Name: Nomenclature of Haloalkanes and Haloarenes
Uploaded: 2019-07-19
Duration: 9 min 1 s
Description: Haloalkanes are formed by replacing a hydrogen atom in an alkane with a halogen atom. Haloarenes are derived by replacing the hydrogen atom attached to the b...

Question

Why are aryl halides less reactive towards nucleophilic substitution reactions than alkyl halides? How can the reactivity of aryl halides be enhanced?

Accepted Answer

Aryl halides do not react with nucleophilic reagents under normal laboratory conditions. The following reasons can be attributed to account for the low reactivity of aryl halides:

1. Resonance effect: In haloarenes, the lone pairs of electrons on the halogen atom are delocalized on the benzene ring. As a result, C-X bond acquires some double bond character, i.e. X is attached to C by little more than a single pair of electrons. On the other hand, in case of alkyl halides (say methyl chloride) carbon is attached to chlorine by a pure single bond. Consequently, C-X bond in aryl halides is little stronger than in alkyl halides, and hence cannot be easily broken.

Also, aryl halides are stabilised by resonance but alkyl halides are not. Consequently, the energy of activation for the displacement of halogen from aryl halides is much greater than that from alkyl halides. Thus, aryl halides are much less reactive than alkyl halides towards nucleophilic substitution reactions.

2. Difference in hybridization of carbon atom in C - X bond : In haloalkanes (e.g. methyl chloride), the halogen is attached to ${sp}^{3}$ hybridized carbon while in haloarenes the halogen is attached to ${sp}^{2}$ hybridized carbon. Since a ${sp}^{2}$ hybridized orbital is smaller as compared to ${sp}^{3}$ orbital of carbon, the C -Cl bond in chlorobenzene should be shorter and hence stronger than in methyl chloride.Thus, difficult to break.

3. Polarity of the carbon-halogen bond. Aryl halides are less polar than akyl halides. The ${sp}^{2}$ hybrid carbon due to greater s-character is more electronegative than a ${sp}^{3}$ hybrid carbon. Therefore, the ${sp}^{2}$ hybrid carbon of C -X bond in aryl halides has less tendency to release electrons to halogen than in ${sp}^{3}$ hybrid carbon in alkyl halides. As a result, the C - X bond in aryl halides is less polar than in alkyl halides.Consequently, the halogen atom present in aryl halides cannot be easily displaced by nucleophiles.

4. Instability of phenyl cation. In haloarenes the phenyl cation formed as a result of self-ionization is not stabilized by resonance because the ${sp}^{2}$ hybridized orbital of carbon having the positive charge is perpendicular to the p-orbitals of the phenyl ring. Therefore, these cations are not formed and aryl halide do not undergo nucleophilic substitution reactions easily.

5. Electronic repulsions: Since arene halides are electron rich molecules due to the presence of pi bonds, they repel the electron rich nucleophiles from attacking them.

The reactivity of aryl halides can be increased by the presence of an electron withdrawing group like nitro group $(- {NO}_{2})$ at ortho and para positions.

Why are aryl halides less reactive towards nucleophilic substitution reactions than alkyl halides? How can the reactivity of aryl halides be enhanced?

Important Questions on Haloarenes (Aryl Halides)

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