Compounds Containing sp³ C—X Bond (X = F, Cl, Br, I)

When the halogen atom (X = F, Cl, Br, or I) is bonded to an $sp^3$-hybridised carbon, the resulting monohalo compound falls into one of three categories, depending on exactly what kind of $sp^3$ carbon is holding the halogen.

(a) Alkyl Halides (Haloalkanes), R—X

Here the halogen is bonded to the carbon of a simple alkyl group, R. Alkyl halides form a homologous series with the general formula

They are further split into three types based on the nature of the carbon atom that bears the halogen:

• Primary (1°) alkyl halide — the halogen is attached to a primary carbon, one bonded to only one other carbon, e.g. $\mathrm{R'-CHH-X}$ (one alkyl group, two hydrogens, on the carbon bearing X).
• Secondary (2°) alkyl halide — the halogen is attached to a secondary carbon, bonded to two other carbons, e.g. $\mathrm{R'-CH(R'')-X}$ (two alkyl groups, one hydrogen).
• Tertiary (3°) alkyl halide — the halogen is attached to a tertiary carbon, bonded to three other carbons, e.g. $\mathrm{R'-C(R'')(R''')-X}$ (three alkyl groups, no hydrogen).

In each case R′, R″, R‴ stand for the alkyl groups attached to that carbon; a primary halide has one such group and two hydrogens on the halogen-bearing carbon, a secondary halide has two such groups and one hydrogen, and a tertiary halide has three such groups and no hydrogen left on that carbon.

(b) Allylic Halides

An allylic halide has the halogen bonded to an $sp^3$-hybridised carbon that sits immediately next to a carbon–carbon double bond ($\mathrm{C=C}$) — that is, on the allylic carbon itself. The double bond is not touched by the halogen; it is the carbon adjacent to it that carries X. This class includes both open-chain and cyclic structures, such as a halomethyl group attached to a propene unit, or a halogen on the carbon next to a double bond within a ring.

(c) Benzylic Halides …