Why aryl halides are less reactive?

Question

Aryl halides are much less reactive towards nucleophilic substitution reactions than haloalkanes. The less reactivity of aryl halides can be described as follows:

1. Withdrawal of electrons by Benzene and Stabilization by resonance:

In aryl halides, the electron pair of halogen atom is in conjugation with π electrons of benzene ring. Thus, halobenzene is a resonance hybrid of the following structures:

The contributing structures II, III and IV indicate that C-X bond has partial double character.

As a result, the C-X bond in halobenzene is shorter and hence, stronger as compared to that in alkyl halides. In chlorobenzene C-Clbond length is 1.69 Å as compared with 1.77 Å in methyl chloride. Thus, cleavage of C-X bond in halobenzene becomes difficult which makes it less reactive towards nucleophilic substitution.

2. Different hybrid state of carbon atom

In haloalkanes, the carbon atom bearing halogen is sp³ hybridized while halogen bearing carbon atom hybridized in halorens, sp³hybrid orbital is smaller in size due to greater s-character as compared with sp³ orbital. As a result bond formed by overlap of sp² hybrid orbital is shorter in size and stronger than the bond formed by overlap of sp3 hybrid orbital. Therefore, C-X bond of haloalkanes is cleaved more easily than in haloalkanes by overlap of sp³ hybrid orbital. Therefore, C-X bond of haloalkanes is cleaved more easily than in haloarenes.

3. Polarity of C-X bond

The C-X bond in haloalkanes is more polar than the C-X bond in haloarenes. (This is supported by dipole moment of these bonds. Dipole moment of chlorobenzene is 1.7 D whereas that of haloalkanes falls in the range (2.0-2.2 D). So greater the polarity of bond hence higher is the reactivity.

The less polarity of C-X bond in aryl halides is due to the fact that electron withdrawing inductive effect of halogen is opposed by the electron releasing resonance effect, as is evident from the positive charge on X in stronger II, III and IV.

In a similar way we can explain the less reactivity of vinyl halides as compared with alkyl or allyl halides.