JEE Mains Chemistry · Organic Chemistry - Some Basic Principles and Techniques

Structural Isomerism

Structural isomers share a molecular formula but join the atoms differently: a different carbon skeleton, a group in a different place, a different functional group, different alkyl groups on either side of one group, or a ring in place of a double bond.

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Why this matters

Thirteen PYQs, four of them asking for a number, and three from 2026. Eight name the kind of isomerism a pair of compounds shows, usually as a match list or a set of statements. Five count the isomers of a formula, from the chain isomers of an alkane to the substituted benzenes of one aromatic formula.

Concept 1 of 2: Kinds of structural isomerism

Two compounds with the same molecular formula are isomers. If their atoms are joined differently, they are structural isomers. To name the kind, ask what changed between the two: the carbon skeleton, the position of a group, the group itself, or only the alkyl groups on either side of it.

Definition

Chain: a different carbon skeleton (straight against branched).
Position: the same skeleton and group, with the group or the multiple bond moved.
Functional group: a different functional group: an alcohol and an ether, an aldehyde and a ketone. A primary and a secondary amine of one formula are counted here too.
Metamerism: the same functional group with different alkyl groups on its two sides. It is seen in ethers, secondary amines and ketones.
Ring-chain: a ring in one isomer where the other has a double bond.
Tautomerism: a hydrogen moves between two atoms and the two forms interconvert, usually keto and enol. It needs a hydrogen on the α-carbon.

Type	What differs	Example pair (one formula)
Chain	The carbon skeleton	Pentane and 2-methylbutane (C₅H₁₂)
Position	Where the group or multiple bond sits	Propan-1-ol and propan-2-ol (C₃H₈O)
Functional group	The functional group itself	Ethanol and methoxymethane (C₂H₆O)
Metamerism	The alkyl groups on either side of one group	Methoxypropane and ethoxyethane (C₄H₁₀O) Two ethers can be metamers. An ether and an alcohol cannot: they are functional isomers.
Ring-chain	A ring in place of a C=C	Cyclopropane and propene (C₃H₆)
Tautomerism	The position of a mobile hydrogen; the forms interconvert	Propanone and prop-1-en-2-ol (keto and enol)

Chain, position, functional, metamer and ring-chain isomers are separate compounds; tautomers exist in equilibrium.

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The same idea in a real exam question:

JEE Mains · 2026 · 21 Jan 2026 Shift 2 · Q27Moderate

Example 1 · Organic Chemistry - Some Basic Principles and Techniques · Structural Isomerism

Match List-I with List-II.

	List-I Pair of Compounds		List-II Type of Isomers
(A)	2-Methylpropene and but-1-ene	(I)	Stereoisomers
(B)	Cis-but-2-ene and trans-but-2-ene	(II)	Position isomers
(C)	2-Butanol and diethyl ether	(III)	Chain isomers
(D)	But-1-ene and but-2-ene	(IV)	Functional group isomers

Choose the correct answer from the options given below:

A moved double bond is position isomerism

But-1-ene and but-2-ene keep one straight four-carbon skeleton and one kind of group; only the C=C moves. They are position isomers, not chain or functional isomers.

Metamers keep one functional group

Metamerism needs the same group with different alkyls around it. An alcohol and an ether of one formula are functional isomers, never metamers.

No α-hydrogen, no tautomer

Keto–enol tautomerism moves a hydrogen from the carbon next to C=O onto the oxygen. A ketone whose α-carbons carry no hydrogen, such as 2,2,4,4-tetramethylpentan-3-one, cannot form an enol.

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Concept 2 of 2: Counting structural isomers of a formula

Start with the degree of unsaturation: it tells you how many rings and π bonds the formula must contain. Then build the isomers in order: the longest chain first, then one carbon shorter with the branch placed in every distinct position, and so on. Never count a renumbered or turned-round copy twice.

Definition

Each ring or double bond counts 1 towards the degree of unsaturation, a triple bond 2 and a benzene ring 4.
Alkanes: $\mathrm{C_4H_{10}}$ has 2 structural isomers, $\mathrm{C_5H_{12}}$ 3, $\mathrm{C_6H_{14}}$ 5 and $\mathrm{C_7H_{16}}$ 9.
Benzene with two groups: ortho (1,2), meta (1,3) and para (1,4), so 3 isomers. With three identical groups: 1,2,3, 1,2,4 and 1,3,5, again 3.
A side chain of three carbons can join the ring at its end (propyl) or at its middle carbon (isopropyl): two different isomers.
A count of structural isomers leaves out stereoisomers unless the question asks for them.

Degree of unsaturation (rings + π bonds)

\text{DoU} = \dfrac{2C + 2 + N - H - X}{2}

Worked example

How many aromatic structural isomers have the formula

\mathrm{C_8H_{10}}

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2025 · 3 Apr 2025 · Q125Moderate

Example 2 · Organic Chemistry - Some Basic Principles and Techniques · Structural Isomerism

The total number of structural isomers possible for the substituted benzene derivatives with the molecular formula

C_{9}H_{12}

is ____ .

Isopropyl and propyl are different side chains

A three-carbon side chain attached through its end carbon gives propylbenzene; attached through its middle carbon it gives isopropylbenzene (cumene). Count both.

A renumbered chain is the same compound

2-Methylpentane and 4-methylpentane are one compound numbered from opposite ends. Always name each isomer properly before you add it to the count.

Nitrogen adds and halogen subtracts in the DoU formula

Each N adds one to the top of the degree-of-unsaturation formula and each halogen counts like a hydrogen. Leaving N out makes a nitrile formula look saturated.

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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 (1)

Counting structural isomers of a formula
Degree of unsaturation (rings + π bonds)
$\text{DoU} = \dfrac{2C + 2 + N - H - X}{2}$

Reference tables (1)

Kinds of structural isomerism6 rows

Type	What differs	Example pair (one formula)
Chain	The carbon skeleton	Pentane and 2-methylbutane (C₅H₁₂)
Position	Where the group or multiple bond sits	Propan-1-ol and propan-2-ol (C₃H₈O)
Functional group	The functional group itself	Ethanol and methoxymethane (C₂H₆O)
Metamerism	The alkyl groups on either side of one group	Methoxypropane and ethoxyethane (C₄H₁₀O) Two ethers can be metamers. An ether and an alcohol cannot: they are functional isomers.
Ring-chain	A ring in place of a C=C	Cyclopropane and propene (C₃H₆)
Tautomerism	The position of a mobile hydrogen; the forms interconvert	Propanone and prop-1-en-2-ol (keto and enol)

Chain, position, functional, metamer and ring-chain isomers are separate compounds; tautomers exist in equilibrium.

Watch out for (6)

A moved double bond is position isomerism
→ Kinds of structural isomerism
Metamers keep one functional group
→ Kinds of structural isomerism
No α-hydrogen, no tautomer
→ Kinds of structural isomerism
Isopropyl and propyl are different side chains
→ Counting structural isomers of a formula
A renumbered chain is the same compound
→ Counting structural isomers of a formula
Nitrogen adds and halogen subtracts in the DoU formula
→ Counting structural isomers of a formula

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