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Imprecise curly arrows in organic mechanisms
“Just over 40% of candidates answered this correctly, suggesting that the understanding of curly arrows is not as well embedded as the recall of curly arrows in familiar mechanisms.”— 9CH0 Paper 2, June 2024
How to fix: Every curly arrow has TWO rules: (1) the tail must start on a lone pair OR the centre of a bond — never on an atom; (2) the head must point to the exact atom or bond receiving the electron pair. For electrophilic substitution on a benzene ring, the first arrow must originate from inside the delocalised ring (not from a C–H bond). Show δ⁺/δ⁻ on every polar bond. Show all lone pairs and charges before drawing arrows. Do not include two-headed arrows in initiation steps of free-radical mechanisms.