2021 H2 Chem P1 Q16 — Alkene Solution

A ✓: The carbon-carbon \(\pi\) bond is a region of high electron density lying above and below the plane of the \(sp^2\) carbons. These \(\pi\) electrons are donated to an electrophile (the \(\delta+\) bromine of a polarised \(\text{Br}_2\) molecule), initiating electrophilic addition.

B ✗: The carbon-carbon \(\sigma\) bond electrons lie between the two nuclei and are held too tightly to be donated to an electrophile.

C ✗: The \(sp^2\) carbon is electron-rich because of the \(\pi\) bond, so it behaves as a nucleophile, not an electrophile.

D ✗: It is the weaker \(\pi\) bond, not the \(\sigma\) bond, that breaks during the addition; the \(\sigma\) framework stays intact.