JEE Chemistry Isomerism Notes Set 02

Structural Isomerism

Stereoisomerism

1. Geometrical (cis-trans, E/Z) isomerism: needs restricted rotation (a double bond or a ring) and two different groups on each doubly bonded carbon. Maleic acid is cis and fumaric acid is trans. The trans isomer is usually more stable and has the higher melting point; the cis isomer has the higher dipole moment.
2. Optical isomerism: a molecule with no plane or centre of symmetry (chiral) is optically active. The commonest cause is a carbon bonded to four different groups. Enantiomers are non-superimposable mirror images with identical physical properties except the direction of rotation. They react differently only with other chiral substances.
3. A molecule with n different stereocentres has 2^n stereoisomers. A meso compound has stereocentres but an internal plane of symmetry, so it is optically inactive (meso-tartaric acid, 2,3-dichlorobutane).
4. Diastereomers are stereoisomers that are not mirror images and differ in all physical properties.
5. A racemic mixture contains equal amounts of the d and l forms, is optically inactive by external compensation, and can be resolved into enantiomers by converting them to diastereomers.
6. Allenes (\( \ce{CH2=C=CH2} \)) are achiral when both ends carry identical groups.
7. Conformations are arrangements produced by rotation about single bonds. Staggered ethane is more stable than eclipsed by about 12 kJ/mol, and the chair form of cyclohexane is the most stable (free of angle and torsional strain).

Counting Isomers

Monochlorination of 2-methylbutane gives four products, of which two are chiral (1-chloro-2-methylbutane and 2-chloro-3-methylbutane). A dibasic or disubstituted benzene with two identical groups has three isomers: ortho, meta and para.