Isomerism

NCERT Reference:Chapter 8 – Organic Chemistry – Page 117–118

Quick Notes

Isomers are compounds with the same molecular formula but different arrangements of atoms. Isomers may differ in structure or spatial orientation.
Two major types:
- Structural Isomerism – different connectivity of atoms.
- Stereoisomerism – same bonding, different 3D arrangement.
Chain isomerism: different carbon skeletons (e.g., straight vs branched).
Position isomerism: same group, different position on chain.
Functional group isomerism: same formula, different functional groups.
Metamerism: different alkyl groups around a functional group (commonly ethers).
Geometrical (cis–trans): arises in alkenes or cyclic compounds with restricted rotation.
Optical isomerism: compounds with chiral centres show mirror-image forms (enantiomers) which rotate plane-polarised light.

Full Notes

Organic molecules often share the same molecular formula but differ in the way atoms are arranged or oriented. Such compounds are called isomers and exhibit isomerism.

Structural Isomerism

Structural isomers differ in how their atoms are bonded or connected. This leads to different physical and chemical properties.

Types of Structural Isomerism:

Chain Isomerism

Occurs due to differences in the carbon chain (straight or branched).

Example Butane vs 2-Methylpropane (C₄H₁₀)

NCERT 11 Chemistry chain isomerism showing butane and 2-methylpropane with straight and branched carbon skeletons.

Position Isomerism

Same functional group or substituent attached at different positions.

Example Bromopentane isomers (C₅H₁₁Br)

NCERT 11 Chemistry position isomerism with 1-bromopentane, 2-bromopentane, and 3-bromopentane showing different bromo positions on the chain.

Functional Group Isomerism

Same molecular formula but different functional groups

Example Propanal vs Propanone (C₃H₆O)

NCERT 11 Chemistry functional group isomerism showing propanal (aldehyde) and propanone (ketone).

Metamerism

Unequal distribution of carbon atoms around a functional group. Common in ethers, amines, and esters

Example Methoxypropane and ethoxyethane

NCERT 11 Chemistry metamerism examples comparing methoxypropane and ethoxyethane with different alkyl groups around the ether linkage.

Stereoisomerism

Stereoisomers have the same structural formula but differ in the arrangement of atoms in space.

Types of Stereoisomerism:

Geometrical Isomerism

Observed in alkenes or cyclic compounds due to restricted rotation around double bonds or ring structures

cis: similar groups on the same side. trans: similar groups on opposite sides.

Example cis–trans (E/Z) in alkenes

NCERT 11 Chemistry geometrical isomerism showing cis-but-2-ene and trans-but-2-ene.

Optical Isomerism

Arises when a compound contains one or more chiral centres (a carbon atom bonded to four different groups). Enantiomers are non-superimposable mirror images and rotate plane-polarised light in opposite directions: (+) or dextrorotatory rotates clockwise, (–) or levorotatory rotates anti-clockwise.

Example Lactic acid CH₃CH(OH)COOH

NCERT 11 Chemistry optical isomerism with lactic acid showing a single chiral carbon and mirror-image enantiomers.

Lactic acid CH₃CH(OH)COOH has one chiral carbon and exists in two enantiomeric forms.

Summary

Isomers share a molecular formula but differ in structure or 3D arrangement.
Structural isomers vary by connectivity including chain, position, functional group, and metamerism.
Stereoisomers have the same connectivity but differ in space as geometrical or optical isomers.
Geometrical isomerism needs restricted rotation and optical isomerism needs chirality.