Compare the structure, bonding, general formulas and reactivity of alkanes, alkenes and alkynes

What this dot point is asking

A hydrocarbon is a compound made only of carbon and hydrogen. The three families covered here differ in the type of carbon-carbon bond they contain, which determines both their general formula and their reactivity.

Alkanes: saturated hydrocarbons

Alkanes contain only single carbon-carbon bonds and so are saturated (each carbon holds the maximum number of hydrogen atoms). Their general formula is $\text{C}_n\text{H}_{2n+2}$ (methane, ethane, propane and so on). Because the C-C and C-H single bonds are strong and non-polar, alkanes are relatively unreactive. Their main reactions are combustion and, with ultraviolet light, substitution with halogens.

Alkenes: a carbon-carbon double bond

Alkenes contain at least one C=C double bond and are unsaturated, with general formula $\text{C}_n\text{H}_{2n}$ (ethene, propene and so on). The double bond is a region of high electron density and is the reactive site. Alkenes readily undergo addition reactions, in which a molecule adds across the double bond, converting it to a single bond.

Alkynes: a carbon-carbon triple bond

Alkynes contain a C≡C triple bond, general formula $\text{C}_n\text{H}_{2n-2}$ (ethyne is the simplest). Like alkenes they are unsaturated and undergo addition, and they are even more unsaturated, so they can add two molecules across the triple bond.

Testing for unsaturation

Bromine water (orange-brown) is decolourised quickly by alkenes and alkynes at room temperature because bromine adds across the multiple bond. Alkanes only decolourise bromine slowly and only in ultraviolet light (a substitution, releasing $\text{HBr}$). This is the standard chemical test to distinguish saturated from unsaturated hydrocarbons.

Combustion: complete versus incomplete

All hydrocarbons burn, but the products depend on the oxygen supply. Complete combustion in plentiful oxygen gives only carbon dioxide and water and releases the most energy, which is why hydrocarbons are valued as fuels. Incomplete combustion in limited oxygen produces carbon monoxide (a toxic gas) and soot (carbon) as well as water, releasing less energy. Balancing a combustion equation reliably is a common calculation: balance carbon first (one $\text{CO}_2$ per carbon), then hydrogen (one $\text{H}_2\text{O}$ per two hydrogens), then count the oxygen atoms needed and supply them as $\text{O}_2$ (using a fractional coefficient if necessary, then doubling through). Unsaturated hydrocarbons tend to burn with a smokier flame than alkanes because their higher carbon-to-hydrogen ratio favours incomplete combustion.

Why this matters

Hydrocarbons are the feedstock of the petrochemical industry and the starting materials for nearly all the organic families studied in Unit 4. The link between unsaturation and reactivity underpins addition reactions, addition polymerisation, and the synthesis pathways examined later.