JEE Chemistry Isomerism Notes Set 01

Types of Isomerism

Isomers are different compounds with the same molecular formula. They differ in physical or chemical properties, or both. The classification is by what kind of difference exists between the molecules.

Structural (constitutional) isomers always have different IUPAC names. Stereoisomers have the same connectivity and differ only in spatial arrangement. Stereoisomers that cannot be interconverted without breaking a bond are called configurational isomers, and these are geometrical and optical isomers. Conformers interconvert by rotation and cannot be isolated at room temperature.

Geometrical (cis–trans) Isomerism

1. Cause: restricted rotation, from a C=C (or C=N, N=N) or from a ring.
2. Condition 1: both carbons of the double bond must each carry two different groups. For example CH₃CH=CHCH₃ qualifies, while CH₂=CHCH₃ does not.
3. Condition 2 for rings: two ring carbons each carrying two different groups, so cis and trans forms exist (for example 1,2-dimethylcyclopropane).
4. Small rings such as cyclohexene and smaller cannot hold a trans C=C because of the strain. trans-Cyclooctene is the smallest trans cycloalkene stable enough to isolate.
5. Naming: cis/trans compares similar groups (same side is cis). E/Z uses CIP priority: if the two higher-priority groups are on the same side, the isomer is Z (zusammen). If they are on opposite sides, it is E (entgegen).

CIP Priority Rules

1. Rule 1: higher atomic number wins. Order for common atoms: I > Br > Cl > S > F > O > N > C > H.
2. Rule 2: for isotopes, higher mass wins (T > D > H).
3. Rule 3: if the first atoms are the same, compare the atoms attached to them, set by set. For example CH₂Cl > CH₂OH > CH₂NH₂ > CH₂CH₃ > CH₃.
4. Rule 4: a double or triple bond is treated as if the atom were duplicated or triplicated. So –CH=CH₂ counts as (C,C,H) at its first carbon and –C≡CH counts as (C,C,C), so the alkynyl group ranks higher.

Counting Geometrical Isomers in Polyenes

Optical Isomerism

1. A molecule is optically active if it rotates the plane of polarised light. Clockwise rotation is dextrorotatory (+ or d), anticlockwise is laevorotatory (− or l).
2. The necessary and sufficient condition for activity is chirality: the molecule cannot be superimposed on its mirror image. If a molecule has a plane of symmetry or a centre of symmetry, it is optically inactive.
3. The common source of chirality is a stereocentre, an atom bonded to four different groups. Carbon is the usual case, but N, P, S, Si and Ge can also be stereocentres when they are tetrahedral with four different groups.
4. Amines with three different groups on N are chiral in shape, but the two forms interconvert rapidly by nitrogen inversion, so they cannot be separated and the amine behaves as a racemic mixture.
5. Enantiomers are non-superimposable mirror images with equal and opposite rotation. Diastereomers are stereoisomers that are not mirror images. A racemic mixture is a 1:1 mixture of enantiomers and shows no net rotation (external compensation).
6. A meso compound has stereocentres but is achiral because of an internal mirror plane (internal compensation). meso-Tartaric acid and meso-butane-2,3-diol are examples.

R and S Configuration and Fischer Projections

1. Step 1: rank the four groups on the stereocentre with the CIP rules.
2. Step 2: place the lowest-priority group at the bottom of a Fischer projection (use two swaps, or an even number of swaps, to keep the configuration).
3. Step 3: trace from priority 1 to 2 to 3. Clockwise is R and anticlockwise is S.
4. Shortcut: if the lowest-priority group is on a horizontal bond (toward you), read the direction and then reverse it.
5. A Fischer projection may be turned by 180° in the plane without changing its meaning. Turning by 90° inverts the configuration.
6. D and L (for sugars and amino acids) refer to the position of OH (or NH₂) on the right (D) or left (L) of the lowest stereocentre in the Fischer projection, not to the direction of rotation.

Number of Optical Isomers

Worked check: butane-2,3-diol has two like stereocentres (n = 2, even), so the number is 2¹ + 2⁰ = 3: one pair of enantiomers (R,R and S,S) and one meso form (R,S).

Chirality Without a Stereocentre

1. Cumulated dienes such as substituted propadienes (C=C=C): if both ends carry two different groups, the two end planes are perpendicular and the molecule is chiral. This happens when the number of C=C bonds is even. With an odd number of cumulated double bonds (for example 1,2,3-butatriene derivatives), the end groups lie in one plane, so the compound shows geometrical isomerism instead.
2. Biphenyls with bulky groups at all four ortho positions: the rings are twisted and rotation about the central bond is blocked. The molecule is chiral if each ring has two different ortho groups. If either ring has two identical ortho groups, it is achiral.
3. Spiro compounds and some cyclic cumulenes can also be chiral because of a chiral axis.

Conformations

1. Rotation about a C–C single bond produces staggered and eclipsed arrangements. In ethane the eclipsed form is about 12 kJ/mol higher in energy.
2. In butane, the order of stability is anti (methyls 180° apart) > gauche (60°) > partially eclipsed > fully eclipsed. The anti form is the most stable because the bulky groups are farthest apart.
3. Molecules such as methane or ammonia cannot show conformational isomerism because they have no single bond between two substituted atoms.

Tautomerism

1. Tautomers are functional isomers in rapid, dynamic equilibrium, formed by migration of a proton (usually from an alpha carbon) with a shift of a double bond. The commonest case is keto–enol tautomerism.
2. Requirement: a C=O (or similar group) with at least one hydrogen on the alpha carbon. A carbonyl with no alpha hydrogen cannot enolise. Bridgehead positions in small bicyclic rings cannot give an enol because the double bond would be too strained.
3. A hydrogen on a carbon conjugated to the C=O through a C=C (a gamma or vinylogous hydrogen) can also be lost, giving an extended enol.
4. Stability: for simple ketones and aldehydes the keto form dominates, because C=O plus C–H is stronger than C=C plus O–H. The enol is favoured when it is aromatic (phenol) or stabilised by hydrogen bonding and conjugation (as in pentane-2,4-dione). Between two enols, the one with the more substituted double bond is more stable.
5. Lactam–lactim tautomerism: 2-pyridone and 4-pyridone with an N–H exist in equilibrium with the corresponding hydroxypyridines. Replacing N–H by N–CH₃ stops it.

Isomer Counting Tips

1. Structural isomers of C₄H₉–: there are 4 butyl groups. So there are 4 aldehydes of formula C₅H₁₀O and 4 alkyl groups to attach in other families.
2. For a given formula, find the degrees of unsaturation, then list by type (double bond, ring, triple bond). For C₃H₆O the total, including enols, ethers and rings, is 9.
3. For a ring with two identical substituents on adjacent carbons, expect a meso cis form and a chiral trans pair (3 stereoisomers in all).