MHT-CET Chemistry · Basic Principles of Organic Chemistry

Isomerism and Stereochemistry

Isomers share a molecular formula: structural isomers differ in connectivity (chain, position, functional group, metamerism), stereoisomers in arrangement — and a carbon with four different groups makes a molecule chiral, giving a pair of non-superimposable mirror-image enantiomers.

All Basic Principles of Organic Chemistry notes MHT-CET Chemistry strategy

Why this matters

9 PYQs, 1 HARD. Three ask which statement about enantiomers is false (the planted one says they are superimposable), one names the Fischer projection from a drawing, three ask which molecule is chiral or how many chiral carbons it has, and two ask for the kind of structural isomerism a pair shows. Learn to spot the carbon with four different groups and the page is done.

Concept 1 of 3

Structural Isomerism: Chain, Position, Functional, Metamerism

Intuition

Same atoms, different wiring. Change the carbon skeleton — chain isomers; move the functional group along the same skeleton — position isomers; change the functional group itself — functional isomers; keep the group and shuffle the alkyls on either side of it — metamers.

Definition

Chain: n-butane and 2-methylpropane; but-1-ene and but-2-ene are POSITION isomers (same chain, C=C moved).
Position: butan-1-ol and butan-2-ol; 1-methoxypropane and 2-methoxypropane.
Functional: same formula, different group — butan-1-ol (alcohol) and 1-methoxypropane (ether), both $\text{C}_4\text{H}_{10}\text{O}$ ; dimethyl ether and ethanol; aldehyde and ketone; acid and ester.
Metamerism: same functional group, different alkyls around it — ethoxyethane $\text{C}_2\text{H}_5\text{OC}_2\text{H}_5$ and methoxypropane $\text{CH}_3\text{OC}_3\text{H}_7$ , both $\text{C}_4\text{H}_{10}\text{O}$ . Seen in ethers, amines, ketones.
Tautomerism (keto–enol) is a special functional isomerism with a mobile hydrogen.

Kinds of structural isomerism

\text{chain} \;|\; \text{position} \;|\; \text{functional group} \;|\; \text{metamerism (alkyls about the group)}

Worked example

Classify the pairs: (i) propanal and propanone; (ii) pentan-2-one and pentan-3-one; (iii) diethylamine and methylpropylamine.

Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 1Basic Principles of Organic ChemistryMODERATE

Which from following pairs of compounds is an example of metamerism?

[Q90 · Shift 1 · 2022]

Calling ether-versus-alcohol pairs metamers

Metamers keep the SAME functional group. Dimethyl ether and ethanol have different groups — functional isomers. Metamerism is two ethers (or two amines) with the alkyls redistributed.

Drill 1 more on structural isomerism: chain, position, functional, metamerism

Concept 2 of 3

Chiral Carbons and Optical Activity

Intuition

A carbon bonded to four DIFFERENT groups is a stereocentre; a molecule with one is chiral and rotates plane-polarised light. Check every candidate carbon: two identical substituents (two methyls, two ethyls, two hydrogens) kill it. Count the carbons that pass.

Definition

Chiral (asymmetric) carbon: four different groups. Test each carbon that carries a halogen or OH first, then the branch points.
2-Bromopropane: C2 has two CH₃ — achiral. 2-Bromo-2-methylbutane: two CH₃ — achiral. 3-Bromopentane: two ethyls — achiral. 2-Bromo-3-methylbutane: C2 has H, Br, CH₃, isopropyl — chiral.
2-Chloro-2-methylbutane is optically INACTIVE (two methyls on C2); 3-chlorohexane, 2-chloropentane, 2-chloro-3-methylbutane are active.
3,4-Dibromohexane $\text{CH}_3\text{CH}_2\text{-CHBr-CHBr-CH}_2\text{CH}_3$ : C3 and C4 each carry H, Br, ethyl and the other half — 2 chiral carbons.
Optical activity needs chirality; a molecule with no stereocentre (and no other chiral element) is optically inactive.

Chirality test

\text{C}abcd \text{ with } a \neq b \neq c \neq d \Rightarrow \text{chiral centre}

Worked example

How many chiral carbons are in 2,3-dichlorobutane and in 2,2-dichlorobutane?

Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 2Basic Principles of Organic ChemistryHARD

Identify the chiral molecule from following.

[Q74 · 9th May Shift 1 · 2024]

Checking only the carbon that carries the halogen

In 2-bromo-3-methylbutane the halogen carbon IS the chiral one, but in general the stereocentre can be elsewhere. Check C2 for two identical groups, then walk the chain.

Drill 2 more on chiral carbons and optical activity

Concept 3 of 3

Enantiomers and the Fischer Projection

Intuition

Enantiomers are non-superimposable mirror images. They match in every scalar property — melting point, density, refractive index, chemical behaviour — and differ only in the SIGN of their optical rotation. A Fischer projection draws the stereocentre as a cross: vertical bonds go back, horizontal bonds come forward.

Definition

Enantiomers: non-superimposable mirror images; identical m.p., b.p., density, refractive index, solubility; same chemical properties (towards achiral reagents); equal and OPPOSITE optical rotation. 'Superimposable mirror images' is always the false statement.
A 1:1 mixture (racemic) is optically inactive by external compensation.
Fischer projection: chiral carbon at the crossing point, main chain vertical (COOH at top, CH₃ at bottom for lactic acid), horizontal groups (H, OH) towards the viewer. Wedge–dash: solid wedge forward, hashed wedge back. Sawhorse and Newman show conformations about a C–C bond, not a single stereocentre.

Fischer convention

\text{vertical bonds away from the viewer;}\quad \text{horizontal bonds towards the viewer}

Worked example

Two samples of 2-butanol have identical boiling points and refractive indices but rotate light +13.5° and −13.5°. What are they, and what is a 1:1 mixture called?

Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 3Basic Principles of Organic ChemistryEASY

Which among the following is NOT a true statement regarding enantiomers?

[Q60 · 9th May Shift 2 · 2023]

Marking 'same chemical properties' as the false statement

Enantiomers DO react alike with ordinary reagents. The false claim is 'superimposable mirror images' — a superimposable mirror image is the same molecule, and the pair would not exist.

Drill 3 more on enantiomers and the fischer projection

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (3)

Structural Isomerism: Chain, Position, Functional, Metamerism
Kinds of structural isomerism
$\text{chain} \;|\; \text{position} \;|\; \text{functional group} \;|\; \text{metamerism (alkyls about the group)}$
Chiral Carbons and Optical Activity
Chirality test
$\text{C}abcd \text{ with } a \neq b \neq c \neq d \Rightarrow \text{chiral centre}$
Enantiomers and the Fischer Projection
Fischer convention
$\text{vertical bonds away from the viewer;}\quad \text{horizontal bonds towards the viewer}$

Watch out for (3)

Calling ether-versus-alcohol pairs metamers
→ Structural Isomerism: Chain, Position, Functional, Metamerism
Checking only the carbon that carries the halogen
→ Chiral Carbons and Optical Activity
Marking 'same chemical properties' as the false statement
→ Enantiomers and the Fischer Projection

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