alkane

Meaning: An alkane is a saturated hydrocarbon: carbon and hydrogen joined by single bonds, with tetrahedral sp³ carbons (about 109.5°). The bonds are nonpolar or nearly so, so alkanes are nonpolar, insoluble in water, and held together by London dispersion forces. Boiling point rises with chain length and falls with branching, because branching reduces surface contact.

Equation: CnH2n+2 for an acyclic alkane, where n = number of carbon atoms (a dimensionless count). A single-ring cycloalkane is CnH2n.

On the MCAT: You may name alkanes (longest chain, lowest locants, suffix -ane), rank boiling points of chains or isomers, or follow free-radical halogenation: initiation by light or heat, propagation, termination. Radical stability runs 3° > 2° > 1° > methyl, so substitution favors the more substituted carbon, and bromination is more selective than chlorination. Conformations also appear: staggered is lower in energy than eclipsed, and anti is lower than gauche in butane.

Common confusion: Alkanes are called unreactive, but they do burn and undergo radical halogenation; they lack the polar bonds and pi bonds that drive ionic and addition reactions. Also, a compact, branched isomer has a lower boiling point than its straight-chain isomer, not a higher one.