ASR BPOC Notes

2025 JC2 H3 BASIC PHYSICAL ORGANIC CHEMISTRY 2025/ASRJC/Chemistry 1 ANDERSON SERANGOON JUNIOR COLLEGE H3 CHEMISTRY BASIC PHYSICAL ORGANIC CHEMISTRY CONTENT 1 Introduction 2 Hammond Postulate 3 Bells-Evan-Polanyi Principle 4 The relationship between Hammond Postulate and the Bells-Evan-Polanyi Principle LEARNING OUTCOMES Students should be able to understand and apply the following concepts involving kinetic control and thermodynamic control to the study of reaction mechanisms: (i) the Hammond postulate: relationship between the transition state and the nearest stable species (ii) the Bell–Evans–Polanyi principle ➢ relationship between activation energy and enthalpy change of reaction ➢ quantitative calculations based on EA = A + B∆Hr 1 INTRODUCTION 1.1 Reaction Mechanism For a chemist involved in synthesis, mechanistic knowledge of a reaction allows intelligent variation of reaction conditions, temperatures and proportions of reagents to maximise yield of pure products. Reaction mechanism is a sequence of elementary steps that are involved in the pathway from reactants to products. Elementary step refers to the simplest step which cannot be further broken down into simpler steps. E.g. Overall reaction: (CH3)3CCl + OH– (CH3)3COH + Cl– (Reactants) (Products) The SN1 mechanism involves 2 elementary steps: Step 1 C Cl CH3 CH3 + − C CH3 CH3 CH3 carbocation intermediate slow CH3 + Cl− + Step 2 OH fast CH3 C CH3 CH3 OH C CH3 CH3 CH3 +

2025 JC2 H3 BASIC PHYSICAL ORGANIC CHEMISTRY 2025/ASRJC/Chemistry 2 1.2 Energy Profile Diagram An energy profile (or reaction profile) diagram is a graph showing how the energy of the reacting system changes as a function of the reaction coordinate. A reaction intermediate is a species that is formed in one step of a reaction mechanism and consumed in a subsequent step. This species can be a molecule, ion or a free radical. A transition state (TS) is defined as the state corresponding to the highest energy along the reaction coordinate of an elementary reaction. TS can neither be isolated nor can it be studied by spectroscopic method. However, TS does have a _______________________ _______________________ which allow chemists to investigate how changes in the reagents and solvent influence the difference in energy between reagents and TS. If these influences make the transition state more stable compared with the reagents, the reaction will be faster. Energy ΔH

2025 JC2 H3 BASIC PHYSICAL ORGANIC CHEMISTRY 2025/ASRJC/Chemistry 3 2 HAMMOND POSTULATE 2.1 Definition Hammond postulate is helpful to understand the relationship between the rate of reaction and the stability of the products/intermediates formed. In 1955, George S. Hammond postulated that: If two states