primary, secondary, tertiary and quaternary carbon
/ TUR-shee-air-ee; KWAH-tur-nair-ee /
Not all carbons in a molecule are alike — some are out at the ends with few carbon neighbours, some are buried in the middle surrounded by other carbons. Chemists give each carbon a rank based on how crowded it is with carbon neighbours: primary, secondary, tertiary, or quaternary. This little classification turns out to predict a great deal about how a molecule reacts.
The rule is simply a headcount of other carbons directly bonded to the carbon in question. A primary (1°) carbon is attached to one other carbon; a secondary (2°) carbon to two; a tertiary (3°) carbon to three; and a quaternary (4°) carbon to four other carbons. (A carbon bonded to no other carbon, like the carbon in methane, is sometimes called methyl or zeroth.) The same labels extend to anything attached to that carbon: a hydroxyl on a 2° carbon is a secondary alcohol, a halogen on a 3° carbon is a tertiary haloalkane. Note that a quaternary carbon has four carbon neighbours and therefore no hydrogen of its own at all.
Why bother? Because this degree of substitution governs reactivity throughout organic chemistry. Carbocations and radicals are more stable the more substituted they are, so tertiary centres favour SN1 and E1 pathways while primary centres favour SN2; steric crowding around a tertiary or quaternary carbon blocks backside attack. A quaternary carbon, having no H, cannot undergo reactions that need a hydrogen on that carbon (it can never become a carbonyl, for instance). Reading off whether a reactive carbon is 1°, 2°, or 3° is often the first thing a chemist does when predicting a reaction's outcome.
In 2-methylbutane, (CH3)2CHCH2CH3: the three CH3 carbons are primary, the CH2 is secondary, and the central CH (bonded to three carbons) is tertiary. In neopentane, C(CH3)4, the central carbon is bonded to four carbons — quaternary, with no hydrogen of its own.
Count carbon neighbours: 1, 2, 3, or 4 gives primary, secondary, tertiary, or quaternary.
Count only carbon neighbours, not hydrogens or other atoms. A quaternary carbon has four carbon neighbours and zero hydrogens, so it cannot do any reaction requiring a hydrogen on that carbon.