Reactivity of C-X Bond

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The halogen atom in haloalkane is very reactive and can be easily replaced by nucleophiles whereas the halogen atom in haloarenes is strongly held to the nucleus and cannot be replaced easily with nucleophiles. The differences between haloalkanes and haloarenes are due to the following reasons:

Resonance effect

There is delocalisation of electrons in haloarenes due to resonance. For example, chlorobenzene is considered to be a resonance hybrid of the following structures:

The contribution of structures III, IV and V imparts a partial double bond character to the carbon-chlorine bond. The shortening of bond length imparts stability to aryl halides and as a result, the bond cleavage becomes rather difficult. The aryl halides are, therefore, less reactive than alkyl halides.

Hybridization states of carbon atom

The carbon atom of the C-X bond in haloalkanes, is sp³ hybridised while in haloarenes the carbon atom is sp² hybridized. The sp² hybridized carbon atom with a greater 's' character is more electronegative. It can hold the electron pair of the bond more tightly than the sp³ hybridized carbon atom in alkyl halides. This has less tendency to release electrons to the halogen. As a result, the bond cleavage in aryl halides is somewhat more difficult than in alkyl halides.

Polarity of carbon-halogen bond

The sp² hybridized carbon atom in the C-X bond in haloarene molecule is more electronegative than the sp³ hybrid carbon atom in alkyl halide.

This carbon has less tendency to release electrons to the chlorine atom and so the C-Cl bond in aryl halides is less polar than in alkyl halides. This is supported by the fact that the dipole moment of chlorobenzene is 1.73 D while the dipole moment of chloroethane is 2.05 D. Lesser the polarity of C-Cl bond, lesser is the reactivity. Thus, haloarenes are less reactive towards the substitution reactions than haloalkanes. However, under drastic conditions, aryl halides undergo substitution reactions.