Isomerism: Same Formula, Different Structure

Introduction

Take the ten letters of a word and rearrange them: the letters are identical, but the new word can mean something completely different. Molecules can do the same trick. Two compounds can contain exactly the same atoms — the same molecular formula — yet join them together in a different order. These compounds are called isomers , and they are different substances with different properties. Isomerism is one of the main reasons carbon forms millions of compounds.

Core explanation

What isomers are. Isomers are compounds that have the same molecular formula but a different structure . At this level we meet structural isomers , in which the atoms are bonded together in a different order. A molecular formula such as C₄H₁₀ tells you what atoms are present; it does not tell you how they are connected.

The isomers of C₄H₁₀. Four carbon atoms can be joined in two ways:

- In a straight chain : CH₃–CH₂–CH₂–CH₃. This is butane . - With three carbons in a chain and one carbon attached to the middle: CH₃–CH(CH₃)–CH₃. This is 2-methylpropane (older name isobutane).

Both molecules contain four carbon atoms and ten hydrogen atoms, and in both every carbon still makes four bonds. But they are different compounds.

The isomers of C₅H₁₂. Five carbons can be arranged in three ways:

Isomer Structure Boiling point --- --- --- Pentane CH₃–CH₂–CH₂–CH₂–CH₃ 36 °C 2-Methylbutane CH₃–CH(CH₃)–CH₂–CH₃ 28 °C 2,2-Dimethylpropane C(CH₃)₄ 10 °C

For comparison, butane boils at about −1 °C and 2-methylpropane at about −12 °C.

Bending is not a new isomer. Because there is free rotation about carbon–carbon single bonds, a straight chain can be drawn bent, zig-zagged or curled into a U shape. It is still the same molecule. To decide whether two drawings are different isomers, trace the longest continuous carbon chain and look at what is attached to it. If the connections are the same, the drawings show the same compound.

Numbers grow fast. As the number of carbon atoms rises, the number of possible structural isomers of an alkane grows rapidly: C₆H₁₄ has 5, C₇H₁₆ has 9, C₁₀H₂₂ has 75, and C₂₀H₄₂ has more than 366 000. This explosion of possibilities is a direct result of catenation and carbon's four bonds.

Same formula, different properties. Isomers of an alkane have very similar chemical reactions — they all burn, for example — but different physical properties, especially boiling point.

Step-by-step reasoning

To find all the structural isomers of an alkane:

1. Draw the longest possible straight chain using all the carbons. 2. Shorten the chain by one carbon and attach that carbon as a branch to an inner carbon (never the end one). 3. Shorten again and try every different branch position. 4. Check each drawing by finding its longest chain; discard any that repeat an earlier structure. 5. Add hydrogens so every carbon has four bonds, and confirm the formula is unchanged.

Visual explanation

Picture five beads threaded on a string in a line — that is pentane. Now take one bead off the end and hang it from the second bead: 2-methylbutane. Finally make a plus sign, one bead in the centre and four around it: 2,2-dimethylpropane. Same five beads, three different shapes, from long and thin to compact and almost spherical. The simulation lets you build and rotate each one.

Real-world analogy

Isomers are like LEGO models built from an identical box of bricks. One child builds a long wall, another builds a compact tower. The parts list is the same, but the finished objects look different and behave differently when you push them.

Real-world example

Camping gas cylinders often contain a mixture of butane and 2-methylpropane. Because 2-methylpropane boils at about −12 °C rather than −1 °C, it keeps vaporising in cold weather when pure butane would struggle, so the stove still lights on a frosty morning.

Why?

Why do branched isomers boil at lower temperatures? A straight chain is long and can lie alongside its neighbours with a large area of contact, so the intermolecular forces between molecules are stronger. A branched molecule is more compact and nearly ball-shaped; it touches its neighbours over a smaller area, so the forces are weaker and less energy is needed to separate the molecules.

Common misconception

"Drawing a chain with a bend in it makes a new isomer." It does not. A chain can rotate freely about its single bonds. Only a change in which atoms are bonded to which creates a different isomer.

Worked example

Question: A student draws CH₃–CH₂–CH(CH₃)–CH₃ and CH₃–CH(CH₃)–CH₂–CH₃ and claims they are two isomers of C₅H₁₂. Are they right?

Reasoning: In both drawings the longest chain is four carbons, with a CH₃ branch on the second carbon from one end. Numbering from the nearer end, both are 2-methylbutane. They are the same molecule drawn from opposite directions.

Answer: No; both drawings represent 2-methylbutane.

Quick check

1. What do structural isomers have in common, and how do they differ? Answer: They have the same molecular formula but their atoms are joined in a different order.

Exam focus

Be ready to draw displayed formulae of the two isomers of butane and the three of pentane, and to explain why more branching gives a lower boiling point. Always check that each carbon has four bonds and that a "new" isomer is not just the same chain bent.

Advanced insight

Structural isomerism is only one kind. Later you will meet stereoisomers, which have the same connections but a different arrangement in space — such as E/Z isomers around a C=C double bond, and optical isomers that are mirror images of each other. Enzymes can tell such mirror-image molecules apart, which matters greatly in medicine.

Summary

Isomers have the same molecular formula but different structures. Structural isomers differ in the order in which atoms are bonded. C₄H₁₀ has two isomers and C₅H₁₂ has three, and the number rises rapidly with chain length. Isomers of alkanes react alike but differ in physical properties: more branching means weaker intermolecular forces and a lower boiling point.

Practice questions

1. Define the term structural isomers. Answer: Compounds with the same molecular formula but a different arrangement (bonding order) of their atoms. 2. Name the two structural isomers of C₄H₁₀. Answer: Butane and 2-methylpropane. 3. Which isomer of C₅H₁₂ has the lowest boiling point, and why? Answer: 2,2-Dimethylpropane, because it is the most compact, has the smallest surface contact between molecules and so the weakest intermolecular forces. 4. Explain why isomers of an alkane have similar chemical properties. Answer: They contain the same types of bond (C–C and C–H single bonds only), so they undergo the same kinds of reaction, such as combustion.