Hydrocarbons and IUPAC Nomenclature

Classifying Hydrocarbons

Hydrocarbons are compounds containing only carbon and hydrogen. They are classified by bonding: alkanes contain only single C-C bonds (saturated), alkenes contain at least one C=C double bond, and alkynes contain at least one $C \equiv C$ triple bond (alkenes and alkynes are unsaturated). Hydrocarbons are further split into aliphatic (chain or non-aromatic ring structures) and aromatic (containing a benzene-type ring).

The general formulas are: alkanes $C_nH_{2n+2}$, alkenes $C_nH_{2n}$, alkynes $C_nH_{2n-2}$ (for a single double or triple bond, respectively, in an open-chain molecule).

Worked example: an alkane with 5 carbons follows $C_nH_{2n+2} = C_5H_{12}$ — this is pentane.

IUPAC Nomenclature of Alkanes

Naming follows a systematic procedure. First, identify the longest continuous carbon chain — this fixes the parent name (meth-, eth-, prop-, but-, pent-, hex-, hept-, oct-, non-, dec- for 1-10 carbons) with the suffix -ane. Second, number the chain from whichever end gives substituents the lowest possible locants. Third, name and locate each substituent (alkyl groups use the suffix -yl, e.g. methyl, ethyl) with a number showing its position, and list multiple identical substituents with prefixes di-, tri-, tetra-. Substituents are listed alphabetically in the final name.

Worked example: a 5-carbon main chain with a methyl group on carbon 2 is named 2-methylpentane. If the same chain had two methyl groups, on carbons 2 and 3, it would be 2,3-dimethylpentane — numbered from the end giving the lower locant set.

Naming Alkenes and Alkynes

Alkenes take the suffix -ene and alkynes take -yne, with a locant number showing where the multiple bond starts (lower of the two carbons involved). The parent chain must include both carbons of the double/triple bond even if a longer alternative chain exists elsewhere.

Worked example: $CH_3-CH_2-CH=CH-CH_3$ is pent-2-ene (or 2-pentene) — the double bond starts at carbon 2 when numbered from the nearer end.

Physical Properties and Trends

Within a homologous series (alkanes, alkenes, or alkynes), boiling point increases with chain length because larger molecules have more surface area and therefore stronger van der Waals (London dispersion) forces between molecules. Branching lowers boiling point relative to a straight-chain isomer of the same formula, because branched molecules pack less efficiently and have reduced surface contact — for example, 2,2-dimethylpropane (b.p. $9.5°C$) boils far lower than its straight-chain isomer pentane (b.p. $36°C$).

Combustion of Hydrocarbons

Complete combustion in excess oxygen produces carbon dioxide and water, releasing energy: $C_3H_8(g) + 5O_2(g) \rightarrow 3CO_2(g) + 4H_2O(l)$ Incomplete combustion (limited oxygen) produces carbon monoxide and/or soot (carbon) instead, e.g. $2C_3H_8 + 7O_2 \rightarrow 6CO + 8H_2O$ — this is why poorly ventilated combustion is dangerous (CO is toxic and colorless).

Why This Matters for Your Exams

Nomenclature questions are graded mechanically: find the longest chain, number for lowest locants, name substituents alphabetically. Practice this as a fixed procedure rather than pattern-matching to memorized examples, because exam structures are frequently drawn to trip up students who number from the wrong end or miss a longer chain running through a "substituent." Combustion equations are almost always balanced the same way — balance carbon first, then hydrogen, then oxygen last.