Structural Isomerism: Same Formula, Different Structure

What Is Structural Isomerism?

Structural isomers (also called constitutional isomers) are molecules that have the same molecular formula but different structural arrangements of atoms. They have different physical and chemical properties despite sharing the same formula.

For example, $C_4H_{10}$ can be either butane (straight chain) or 2-methylpropane (branched). Same atoms, different connectivity.

Learning Goals: By the end of this guide, you should be able to:

1. Define structural isomerism and distinguish it from stereoisomerism.
2. Identify and draw all three types: chain, position, and functional group isomers.
3. Systematically find all possible isomers for a given formula.
4. Predict how isomer type affects physical and chemical properties.

The Three Types

1. Chain Isomers

Different arrangements of the carbon skeleton — the same molecular formula, but different chain length and branching patterns.

Example: $C_5H_{12}$ (three chain isomers)

Trend: More branching → more compact shape → less surface area → weaker London forces → lower boiling point.

2. Position Isomers

Same carbon skeleton and same functional group, but the functional group is at a different position on the chain.

Example: $C_3H_7OH$ (two position isomers)

These have similar but not identical properties — propan-2-ol is oxidised to a ketone, while propan-1-ol is oxidised to an aldehyde.

3. Functional Group Isomers

Same molecular formula but different functional groups entirely. These have very different chemical properties.

Example: $C_2H_6O$ (two functional group isomers)

Systematic Isomer Finding

A Step-by-Step Method

1. Start with the longest possible chain (straight chain isomer).
2. Shorten by one carbon, add it as a branch. Move the branch along the chain to find all position isomers.
3. Shorten by two carbons and explore branches of two.
4. Consider functional group isomers (e.g., alcohol vs. ether for $C_nH_{2n+2}O$).
5. Check for duplicates — rotate each structure mentally. If two have the same connectivity, they're the same molecule.

Example: Find All Isomers of $C_4H_8$

Since the formula is $C_nH_{2n}$, these could be alkenes or cycloalkanes:

Alkenes (contain $C=C$):

1. But-1-ene: $CH_2=CHCH_2CH_3$
2. But-2-ene: $CH_3CH=CHCH_3$
3. 2-methylpropene: $CH_2=C(CH_3)_2$

Cycloalkanes (ring structure): 4. Cyclobutane 5. Methylcyclopropane

Total: 5 structural isomers.

Worked Examples

Example 1: Drawing Isomers of $C_3H_6O$

Solution: This formula allows several functional groups:

Functional group isomers:

1. Propanal $CH_3CH_2CHO$ (aldehyde)
2. Propanone $CH_3COCH_3$ (ketone)
3. Prop-2-en-1-ol $CH_2=CHCH_2OH$ (unsaturated alcohol)
4. Allyl alcohol / methoxyethene — and several more

The aldehyde and ketone are functional group isomers of each other.

Example 2: Predicting Boiling Points

Question: Which has the higher boiling point: pentane or 2,2-dimethylpropane?

Solution: Both are $C_5H_{12}$. Pentane is a straight chain with more surface area contact between molecules → stronger London forces → higher boiling point (36°C vs. 10°C).

Example 3: Distinguishing Isomers by Reaction

Question: How would you distinguish ethanol ($C_2H_5OH$) from dimethyl ether ($CH_3OCH_3$)?

Solution:

• Add sodium metal: ethanol reacts (bubbles of $H_2$), dimethyl ether does not.
• Add acidified dichromate: ethanol turns it from orange to green (oxidation), dimethyl ether shows no colour change.

Common Mistakes

1. Drawing the same molecule twice — Students often draw what looks like a different structure but is actually just the same molecule rotated or flipped. Always check: is the connectivity different?

2. Missing functional group isomers — When asked for all isomers of $C_2H_6O$, many students draw only ethanol and forget dimethyl ether. Always consider different functional groups.

3. Confusing structural isomerism with stereoisomerism — Structural isomers differ in connectivity (which atom is bonded to which). Stereoisomers (cis/trans, optical) have the same connectivity but differ in spatial arrangement.

4. Not systematically searching — Random drawing misses isomers. Use the systematic method: longest chain → shorten and branch → check functional groups.

Exam Tips (A-Level / AP / IB)

• When asked to "draw all isomers", work systematically from longest chain to shortest, then consider functional group changes.
• Always name your isomers — it helps avoid duplicates and shows the examiner you understand the differences.
• For "explain the difference in boiling point" between isomers: it's always about surface area affecting London forces (for chain isomers) or hydrogen bonding capability (for functional group isomers).

Frequently Asked Questions

How many structural isomers does hexane have?

$C_6H_{14}$ has 5 structural isomers: hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, and 2,3-dimethylbutane.

What is the difference between structural isomers and stereoisomers?

Structural isomers differ in which atoms are connected to which (connectivity). Stereoisomers have the same connectivity but differ in how atoms are arranged in space (e.g., cis-trans isomers, optical isomers).

Do structural isomers have the same chemical properties?

Not necessarily. Chain and position isomers have similar chemical properties but different physical properties. Functional group isomers have completely different chemical and physical properties.

Related Topics

• Hydrocarbons — The parent molecules that exhibit chain isomerism.
• Hydrocarbon Derivatives — Functional groups that create functional group isomers.
• Molecular Coplanarity — How 3D shape relates to isomer properties.