JEE Chemistry Isomerism Notes Set 04

Types of Isomerism

Isomers are compounds with the same molecular formula but different properties. Structural isomers differ in the order of bonding of atoms (chain, position, functional group, ring-chain, metamerism and tautomerism). Stereoisomers have the same connectivity but differ in the arrangement of atoms in space (geometrical, optical and conformational).

Structural Isomerism

1. Chain isomerism: different carbon skeletons, for example n-butane and isobutane; n-pentane, isopentane and neopentane.

2. Position isomerism: the same skeleton but a different position of a substituent, double bond or functional group, for example 1-chloropropane and 2-chloropropane; pentan-2-one and pentan-3-one; o-, m- and p-dibromobenzene.

3. Functional isomerism: different functional groups: alcohols and ethers (C₂H₅OH, CH₃OCH₃), aldehydes and ketones (propanal, propanone), carboxylic acids and esters, nitriles and isonitriles, and primary, secondary and tertiary amines.

4. Ring-chain isomerism: an open chain compound and a ring compound, for example propene and cyclopropane.

5. Metamerism: different alkyl groups on either side of a polyvalent functional group such as –O–, –CO– or –NH–: ethoxyethane and methoxypropane.

6. Tautomerism: rapidly interconverting functional isomers in dynamic equilibrium, due to migration of a hydrogen atom. Keto–enol tautomerism needs a carbonyl compound with an α-hydrogen: acetone ⇌ propen-2-ol. Enol content is large when the enol is stabilised by hydrogen bonding or conjugation (pentane-2,4-dione about 80%, phenol almost entirely enol). Nitroalkanes with an α-hydrogen show nitro–aci-nitro tautomerism.

Counting: alkanes C₄H₁₀, C₅H₁₂, C₆H₁₄ have 2, 3 and 5 isomers; C₄H₉Br has 4; C₃H₉N has 4 amines; C₄H₁₀O has 7 (4 alcohols and 3 ethers); C₇H₈O has 5 aromatic isomers.

Geometrical Isomerism

It arises from restricted rotation about a double bond (or in a ring) when each doubly bonded carbon has two different groups. The cis isomer has similar groups on the same side and the trans isomer on opposite sides. In the E/Z system, groups on each carbon are ranked by the CIP rules, with Z when the two higher-priority groups are on the same side and E when they are on opposite sides.

Properties: cis isomers are usually more polar (higher dipole moment and boiling point) while trans isomers are more symmetrical and have higher melting points (fumaric acid 287 °C against maleic acid 135 °C). Maleic acid forms a cyclic anhydride on heating, fumaric acid does not. For a polyene with n double bonds having different ends there are 2ⁿ geometrical isomers; if the molecule is symmetrical the number is fewer (hexa-2,4-diene has 3). Oximes show syn/anti isomerism and cycloalkanes show cis–trans isomerism.

Optical Isomerism

A molecule is optically active if it is chiral, that is, not superimposable on its mirror image. A carbon bearing four different groups is a stereocentre. Enantiomers have identical physical properties but rotate plane-polarised light equally in opposite directions (dextro, d or (+), and laevo, l or (−)). A 50 : 50 mixture is a racemic mixture (external compensation, can be resolved); a meso compound has stereocentres but an internal plane of symmetry, so it is inactive (internal compensation).

Counting: with n different stereocentres there are 2ⁿ stereoisomers (2ⁿ⁻¹ pairs of enantiomers). If the molecule is symmetrical, meso forms reduce the number: tartaric acid and butane-2,3-diol have 3 stereoisomers (d, l and meso). Aldohexoses (4 centres) have 16. Diastereomers are stereoisomers that are not mirror images and have different physical properties.

Other chiral systems: allenes with two different groups on each end (CH₃CH=C=CHCH₃) and hindered biphenyls such as 6,6′-dinitrodiphenic acid are chiral without a stereocentre.

R/S nomenclature: assign CIP priorities; view with the lowest priority group pointing away. Clockwise 1 → 2 → 3 is R, anticlockwise is S. Priority rules: higher atomic number first; at a tie go outward; double bonds are duplicated (–CHO > –CH₂OH). Fischer projections: a 180° rotation in the plane keeps the configuration, and exchanging two groups inverts it.

Conformational Isomerism

Conformers arise from rotation about C–C single bonds. In ethane the staggered form is more stable than the eclipsed by about 12 kJ mol⁻¹ (torsional strain) but rotation is still free at room temperature. In n-butane about C2–C3 the energy order is anti < gauche < partially eclipsed < fully eclipsed, with the anti conformer the most stable. In cyclohexane the chair is the most stable (free from angle and torsional strain, with 6 axial and 6 equatorial hydrogens), followed by the twist boat and the boat; the half chair is the least stable. A bulky substituent prefers the equatorial position to avoid 1,3-diaxial interactions (the methyl group in methylcyclohexane prefers equatorial by about 7 kJ mol⁻¹).