What comes after the primary secondary tertiary quarters

C atoms: primary, secondary, tertiary and quaternary

Primary carbon atoms are linked to exactly one other carbon atom. In the case of alkanes, the external carbon atoms are always primary carbon atoms. The carbon atom in methane is not a primary carbon atom because it is not connected to a single other carbon atom.

Secondary carbon atoms are linked to exactly two other carbon atoms. In the case of unbranched alkanes, the inner carbon atoms are always secondary carbon atoms.

Tertiary carbon atoms occur in branched alkanes (and of course in other organic compounds as well). A tertiary carbon atom is always connected to three other carbon atoms.

Quaternary carbon atoms are also found in some branched alkanes and other compounds. Quaternary carbon atoms are connected to exactly four other carbon atoms.

Stability, reactivity

Tertiary carbon atoms can accept a loss of electrons more easily than secondary or even primary carbon atoms. Tertiary radicals form much more easily than secondary ones, and the same applies to the formation of tertiary carbo-cations.

The reason for this lies partly in the positive inductive effect (+ I effect) exerted by the three alkyl groups of a tertiary radical or carbenium ion, as well as in factors such as hyperconjugation.

A famous textbook experiment demonstrates the different stability and reactivity of primary, secondary and tertiary carbon atoms. Lye is added to three different bromobutanes. The bromine atoms are then substituted (replaced) by OH groups. This happens particularly quickly with tertiary bromobutane, 2-bromo-2-methylpropane. With the secondary 2-bromo-butane the reaction takes longer, and the primary 1-bromo-butane only reacts very slowly.

The reason is the different stability of the carbon atoms on which the bromine atom is located. In the case of 2-bromo-2-methylpropane, the bromine atom is on a tertiary carbon atom. Here the bromine atom can easily be split off as a bromide ion. The resulting tertiary carbenium ion is well stabilized by the three adjacent alkyl groups. These push the electron density towards the tertiary carbon atom, so that the energetically unfavorable positive charge on this carbon atom is somewhat reduced. The carbenium ion of the secondary bromobutane is only stabilized by two alkyl groups, and the carbenium ion of the primary bromobutane is only stabilized by one alkyl group. Only bromomethane whose carbon atom is not supported by any alkyl group would react even worse.