Classification Based on Functional Groups

The second way to classify an organic compound is by its functional group -- the reactive atom or group of atoms that gives the molecule its characteristic chemistry, independent of how long or short the rest of the carbon skeleton is. Table 11.1 in this section catalogues roughly thirty such classes, each with its defining functional group and general formula (written with R standing for a generic alkyl group).

Working through the families the table covers: alkenes and alkynes are defined purely by their carbon-carbon multiple bond (a double bond, general formula CnH2n, for alkenes; a triple bond, CnH2n-2, for alkynes) rather than by a heteroatom-bearing group. Alkyl halides (R-X) carry a halogen (F, Cl, Br or I) in place of a hydrogen. Alcohols (R-OH) and ethers (R-O-R') both build on an oxygen singly bonded to carbon, differing in whether that oxygen also bears a hydrogen (alcohol) or bridges two carbon chains (ether). The carbonyl-based families -- aldehydes (R-CHO), ketones (R-CO-R'), carboxylic acids (R-COOH), esters (RCOOR'), acid anhydrides (R-CO-O-CO-R'), acyl chlorides (R-COX) and amides (R-CO-NH2) -- all share a C=O group but differ in what else is attached to that carbonyl carbon (H, a second carbon chain, OH, OR, another acyl group, a halogen, or NH2 respectively). Sulphonic acids (R-SO3H) and nitro compounds (R-NO2) carry sulfur- and nitrogen-oxide groups respectively. The nitrogen-based families multiply further: amines (R-NH2), nitriles/cyanides (R-CN, carbon triple-bonded to nitrogen) and their less-common isomeric relatives isocyanides (R-NC), cyanates (R-OCN), isocyanates (R-NCO), thiocyanates (R-SCN) and isothiocyanates (R-NCS) -- each swapping which atom of a small linear group bonds to the parent carbon chain. Sulfur-analogue families mirror the oxygen ones: thiols (thioalcohols, R-SH) parallel alcohols, and thioethers (R-S-R') parallel ethers. Rounding out the table: imines (R-CH=NH, a carbon-nitrogen double bond), nitroso compounds (R-NO), hydrazines (R-NH-NH2) and hydrazo compounds (R-NH-NH-R, two nitrogens dire …