Paper 1 Q6 — SN1 vs SN2: steric and electronic

Reaction mechanisms — deducing SN1/SN2 from structure and data

What the examiners said

“Candidates clearly knew the theoretical differences between SN1 and SN2 mechanisms; this question required them to make deductions based on that knowledge and the data… Most answers gave only incomplete comparisons… for which both steric and electronic factors were at play.”

Try the question (reconstruction)

A tertiary halogenoalkane (e.g. 2-bromo-2-methylpropane) reacts far faster than a primary one under the same nucleophilic-substitution conditions with a weak nucleophile. Which mechanism dominates, and why?

Examiner report: “Complete answers needed BOTH steric and electronic factors.”

Explore — why substrate structure picks the mechanism

Move from primary to tertiary substrate. Watch the SN2 rate fall (steric hindrance) while the SN1 rate rises (carbocation stability). The dominant mechanism is whichever is faster.

Substrate: primary

Primary → SN2 (open to attack, no stable cation). Tertiary → SN1 (stable cation, too hindered for SN2). Both factors reinforce each other.

Original reconstruction inspired by the misconceptions described in the Singapore-Cambridge GCE A-Level H3 Chemistry 9813 November 2023 Examiner Report. Not the actual examination question. For classroom exploration.

Made by lookang, using Claude Fable 5. More resources: iwant2study.org