Alkyl halides (also known as haloalkanes) are hydrocarbon compounds in which one or more of the hydrogen atoms have been replaced by a halogen atom (iodine, bromine, chlorine or fluorine). Incorporating halogen atoms into a hydrocarbon changes the compounds' physical properties - affecting size, electronegativity and bond length and strength.
Alkyl halides are ideal substrates for reactions that require a good leaving group. The high reactivity of alkyl halides can be explained in terms of the nature of the C-X bond. The differences in electronegativity between the carbon and halogen atoms create a highly polarized bond resulting in a slightly electropositive carbon and slightly electronegative halogen. This electron-deficient carbon becomes a hotspot for nucleophilic attack, making alkyl halides excellent substrates for nucleophilic substitution and elimination reactions.
Alkyl halides are classified according to the connectivity of the carbon atom that carries the halogen atom:
Primary the carbon attached to the halogen is only attached to one other alkyl group
Secondary the carbon attached to the halogen is attached to two other alkyl groups
Tertiary the carbon attached to the halogen attached to three other alkyl groups
In general - due to the steric bulk of three alkyl groups surrounding the halogen in tertiary alkyl halides - tertiary alkyl halides are far less reactive than the other classes and may only participate in elimination reactions. The general reactivity trend across alkyl halides classes is Primary > Secondary > Tertiary. However - this alkyl halide reactivity trend is reversed if the rate of a specific reaction (e.g. SN1 reaction) is determined by the formation of the most stable carbocation. In these situations, tertiary alkyl halides are highly favored as they would form the most stable reactive intermediate.
Other factors - like which C-X bond you need to break - also contribute to the alkyl halides' reactivity and the likelihood of a reaction occurring or not.
In selecting the best halide starting material for a reaction - we also need to consider the strength of the C-X bond we are trying to break. C-F bonds are so strong that fluoroalkanes very rarely react, so don't make great starting materials. C-X bond strength falls as we go down the periodic table, meaning the weakly-bonded iodide is the most willing leaving group, closely followed by bromide.
Figure 1: Trend in C-X bond strength