Structural Isomers

Take the letters A, T, C. You can spell “cat,” “act,” or “tac.” Same letters, completely different words with different meanings. Constitutional isomers work the same way - same atoms, but connected in a different order, creating entirely different molecules.

Constitutional isomers (also called structural isomers) share the same molecular formula but differ in how their atoms are bonded together. This is the most fundamental type of isomerism. If the connectivity is different, every property can be different: boiling point, melting point, solubility, reactivity, and biological activity.

Three Flavors of Constitutional Isomers

Constitutional isomers come in three subcategories. The MCAT will not usually ask you to name the subcategory, but understanding them helps you quickly identify isomeric relationships.

Chain (Skeletal) Isomers

Chain isomers have the same molecular formula but differ in the arrangement of the carbon skeleton - one might be a straight chain while another is branched.

Example: C5H12 has three chain isomers:

• Pentane - five carbons in a straight chain
• Isopentane (2-methylbutane) - four-carbon chain with a methyl branch at carbon 2
• Neopentane (2,2-dimethylpropane) - three-carbon chain with two methyl branches at carbon 2

All three have the formula C5H12, but their carbon skeletons are arranged differently. Pentane has the highest boiling point (36 C) because its longer, unbranched shape allows more surface area for London dispersion forces. Neopentane has the lowest boiling point (9.5 C) because its compact, spherical shape minimizes intermolecular contact.

Positional Isomers

Positional isomers have the same carbon skeleton and the same functional group, but the functional group is attached at a different position on the chain.

Example: C3H8O has two positional isomers (among its alcohol forms):

• 1-propanol - OH group on carbon 1 (primary alcohol)
• 2-propanol - OH group on carbon 2 (secondary alcohol)

Both are alcohols with the same carbon chain, but the hydroxyl group sits at a different position. This changes their reactivity: primary alcohols can be oxidized to aldehydes and then carboxylic acids, while secondary alcohols oxidize to ketones.

Functional Group Isomers

Functional group isomers have the same molecular formula but contain entirely different functional groups. This is the most dramatic type of constitutional isomerism.

Example: C2H6O can be:

• Ethanol (CH3CH2OH) - an alcohol with a boiling point of 78 C
• Dimethyl ether (CH3OCH3) - an ether with a boiling point of -24 C

Same formula, but one is an alcohol and the other is an ether. Their properties are wildly different. Ethanol is a liquid at room temperature, forms hydrogen bonds, dissolves in water, and can make you drunk. Dimethyl ether is a gas at room temperature, cannot hydrogen-bond with itself, and is used as an aerosol propellant.

More functional group isomer pairs to know:

Degrees of Unsaturation (Index of Hydrogen Deficiency)

When you see a molecular formula on the MCAT, calculating degrees of unsaturation helps you quickly determine how many rings and/or pi bonds the molecule contains. This narrows down which constitutional isomers are possible.

The formula for a molecule with C, H, N, and O:

How to use it:

• DoU = 0 means the molecule is fully saturated (no rings, no double bonds)
• DoU = 1 means one ring OR one double bond
• DoU = 4 usually signals a benzene ring (three double bonds + one ring)

Example: Benzene (C6H6): DoU = (2(6) + 2 - 6) / 2 = $\frac{8}{2}$ = 4. That accounts for three C=C double bonds and one ring.

How Many Constitutional Isomers Exist?

The number of constitutional isomers increases dramatically with molecular size:

You don’t need to memorize these numbers, but appreciate the pattern: as the number of carbons grows, the number of possible arrangements explodes. This is why systematic naming (IUPAC nomenclature) exists - common names would be impossibly ambiguous.

Identifying Constitutional Isomers on the MCAT

Here is a reliable strategy for determining whether two molecules are constitutional isomers:

1. Check the molecular formula. If the formulas are different, they are not isomers of any kind.
2. Check the connectivity. Trace the bonds. Are the atoms connected to the same partners? If not, they are constitutional isomers.
3. Don’t be fooled by rotation. If you can convert one drawing into the other just by rotating it (without breaking bonds), they are the same molecule, not isomers.