Carbon Centers:
One of the easiest methods of ascertaining whether a carbon center is 1?, 2?, 3? or 4? is to count the number of bonds that are leading from that carbon center to other carbon atom. A methyl group (CH3) is a primary or 1? carbon center, a methylene group (CH2) is a secondary or 2? carbon center, a methine group (CH) is a tertiary or 3? carbon center, and a carbon center with four alkyl substituents (C) is a quaternary or 4? carbon center.
Figure: Carbon centers; (a) primary; (b) secondary; (c) tertiary; (d) quaternary.
Figure: Primary, secondary, tertiary and quaternary carbon centers.