Exam Master Review Sheet - Isomerism

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EXAM MASTER REVIEW SHEET

Isomerism in Organic Compounds

Structural & Stereo Classification for JEE and NEET

1. Structural Isomerism

Same molecular formula, different connectivity.

Chain

Difference in the nature of carbon skeleton (e.g., Pentane & Isopentane).

Position

Difference in position of functional group or substituent on same chain (e.g., Propan-1-ol & Propan-2-ol).

Functional

Difference in functional groups (e.g., Ethanol & Methoxymethane).

Metamerism

Unequal distribution of carbon atoms around polyvalent functional groups (e.g., Ethers, Ketones, Amines).

Tautomerism (Desmotropism)

A special functional isomerism where isomers exist in dynamic equilibrium due to migration of a proton (usually 1,3-shift).

Keto Form \(\rightleftharpoons\) Enol Form

Enol content \(\uparrow\) with aromaticity or intramolecular H-bonding (e.g., Acetylacetone).

2. Geometrical Isomerism (GI)

Occurs due to Restricted Rotation (Double bonds or Cyclic structures).

E / Z System

Based on CIP (Cahn-Ingold-Prelog) Priority Rules.
• Z (Zusammen): Higher priority groups on same side.
• E (Entgegen): Higher priority groups on opposite sides.

Physical Properties

Cis-isomer: Higher Dipole Moment (\(\mu\)), higher B.P.
Trans-isomer: Better packing, higher M.P., higher Stability.

Calculation of No. of GI:

For a polyene with \(n\) double bonds showing GI:

Unsymmetrical Ends

\[ 2^n \]

Symmetrical Ends

\[ 2^{n-1} + 2^{p-1} \]

(\(p=n/2\) if even, \(p=(n+1)/2\) if odd)

3. Optical Isomerism

Specific Rotation \([\alpha]\) \[ [\alpha]_T^\lambda = \frac{\alpha_{obs}}{l \times c} \]

\(l\) in dm, \(c\) in g/mL

Optical Purity (\(\% ee\)) \[ \% ee = \frac{d - l}{d + l} \times 100 = \frac{\alpha_{mix}}{\alpha_{pure}} \times 100 \]

Term	Definition	Characteristics
Enantiomers	Non-superimposable mirror images.	Identical physical props (except rotation).
Diastereomers	Stereoisomers not related as mirror images.	Different physical props (M.P., B.P., solubility).
Meso Compound	Acyclic/cyclic with chiral centers but has a Plane of Symmetry.	Optically Inactive (Internal Compensation).

4. Conformational Isomerism

Stability Orders

n-Butane:

Anti > Gauche > Partially Eclipsed > Fully Eclipsed

Cyclohexane:

Chair > Twist Boat > Boat > Half Chair

*Gauche Effect: In \(O-CH_2-CH_2-F\), Gauche is more stable due to H-bonding or dipole interactions.*

5. Calculation Formulas for Optical Isomers

Molecule Nature	Optically Active (\(a\))	Meso (\(m\))	Total (\(a+m\))
Unsymmetrical (\(n\) chiral centers)	\(2^n\)	\(0\)	\(2^n\)
Symmetrical (\(n\) is Even)	\(2^{n-1}\)	\(2^{(n/2)-1}\)	\(a + m\)
Symmetrical (\(n\) is Odd)	\(2^{n-1} - 2^{(n-1)/2}\)	\(2^{(n-1)/2}\)	\(2^{n-1}\)

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