Consider the following reactions. Which of these reactions are possible?

Step 1: Evaluate Each Reaction

(A) PhCl + benzene with $\ce{AlCl3}$ (Friedel-Crafts): Aryl chlorides do NOT readily undergo FC alkylation because aryl carbocations are too unstable. (A) Not POSSIBLE

(B) $\ce{Cl2}$ excess with benzene + $\ce{AlCl3}$: Multiple chlorination of benzene gives polysubstituted products. Hexachlorobenzene ($\ce{C6Cl6}$) is the final product with excess Cl2 and $\ce{AlCl3}$. (B) POSSIBLE

(C) Vinyl chloride + benzene + $\ce{AlCl3}$: Vinyl chlorides do NOT form stable vinyl carbocations ($\ce{CH2=CH^+}$ is very unstable). FC alkylation with vinyl halides does NOT work. (C) NOT POSSIBLE)

(D) Allyl chloride + benzene + $\ce{AlCl3}$: Allyl chloride forms a stable allylic carbocation, which is resonance stabilized). FC alkylation proceeds. (D) POSSIBLE)

Possible reactions: B and D = option (a).

Key Points:

• FC alkylation: requires stable carbocation from alkyl halide
• Vinyl halides: no FC (vinyl carbocation too unstable)
• Allyl halides: yes (allylic cation stabilized by resonance)
• Aryl halides: no FC (aryl carbocation very unstable)