Question

Why are Haloarenes less reactive than haloalkane?

Answer 1

Haloarenes are more stable than the haloalkane that’s why haloarenes are less reactive than the haloalkane. The reasons for being stable are dependent on mainly three factors that are polarity of carbon-halogen bond, hybrid state of the carbon atom in the haloarenes and the presence of resonance.

Complete answer:

Polarity of carbon-halogen bond- As we know that if the polarity is greater than the reactivity is increased accordingly. Here we see that the dipole moment of ${\text{C - X}}$ bond in haloarenes are less than that of the haloalkane, (For example-dipole moment for ${\text{C - X}}$ in haloalkane is in the range of 2.0-2.2D while the dipole moment for the chlorobenzene is 1.7D) hence the greater is the polarity of the haloalkane makes them more reactive.

The hybrid state of carbon atom- Haloarenes exhibits the $s{p^2}$ hybridisation while haloalkane exhibits $s{p^3}$ hybridisation. The bond length in the $s{p^2}$ hybridisation is less than that of the $s{p^3}$ hybridisation, that’s why the bond formed by the haloarenes are more strong than the bond formed by the haloalkane, so the haloarenes are less reactive.

Stabilisation by resonance- in the haloarenes the $\pi$ electron of the benzene ring is in conjunction with the electron pair of the halogen in the haloarenes. Thus, haloarenes is a resonance hybrid and hence the ${\text{C - X}}$ bond has partial double bond character and this makes the haloarene more stable and less reactive.

Note: Also one more reason for the haloarenes to be less reactive than the haloalkane is that, formation of the phenyl cation (short lived electrophiles that can be generated easily by photolysis of the aromatic halides) due to heterolytic cleavage is highly unstable and the electronic repulsion between neophile and haloarene are responsible for the less reactivity of the haloarenes than haloalkane.