Classification and Nomenclature of Haloalkanes and Haloarenes

Organic compounds in which one or more hydrogen atoms of a hydrocarbon are replaced by a halogen atom (fluorine, chlorine, bromine or iodine) are called organohalogen compounds. They fall into two structurally distinct families that behave quite differently in reactions.

Haloalkanes (alkyl halides) have the halogen attached to an $sp^3$-hybridised carbon of a saturated chain -- the general formula for a simple monohaloalkane is $\text{C}_n\text{H}_{2n+1}\text{X}$. They are further classified by the nature of the halogen-bearing carbon: primary ($1^\circ$) if that carbon is attached to only one other carbon, secondary ($2^\circ$) if attached to two, and tertiary ($3^\circ$) if attached to three. A haloalkane may also be classed by the number of halogens it carries: monohalogen ($\text{CH}_3\text{Cl}$), dihalogen (e.g. $\text{CH}_2\text{Cl}_2$, or a 1,2-dihaloalkane), or polyhalogen ($\text{CHCl}_3$, $\text{CCl}_4$).

Haloarenes (aryl halides) have the halogen attached directly to an $sp^2$-hybridised carbon of an aromatic ring, as in chlorobenzene. A related but distinct class is the vinylic halides, where the halogen sits on an $sp^2$ carbon of a $\text{C=C}$ double bond outside a ring (e.g. $\text{CH}_2{=}\text{CHCl}$); vinylic and aryl halides share the key property that the halogen-bearing carbon is $sp^2$, which is what makes both classes resistant to the substitution chemistry that haloalkanes undergo readily.

IUPAC nomenclature. A haloalkane is named by selecting the longest continuous chain containing the halogen-bearing carbon as the parent, numbering the chain to give the lowest locant set to all substituents taken together, and citing each halogen as a prefix (fluoro-, chloro-, bromo-, iodo-) in alphabetical order along with any alkyl substituents. For example, $\text{CH}_3\text{CH}(\text{Cl})\text{CH}_2\text{CH}_3$ is numbered from the end nearer the chlorine to give locant 2, so its name is 2-chlorobutane; the chlorine sits on a carbon attached to two other carbons, so this is a secondary haloalkane. When two different halogens compete for the lowest locant and either numbering direction gives the same overall locant set, the substituent cited first alphabetically receives the lower number.

Haloarenes are named as substituted benzenes: chlorobenzene, bromobenzene, or, when disubstituted, by ortho/meta/para prefixes or numerical locants (e.g. 1-bromo-4-chlorobenzene for the para-disubstituted compound, with the two substituents cited alphabetically and given the lowest locant set $\{1,4\}$). A few haloarenes retain traditional common names in casual use (o-, m-, p-dichlorobenzene), but the numbered IUPAC name is unambiguous and preferred.