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Randy

SN1 vs SN2 Mechanisms – The 1 Reason Candidates Lose 6–8 Marks in Paper 2 Every year, the GCE A-Level Chemistry examiners report that the biggest single area of lost marks is incorrect or incomplete curly arrow notation in organic mechanisms. The Two Mechanisms You Must Master SN2 Substitution Nucleophilic Bimolecular - Substrate: Primary halogenoalkanes e.g. bromoethane, 1-chloropropane - Step: Single concerted step. The nucleophile attacks from the back while the leaving group departs. - Curly arrows needed: 1. From lone pair on nucleophile OH⁻, CN⁻, NH₃ → the carbon attached to the halogen. 2. From C–X bond → the halogen atom showing the electrons going with the leaving group. Marking tip: Examiners award the "arrow from nucleophile" mark even if the product is wrong, provided the arrow is drawn correctly. SN1 Substitution Nucleophilic Unimolecular - Substrate: Tertiary sometimes secondary halogenoalkanes - Steps: Two clear steps. 1. Heterolytic fission of C–X bond → carbocation + X⁻ this is the slow, rate-determining step. 2. Nucleophile attacks the planar carbocation from either side. Most common fatal mistake: Drawing the first arrow starting from the halogen instead of the C–X bond, or forgetting to show the carbocation with a positive charge. Examiner Pet Peeves from 2022–2024 reports - Arrows not starting from a lone pair or bond starting from a + charge - Missing the "fish-hook" or using single-headed arrows for pair movement - Not showing the correct stereochemistry in SN2 inversion - Writing "slow" and "fast" steps in the wrong order Would you like me to post the exact 2023 Paper 2 Question 4b mechanism that most candidates got zero on?

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