Isomers: Chain, Position and Ring-Chain

Two compounds can share one molecular formula and still be different substances. This part defines isomers, maps every type of isomerism, and covers the first three structural types: chain, position and ring-chain.

What isomers are

Same formula, different compounds

Ethyl alcohol and dimethyl ether are both C₂H₆O. Ethanol is a liquid (b.p. 351 K) that reacts with sodium; the ether is a gas (b.p. 249 K) that doesn't. With HI, ethanol gives C₂H₅I and the ether gives CH₃I. Same formula, different structure and properties: isomers.

Ethanol and dimethyl ether with a property table. — The –OH in ethanol and the –O– in the ether are lit.

Classification tree of isomerism. — The series covers every branch of this tree.

The map

Types of isomerism

Structural isomers differ in which atoms are bonded: chain, position, ring-chain, functional, metamerism and tautomerism. Stereoisomers have the same bonds arranged differently in space: configurational (geometrical, optical; interconvert only by breaking a bond) and conformational (interconvert by rotating single bonds).

Chain isomerism

A different carbon skeleton

C₄H₁₀ is n-butane (straight chain) or isobutane, 2-methylpropane (branched). Branching makes the molecule more compact, so isobutane boils lower (261 K vs 272 K).

n-Butane and isobutane. — Moving one carbon to the middle makes a branch.

Ring-chain isomers of C₄H₈ and C₆H₁₂. — C₄H₈ and C₆H₁₂ ring-chain isomers.

Ring-chain isomerism

Open chain or ring

A ring needs two fewer H than an open chain, exactly like a double bond, so alkenes and cycloalkanes share formulas: C₃H₆ propene / cyclopropane, C₃H₄ propyne / cyclopropene, C₄H₈ but-1-ene / cyclobutane / methylcyclopropane, C₆H₁₂ hex-1-ene / cyclohexane / methylcyclopentane. If ring-chain isn't an option, call them functional isomers.

Worked examples

From the video

1. Write the chain isomers of C₄H₁₀.

n-Butane and isobutane (2-methylpropane).

JEE-style question

Your turn

n-Propyl alcohol and isopropyl alcohol are:

(a)position isomers

(b)chain isomers

(c)tautomers

(d)geometrical isomers

Show the answer and the traps

The skeleton is the same three-carbon chain; only the OH moves. Position isomers: option (a).

Not chain isomers: the skeleton doesn't change. Not tautomers: nothing interconverts. Not geometrical: there's no double bond or ring.

Watch out

Common mistakes

Calling "but-3-ene" a new isomer.Number from the other end first: it is but-1-ene.

Calling an alkene and a cycloalkane chain isomers.They are ring-chain isomers (functional isomers if ring-chain is not an option).

Practice

Try these

1. But-1-ene and but-2-ene are which type of isomers?

Position isomers: same skeleton, the C=C moves.

2. Cyclopentane and pent-1-ene (both C₅H₁₀) are which type of isomers?

Ring-chain isomers (functional isomers if ring-chain is not offered).