ASRJC H2 Chem 8b. Isomerism

2024 JC1 H2 Isomerism ©2024ASRJC/CHEM 1 ANDERSON SERANGOON JUNIOR COLLEGE JC1 H2 CHEMISTRY ISOMERISM Contents • Isomerism: constitutional (structural); cis–trans; enantiomerism Learning Outcomes (LOs) Candidates should be able to: (a) describe constitutional (structural) isomerism (b) describe cis–trans isomerism in alkenes, and explain its origin in terms of restricted rotation due to the presence of  bonds [use of E, Z nomenclature is not required] (c) explain what is meant by a chiral centre (d) deduce whether a given molecule is chiral based on the presence or absence of chiral centres and/or a plane of symmetry (e) recognise that an optically active sample rotates plane–polarised light and contains chiral molecules (f) recognise that enantiomers have identical physical properties except in the direction in which they rotate plane– polarised light [usage of the term diastereomers is not required] (g) recognise that enantiomers have identical chemical properties except in their interactions with another chiral molecule (h) recognise that different stereoisomers exhibit different biological properties, for example in drug action (i) deduce the possible isomers for an organic molecule of known molecular formula (j) identify chiral centres and/or cis–trans isomerism in a molecule of given structural formula References 1. Chemistry for Advanced Level, Cann and Hughes, Murray 2. Organic Chemistry, McMurry, Brooks/ Cole 3. Understanding Advanced Physical Inorganic Chemistry, Jeanne Tan and Kim Seng Chan

2024 JC1 H2 Isomerism ©2024ASRJC/CHEM 2 I. INTRODUCTION • Isomerism is the phenomenon in which two or more compounds possessing the same molecular formula exist in different forms due to different arrangements of atoms in their molecules. These compounds are known as isomers. • Isomerism in organic compounds can be classified into 2 broad categories which in turn can further differentiate into 5 types as shown: (A) Constitutional (structural) isomerism • Constitutional ( structural) isomers are compounds with the same molecular formula but different structural formulae. • Structural isomers can be members of different homologous series (i.e. functional groups), or they may be members of the same series. As structural isomers have different structures, they have different properties. • There are 3 types of structural isomerism. 1. Chain isomerism • Chain isomers have the same number of carbon atoms, but their carbon backbones are different. • Example of chain isomers with the formula C4H10 butane CH3 CH2 CH2 CH3 2−methylpropane CH3 CH CH3 CH3 Isomerism Stereoisomerism Constitutional (structural) Isomerism Chain isomerism Positional isomerism Functional group isomerism Cis–trans isomerism Enantiomerism

2024 JC1 H2 Isomerism ©2024ASRJC/CHEM 3 Checkpoint 1 Consider structures A and B A B CH3