1. Alkanes — The Basics
Hydrocarbons are compounds made of only carbon and hydrogen — nothing else.
Within hydrocarbons, alkanes are the family where every carbon-carbon bond is a
single bond. Because every carbon has used up all four of its bonds
connecting to either H or another C (no double bonds, no room for more atoms), we call
alkanes saturated — like a sponge that's already soaked up as much water
as it can hold.
Alkanes and alkenes are called aliphatic (chain-like, non-ring aromatic
structures), while compounds with a benzene ring are called aromatic (arenes).
Naming rule: alk + ane
The "alk" part tells you how many carbons; the "ane" part tells you it's all single bonds.
| Carbons | Prefix | Example |
|---|
| 1 | meth | methane, CH₄ |
| 2 | eth | ethane, C₂H₆ |
| 3 | prop | propane, C₃H₈ |
| 4 | but | butane, C₄H₁₀ |
| 5 | pent | pentane, C₅H₁₂ |
| 6 | hex | hexane, C₆H₁₄ |
Memory trick
After 5 carbons, the names match the maths polygon names: pent(agon), hex(agon), hept(agon), oct(agon)... Same logic, different subject!
Linear, branched, or cyclic
Alkanes don't have to be a straight chain. They can be:
- Linear (straight-chain): carbons connected one after another, e.g. butane.
- Branched: a side chain comes off the main chain.
- Cyclic (cycloalkanes): the carbon chain loops round into a ring. These follow a different general formula:
CₙH₂ₙ (same formula as alkenes — but don't be fooled, cycloalkanes are still fully saturated because the "missing" two hydrogens went into forming the ring, not a double bond).
Cycloalkanes are named cyclo + alk + ane, e.g. cyclohexane, C₆H₁₂.
Practice Question 1
What is the molecular formula of an alkane with 9 carbon atoms? What would the molecular formula be if it were a cycloalkane instead?