ASR Further Organic Mechanisms Notes

2025/ASRJC/Chemistry 1 ANDERSON SERANGOON JUNIOR COLLEGE 2025 H3 CHEMISTRY FURTHER ORGANIC MECHANISMS Learning Outcomes Nucleophilic Substitution Students should be able to: (a) explain how the relative rate of nucleophilic substitution is affected by the nature of the (i) nucleophile (ii) leaving group (iii) substituents (b) describe and compare the mechanisms and kinetics of SN1 and SN2 reactions, in terms of (i) the energy profile and rate law, including steady state approximation in S N1 [mathematical treatment of steady state is not required] (ii) stereochemistry, including ion pair interactions in SN1 (iii) substituents effects (c) explain the factors affecting competition between S N1 and S N2 mechanisms [solvent effects are not required] Elimination Students should be able to: (a) understand and apply the following concepts to the study of elimination reactions: (i) syn– and anti– elimination; and its effect on stereoselectivity (ii) regioselectivity: Zaitsev (thermodynamic) and Hofmann (kinetic) product(s) (b) describe and compare the mechanisms and kinetics of E1 and E2 reactions, in terms of (i) the energy profile and rate law (ii) regioselectivity (c) explain the E2/SN2 competition, in terms of (i) substrate effects (ii) base effects References 1. Organic Chemistry, 7th edition, McMurry J (2008) 2. Organic Chemistry, J Clayden 3. Organic Chemistry as a Second Language, David R. Klein 4. ChemgaPedia

2025/ASRJC/Chemistry 2 1. Nucleophilic substitution of halogenoalkanes C+−X– covalent bond is polar in halogenoalkane, thus the carbon is electron –deficient and attracts nucleophiles to undergo two main types of reactions: either substitution of the halogen or undergo base–induced elimination to yield an alkene. Nucleophilic substitution C XNu - +  - CNu + X - Elimination H X Nu Nu H X Depending on the substrates and conditions used, a mixture of both substitution and elimination products can form. Reagents and conditions: NaOH(aq), heat under reflux + OH- RCH2 OH + Br- substrate leaving group nucleophile RCH2 Br 1.1 SN2 mechanism This is a one–step reaction mechanism. The nucleophile, OH –, approaches the substrate, RCH2CH2Br, from a direction opposite the leaving group, in this case Br. The C–O bond is partially formed and the C –Br bond is partially broken in the transition state. The transition state contains a pentavalent carbon atom. OH-  - + - C R H R1 HO Br C R H R1 HO transition state + Br - nucleophile substrate leaving group C R H R1 Br polar bond A transition state is an unstable species that has very short lifetime and thus cannot be isolated. It lies in an energy maximum.

2025/ASRJC/Chemistry