2 Isomerism (Organic Chem NJC)

National Junior College SH1 H2 Chemistry 1 2 ISOMERISM Note: bonds have to be broken in order for isomer A to be converted to isomer B. 2.1 INTRODUCTION Isomerism refers to the existence of two or more stable compounds with the same molecular formula but different arrangement of the atoms. These compounds are known as isomers. There are 2 major types of isomerism: constitutional (structural) isomerism and stereoisomerism.

National Junior College SH1 H2 Chemistry 2 2.2 STRUCTURAL ISOMERISM (CONSTITUTIONAL ISOMERISM) Success Criteria: • I can describe and draw constitutional (structural) isomers Structural isomers have the same molecular formula but different structural formulae; i.e., at least one atom is bonded to a different atom in one isomer compared to another. (i) Chain Isomers e.g. structural isomers of C5H12 Pentane (b.p. 36 C) Straight Chain 2,2-dimethylpropane (b.p. 10 C) Branched Both share the same molecular formulae but differ in the longest continuous hydrocarbon chain. (ii) Positional Isomers e.g. pentan-3-one vs pentan-2-one CH3CH2CCH2CH3 O pentan-2-one pentan-3-one Both share the same molecular formulae but different location of C=O group on the longest continuous hydrocarbon chain. Chain and positional isomers have different physical properties such as density, solubility, melting and boiling points as the surface area of contact between molecules are different. These isomers have similar chemical properties as they have the same functional groups. However, the shape of a molecule often determines its chemical reactivity. An easy- to-reach functional group or other side chain reacts more readily than one that experiences steric hindrance. (i.e., one that is “buried” in the middle of a long C chain or “protected” by bulky groups). (Concept of steric hindrance will be covered in details in later chapter.)

National Junior College SH1 H2 Chemistry 3 (iii) Functional group isomers There are 4 common pairs of functional group isomers. • alcohols and ethers with general formula CnH2n+2O CH3CH2OH, ethanol and CH3OCH3, dimethyl ether • aldehydes and ketones with general formula CnH2nO • alkenes and cycloalkanes with general formula CnH2n CH3CH2CH2CH=CH2 • Carboxylic acids and esters with general formula CnH2nO2 Functional group isomers have different physical and chemical properties. Note: Consider 1) chain isomerism - vary no. of C atom in longest chain by moving C atoms to become substituent (e.g. branch) 2) positional isomerism - vary positional number of principal functional group and substituents 3) functional group isomerism -vary arrangement of atoms into a different functional group Checkpoint 1 (a) Draw all the structural isomers of molecular formula C6H14. (b) Draw all the structural isomers of an ester of molecul