene (isobutylene) rather than $\textit{tert}$-butylbenzene. Friedel-Crafts alkylation requires a Lewis acid catalyst (such as $AlCl_3$) rather than a strong base. Hence, $\textit{tert}$-butylbenzene is not formed.

2. Option (B): Isobutyl chloride, $(CH_3)_2CHCH_2Cl$, reacts with $AlCl_3$ to generate an initial primary carbocation-like species: $(CH_3)_2CHCH_2Cl + AlCl_3 \longrightarrow (CH_3)_2CH\overset{+}{C}H_2 + AlCl_4^-$ This primary carbocation undergoes a rapid $1,2$-hydride shift to form the significantly more stable tertiary $\textit{tert}$-butyl carbocation: $(CH_3)_2CH\overset{+}{C}H_2 \xrightarrow{1,2\text{-hydride shift}} (CH_3)_3\overset{+}{C}$ Electrophilic attack of the $\textit{tert}$-butyl carbocation on benzene gives $\textit{tert}$-butylbenzene as the major product.

3. Option (C): Protonation of 2-methylpropene, $(CH_3)_2C=CH_2$, by strong acid ($H_2SO_4$) generates the stable $\textit{tert}$-butyl carbocation according to Markovnikov's rule: $(CH_3)_2C=CH_2 + H^+ \longrightarrow (CH_3)_3\overset{+}{C}$ Subsequent electrophilic aromatic substitution on benzene yields $\textit{tert}$-butylbenzene as the major product.

4. Option (D): Isobutyl alcohol, $(CH_3)_2CHCH_2OH$, reacts with the Lewis acid catalyst $BF_3 \cdot OEt_2$. Coordination of $BF_3$ to the oxygen atom and departure of the leaving group yields a primary carbocation, which undergoes a $1,2$-hydride shift to yield the more stable $\textit{tert}$-butyl carbocation: $(CH_3)_2CHCH_2OH \xrightarrow{BF_3} (CH_3)_2CH\overset{+}{C}H_2 \xrightarrow{1,2\text{-hydride shift}} (CH_3)_3\overset{+}{C}$ Reaction of this carbocation with benzene yields $\textit{tert}$-butylbenzene as the major product.

Thus, reactions (B), (C), and (D) produce $\textit{tert}$-butylbenzene as the major product.

Correct Answer: B, C, D