2023 H2 Chem P1 Q20 — Arene Solution

The answer is C.
The delocalised \(\pi\) electrons of benzene are an electron-rich region. In Friedel-Crafts alkylation, the carbocation is the electrophile, and the \(\pi\) electrons donate an electron pair to it to form a new C-C bond (electrophilic substitution).
• A is wrong: with \(\text{Br}_2\) and \(\text{AlBr}_3\), benzene undergoes electrophilic substitution; \(\text{AlBr}_3\) generates the electrophile \(\text{Br}^+\), which is attacked by the \(\pi\) electrons.
• B is wrong: benzene is planar, but the delocalised \(\pi\) system gives it extra stability, so addition of hydrogen does not happen easily; it needs a catalyst (such as Ni or Pt), high temperature and high pressure.
• D is wrong: the sideways overlap of p orbitals delocalises the 6 \(\pi\) electrons over the whole ring, so there are no separate single and double bonds; all six C-C bonds are equal in length, intermediate between a single and a double bond.