A saturated hydrocarbon has only single bonds between carbon atoms, and an unsaturated hydrocarbon has at least one C=C double bond. Alkanes are saturated and alkenes are unsaturated. The formulae give a quick check: alkanes fit CₙH₂ₙ₊₂ and alkenes fit CₙH₂ₙ.
This lesson sits in organic families and naming and follows drawing small molecules with correct bonding.
Why does the double bond matter?
Each carbon forms four bonds. In an alkane, every carbon is joined to as many hydrogen or carbon atoms as possible, so the molecule is “full”. In an alkene, two carbons share a double bond, which uses up an extra bond and leaves room for two fewer hydrogens.
That is the reason the two formulae differ by exactly two hydrogens for the same number of carbons. The double bond is also where alkenes react, which is why they behave differently from alkanes in tests and in organic reactions.
| Carbons (n) | Alkane (CₙH₂ₙ₊₂) | Alkene (CₙH₂ₙ) |
|---|---|---|
| 2 | C₂H₆ | C₂H₄ |
| 3 | C₃H₈ | C₃H₆ |
| 4 | C₄H₁₀ | C₄H₈ |
How do you decide, step by step?
- From a drawing: look for a C=C. If there is one, it is unsaturated; if every carbon-carbon bond is single, it is saturated.
- From a formula: count carbons (n), work out 2n + 2 and compare it with the hydrogens given.
- If hydrogens equal 2n + 2, the hydrocarbon is saturated.
- If hydrogens equal 2n, it is unsaturated with one double bond.
- From a test result: a hydrocarbon that decolourises bromine water is unsaturated.
Worked example
State whether each hydrocarbon is saturated or unsaturated: A: C₅H₁₂, B: C₃H₆, C: a compound that turns bromine water from orange-brown to colourless.
A. n = 5, so 2n + 2 = 12. The formula has 12 hydrogens, so A is an alkane and saturated.
B. n = 3, so 2n + 2 would be 8, but B has only 6. That equals 2n, so B has one C=C, making it an alkene (propene) and unsaturated.
C. Decolourising bromine water is the test result for a C=C double bond, so C is unsaturated. A saturated compound would leave the colour unchanged.
As a check, draw B. Three carbons with one double bond give CH₂=CHCH₃.
Hydrogens: 2 + 1 + 3 = 6. That matches C₃H₆.
The mistake to watch for
A typical error is to say that an alkene is unsaturated “because it has more hydrogens”.
Mistaken answer: Propene is unsaturated because it has more hydrogen atoms than propane.
The student reversed the facts. Propene, C₃H₆, has fewer hydrogens than propane, C₃H₈.
The correction is that an alkene is unsaturated because it contains a C=C double bond, and so holds fewer hydrogens for its number of carbons. A second slip is to think any double bond makes a molecule unsaturated in this sense; in these questions, look specifically for the double bond between carbon atoms.
Check yourself
1. C₄H₈ and C₄H₁₀ are both hydrocarbons. Which is saturated? Give the evidence from the formulae.
Show answer
For n = 4, 2n + 2 = 10. C₄H₁₀ fits, so C₄H₁₀ is saturated. C₄H₈ has two fewer hydrogens (2n = 8), so it contains a C=C and is unsaturated.
2. A hydrocarbon has 6 carbons and is saturated. Write its molecular formula.
Show answer
2n + 2 = 2(6) + 2 = 14, so the formula is C₆H₁₄.
3. Is propan-1-ol, CH₃CH₂CH₂OH, saturated or unsaturated? Explain.
Show answer
It is saturated. All the bonds between its carbon atoms are single, and there is no C=C. The oxygen is in an -OH group and does not make the carbon chain unsaturated.
Where this leads next
The next lesson, converting between formula representations, ties together molecular, structural and displayed formulae. You can test the whole module in the organic families and naming practice set.
If the formula patterns make sense in a lesson but slip away when a question gives only a formula, online one-to-one Chemistry tuition lets a teacher give you varied examples and watch your method.