Isomerism

Same formula, different molecule. How identical atoms build compounds with completely different properties.

High schoolIntro OrganicUni Year 1
⏱️ About 16 min

C₄H₁₀ could be butane, the fuel in a lighter — or it could be isobutane, which boils at a different temperature and branches differently. Same atoms, same count, genuinely different substances. That is isomerism, and it is why carbon chemistry is so endlessly rich.

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The big idea: Isomers are different compounds that share the same molecular formula but differ in how their atoms are arranged. Structural isomers differ in connectivity; stereoisomers have the same connectivity but a different arrangement in space. Different arrangement means different properties.
🎯 By the end, you'll be able to
  • Define isomers as different compounds with the same molecular formula
  • Distinguish structural isomers (connectivity) from stereoisomers (3D arrangement)
  • Explain cis–trans isomerism at a C=C double bond and enantiomers (mirror images)
  • Use degrees of unsaturation to gauge rings and multiple bonds
📎 Helpful to know first

Same formula, different compound

A molecular formula counts atoms but does not say how they are joined. Isomers are molecules with the same molecular formula but a different arrangement of atoms — and because the arrangement differs, they are genuinely different compounds with their own melting points, boiling points and reactions.

There are two broad kinds: structural isomers, which differ in what is connected to what, and stereoisomers, which have the same connections but a different arrangement in 3D space.

⚠️ Isomers are not the 'same molecule drawn twice'
This is the classic trap. Butane and isobutane (both C4H10) are different substances, not two pictures of one molecule. You cannot turn one into the other by rotating or flipping the drawing — you would have to break bonds and reconnect them.

Structural isomers: different connectivity

Structural (constitutional) isomers have their atoms bonded in a different order. C4H10 can be a straight chain (butane) or a branched chain (2-methylpropane, "isobutane"). Both obey the formula; their skeletons differ. As chains get longer the number of possible structural isomers explodes.

\[ \ce{CH3CH2CH2CH3}\qquad \ce{CH3-CH(CH3)-CH3} \]
Two structural isomers of C₄H₁₀: straight-chain butane and branched 2-methylpropane.

Stereoisomers: same connections, different in space

Stereoisomers share the same connectivity but arrange atoms differently in three dimensions. Two important types:

  • Cis–trans (geometric) isomers — a C=C double bond cannot rotate freely, so groups are locked on the same side (cis) or opposite sides (trans) of the double bond.
  • Enantiomers — non-superimposable mirror images, like your left and right hands. They share nearly all properties but interact differently with other handed molecules, which matters enormously in biology and medicine.
✨ Why C=C locks, but C–C spins
A single C–C bond lets the two carbons rotate freely, so cis and trans would just interconvert — no separate isomers. A C=C double bond is rigid: it cannot twist without breaking. That rigidity is exactly what freezes the groups into fixed cis or trans positions.

Counting unsaturation as a clue

Before you draw isomers, it helps to know how many rings or multiple bonds a formula must contain. The degrees of unsaturation (also called the index of hydrogen deficiency) counts them. For a molecule of carbon and hydrogen only:

\[ \text{DoU} = \frac{2C + 2 - H}{2} \]
Each degree of unsaturation is one ring or one double bond; a triple bond counts as two.
📝 Worked example: How many degrees of unsaturation does C₅H₈ have, and what does that tell you?
  1. Use DoU = (2C + 2 − H)/2 with C = 5, H = 8.
  2. = (2·5 + 2 − 8)/2 = (10 + 2 − 8)/2.
  3. = 4/2 = 2 degrees of unsaturation.
  4. Two degrees means the molecule must contain some combination of rings and/or double bonds totalling 2 — for example, two double bonds, or one ring plus one double bond, or one triple bond.
✓ 2 degrees of unsaturation — e.g. two C=C bonds, a ring + a C=C, or one C≡C.
✏️ Practice: Calculate the degrees of unsaturation for benzene, C₆H₆. Use DoU = (2C + 2 − H)/2.
degrees
Solution
  1. Substitute C = 6, H = 6: DoU = (2·6 + 2 − 6)/2.
  2. = (12 + 2 − 6)/2 = 8/2.
  3. = 4 degrees — consistent with benzene's ring (1) plus its three double bonds (3).

Check your understanding

1. Two isomers share the molecular formula C₄H₁₀. What must be true?
Isomers are genuinely different compounds — same formula, different arrangement. They cannot be interconverted just by rotating a drawing; that would require breaking bonds.
2. What is the difference between structural isomers and stereoisomers?
Structural isomers bond their atoms in a different order (different connectivity). Stereoisomers keep the same connectivity but arrange atoms differently in space.
3. Why can cis–trans isomerism exist across a C=C double bond but not across a C–C single bond?
A C=C double bond is rigid — it cannot twist without breaking — so groups stay locked cis or trans. A single bond rotates freely, so no distinct isomers form.
🔎 Related lessons
Previous: IUPAC Nomenclature Next: Reaction Types & Mechanisms Functional Groups
✅ Key takeaways
  • Isomers are different compounds that share the same molecular formula.
  • Structural isomers differ in connectivity (e.g. butane vs 2-methylpropane).
  • Stereoisomers share connectivity but differ in 3D: cis–trans across a rigid C=C, or enantiomers (mirror images).
  • A C=C double bond cannot rotate, which is what allows cis–trans isomers.
  • Degrees of unsaturation = (2C + 2 − H)/2 counts rings and multiple bonds.
➡️ Different arrangements react differently — which brings us to the reactions themselves. Next, the handful of reaction types that account for most of what organic molecules actually do.
Want to test yourself on this? Try the Chemistry practice test →