Chapter 18 · Organic Chemistry
18.4Hydrocarbons: Compounds Containing Only Carbon and Hydrogen
8 min · two checks
Predict
What is the difference between an alkane, an alkene, and an alkyne?
The idea
- Classify hydrocarbons by bond type.
- Recognize saturated versus unsaturated.
A hydrocarbon contains only carbon and hydrogen. Alkanes are saturated: every carbon–carbon link is a single bond, and each carbon holds as many hydrogens as it can. Alkenes are unsaturated because a double bond could, in principle, add two more atoms. Alkynes are unsaturated at a triple bond. Aromatic hydrocarbons, such as benzene, are a special unsaturated family saved for later; they do not behave like ordinary alkenes.
The general formula for a straight-chain alkane is CₙH₂ₙ₊₂. Each ring or each double bond reduces the hydrogen count by two. You can use that as a check on a drawing. Petroleum is mostly alkanes. Fats are not hydrocarbons — they contain oxygen — but the long tails of fatty acids are hydrocarbon-like, and saturation language shows up again in nutrition.
Keep these
- Alkane: single bonds only. Alkene: a double bond. Alkyne: a triple bond.
- Saturated means no room to add more hydrogen without breaking the carbon framework.
- Acyclic alkanes: CₙH₂ₙ₊₂.