Question

What is a saturated carbon compound? What is an unsaturated one?

Answer 1

Saturated carbon compound
- A compound whose carbon framework contains only single bonds between carbons (C–C) — no carbon–carbon double (C=C) or triple (C≡C) bonds.
- In hydrocarbons this means each carbon has the maximum possible number of hydrogens. For an acyclic (open-chain) alkane the formula is CnH2n+2. Cycloalkanes (rings with only single bonds) are also considered saturated even though they have fewer H than the open-chain formula.
- Example: ethane (C2H6), propane (C3H8), cyclohexane (C6H12).

Unsaturated carbon compound
- A compound that contains one or more carbon–carbon multiple bonds (double or triple) or conjugated/aromatic π-systems. These reduce the number of hydrogens attached to carbon compared with the fully saturated formula.
- Unsaturated compounds readily undergo addition reactions across the multiple bonds.
- Examples: ethene (C2H4, one C=C), acetylene/ethyne (C2H2, one C≡C), benzene (aromatic π-system).

How to tell from a formula
- For simple acyclic hydrocarbons, if H < 2n+2 (for n carbons) the molecule has at least one degree of unsaturation (double bond, triple bond, or ring). More detailed counting uses the index/degree of hydrogen deficiency.

Note
- A molecule can contain heteroatoms or functional groups (–OH, –Cl, etc.) and still be called saturated as long as there are no C=C or C≡C bonds in the carbon skeleton.
- In biochemistry/lipids, “saturated” and “unsaturated” fatty acids use the same idea: saturated fatty acids have no C=C bonds in the acyl chain; unsaturated ones have one or more.