VJC Isomerism Lecture Notes

VICTORIA JUNIOR COLLEGE CHEMISTRY DEPARTMENT ISOMERISM ______________________________________________________________________________ LECTURE OUTLINE 1 Constitutional (Structural) Isomerism 2 Stereoisomerism 2.1 Cis-trans Isomerism 2.2 Chirality and Enantiomerism 3 Molecules Exhibiting Multiple Isomerism 4 Differences between Properties of Enantiomers ASSESSMENT OBJECTIVES 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 Scan here for to ask question related to the lecture

2 ISOMERISM Isomerism is the occurrence of two or more compounds with the same molecular formula but having different structures or spatial orientation. There are two main types of isomerism. Firstly, c onstitutional (structural) isomerism , which is further divided into chain isomerism, positional isomerism and functional group isomerism. Secondly, stereoisomerism, which is further divided into cis-trans isomerism and enantiomerism. 1 Constitutional (Structural) Isomerism Candidates should be able to: (a) describe constitutional (structural) isomerism It refers to isomers containing the same number of each kind of atom but differing in regard to how the atoms are linked to one another, i.e. they have the same molecular formula but different structural formula . There are three types: chain, position al and functional group isomerism.  Chain isomerism The isomers differ in the way carbon atoms are linked together. They possess the same functional group or homologous series. E.g. C5H12 CH3–CH2–CH2–CH2–CH3 C CH3 CH3 H CH2 CH3  Positional isomerism The isomers have the same functional group but differ in its position along the same carbon chain length. E.g. C3H8O CH3–CH2–CH2–OH E.g. C7H7Cl (contains benzene ring) CH3 Cl CH3 Cl CH2Cl Exercise Draw all possible posit