Paper 1 Q18 — Counting all the isomers

Isomerism — enumerating straight- and branched-chain structures systematically

What the examiners said

“The most common incorrect choice, option B, identified only three of the five possible structures… Candidates who selected option A most likely identified only two straight chain structures.”

Try the question (reconstruction)

How many structural isomers (chain and position) have the molecular formula C₄H₉Cl?

Examiner report: “Common errors found only three (missing a branched isomer) or only two (straight-chain only).”

Explore — build every skeleton first

Toggle the carbon skeleton, then place the chlorine at each distinct carbon. The tally shows how missing a skeleton loses isomers.

Systematic method: enumerate all carbon skeletons first, then place the substituent at each non-equivalent position. Name each to catch duplicates.

Original reconstruction inspired by the misconceptions described in the Singapore-Cambridge GCE A-Level H2 Chemistry 9729 November 2023 Examiner Report. Not the actual examination question. For classroom exploration.

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