Alkyl Halides

An alkyl halide is a halogen derivative of an alkane: one or more hydrogens of a saturated hydrocarbon have been replaced by a halogen, and the monohalo family is written $R-X$, with the general formula $C_nH_{2n+1}X$. Count the halogens first. A molecule carrying one halogen is a monohaloalkane, two gives a dihaloalkane, three a trihaloalkane, and more a polyhaloalkane. Then classify by the carbon that holds the halogen. That carbon is primary when it carries one alkyl group or none, secondary when it carries two, and tertiary when it carries three. The class belongs to that carbon, never to the molecule's overall shape, which is why neopentyl chloride is a primary alkyl halide even though its skeleton is branched: the halogen sits on a $CH_2$ group whose only carbon neighbour is the quaternary centre.

Two naming systems run side by side. The common system names the alkyl group first and the halide second, so $CH_3CH_2Cl$ is ethyl chloride, and a straight-chain group takes the prefix n-, as in n-butyl bromide. IUPAC treats the halogen as a substituent on the alkane: choose the longest continuous chain that contains the halogen, number it so the halogen has the lowest locant, and write the halogen as a prefix, chloro for $Cl$ and bromo for $Br$. That gives 1-bromobutane, 2-bromo-3-methylbutane and 2-methyl-2-chloropropane. Both systems are accepted, and the exam leans on the common names for simple members. Halothane, the inhaled anaesthetic, is $CF_3-CHCl-Br$, a halo derivative of ethane and not of methane.