Isomerism

1. Structural Isomerism

Same molecular formula, different connectivity. Includes Chain, Positional, Functional, Metamerism, and Tautomerism.

2. Stereoisomerism

• Geometrical Isomerism: Restrained rotation (C=C or rings). Cis/Trans or E/Z nomenclature.
• Optical Isomerism: Chirality and non-superimposable mirror images. A molecule with one chiral center is always optically active.

3. Counting Stereoisomers

For a molecule with $n$ dissimilar chiral centres, the maximum number of stereoisomers is $2^n$. With symmetric molecules (like tartaric acid) some are meso, so the number is smaller: tartaric acid has 3 stereoisomers (d, l and meso), not 4.

4. Enantiomers, Diastereomers and Racemic Mixtures

• Enantiomers: non-superimposable mirror images; identical physical properties except the direction of rotation of plane-polarised light.
• Diastereomers: stereoisomers that are not mirror images; different physical properties (cis/trans isomers are diastereomers).
• Racemic mixture: equal amounts of two enantiomers; optically inactive by external compensation.

5. R/S Configuration (CIP rules)

Rank the four groups by atomic number of the first atom (higher = higher priority). Point the lowest-priority group away and trace 1 → 2 → 3: clockwise is R, anticlockwise is S. R/S is unrelated to the sign of rotation (+/−) and to D/L.

6. Conformations

Rotation about C–C single bonds gives conformers. In ethane, the staggered form is more stable than the eclipsed form by about 12.5 kJ/mol (torsional strain). In cyclohexane, the chair form is the most stable; bulky substituents prefer equatorial positions.

7. Geometrical Isomers in Detail

Cis isomers usually have higher dipole moments and boiling points; trans isomers pack better and usually have higher melting points. Example: cis-but-2-ene boils higher than trans-but-2-ene, while trans melts higher.

Practice questions

1. The number of stereoisomers possible for 2,3-dibromobutane is:

   1. 2
   2. 3
   3. 4
   4. 6
   Answer

   B. Two similar chiral centres: a pair of enantiomers plus one meso form.

2. A racemic mixture is optically inactive because of:

   1. Internal compensation
   2. External compensation
   3. Absence of chiral centres
   4. A plane of symmetry
   Answer

   B. Equal amounts of (+) and (−) enantiomers cancel each other's rotation.

3. Which of the following shows geometrical isomerism?

   1. Propene
   2. But-1-ene
   3. But-2-ene
   4. 2-Methylpropene
   Answer

   C. Each doubly bonded carbon in but-2-ene carries two different groups (H and CH₃).

4. The most stable conformation of cyclohexane is:

   1. Boat
   2. Twist-boat
   3. Chair
   4. Half-chair
   Answer

   C. The chair form has no angle strain and all C–H bonds are staggered.

5. Diastereomers differ from each other in:

   1. Only optical rotation
   2. Physical properties such as melting point
   3. Molecular formula
   4. Connectivity of atoms
   Answer

   B. Unlike enantiomers, diastereomers have different physical properties.