JEE Chemistry Organic Chemistry Some Basic Principles and Techniques Notes Set 26

Isomerism and Its Classes

Isomers are compounds that share a molecular formula but differ in the arrangement of atoms, and so differ in properties. Structural (constitutional) isomers differ in the way atoms are connected; stereoisomers have the same connectivity but differ in spatial arrangement.

Structural Isomerism

Chain isomers differ in the carbon skeleton (butane and 2-methylpropane). Position isomers keep the skeleton but shift the functional group, multiple bond or substituent (propan-1-ol and propan-2-ol). Functional isomers have different functional groups (ethanol and dimethyl ether). Metamers have the same functional group but different alkyl groups around it (diethyl ether and methyl propyl ether). In ring-chain isomerism one isomer is open-chain and the other is cyclic (propene and cyclopropane). Tautomers interconvert rapidly by shift of a hydrogen, as in keto and enol forms. Useful counts: C\(_5\)H\(_{12}\) has 3, C\(_6\)H\(_{14}\) has 5 and C\(_7\)H\(_{16}\) has 9 chain isomers.

Geometrical Isomerism

Restricted rotation about a C=C, C=N or a ring bond gives cis-trans or E-Z isomerism when each end carries two different groups. In the E-Z system, groups on each end are ranked by CIP priority (higher atomic number first); the Z isomer has the two higher-priority groups on the same side and the E isomer has them on opposite sides. Trans isomers generally have a lower dipole moment and a higher melting point than cis isomers, while cis isomers usually have higher boiling points because of higher polarity. For a polyene with n double bonds each needing different ends, the number of geometrical isomers is \( 2^n \), reduced when the molecule is symmetrical.

Optical Isomerism

A molecule that cannot be superimposed on its mirror image is chiral and rotates plane-polarised light. The usual cause is a carbon bonded to four different groups (a stereocentre). For n different stereocentres there are \( 2^n \) stereoisomers. Enantiomers are mirror images with equal and opposite rotation; diastereomers are stereoisomers that are not mirror images. A meso compound has stereocentres but an internal plane of symmetry, so it is inactive by internal compensation. An equimolar mixture of enantiomers is a racemic mixture and is inactive by external compensation. Chirality can also occur without a chiral carbon, as in allenes and hindered biphenyls.

Fischer Projection and R-S Configuration

In a Fischer projection the vertical bonds go away from the viewer and the horizontal bonds come toward the viewer. Rotating the projection by 180° in the plane gives the same molecule, while swapping any two groups gives the enantiomer. To assign R or S, rank the groups by CIP priority. If the lowest priority group is on a vertical bond, the direction 1 to 2 to 3 gives the configuration directly (clockwise is R). If it is on a horizontal bond, reverse the apparent result.

Conformational Isomerism

Conformers arise by rotation about a single bond and interconvert easily, so they cannot be separated. Ethane has staggered (dihedral angle 60°) and eclipsed (0°) forms; the staggered form is more stable by about 12.5 kJ/mol because the eclipsed form has torsional strain. Bond lengths and bond angles do not change. For n-butane the order of stability is anti, then gauche, then partially eclipsed, then fully eclipsed.