Question

The main difference in ${\text{C - X}}$ bond of a haloalkane and a haloarene is:
A. ${\text{C - X}}$ bond in haloalkanes is shorter than haloarenes.
B. In haloalkanes, the ${\text{C}}$ attached to halogen in ${\text{C - X}}$ bond is ${\text{s}}{{\text{p}}^3}$ hybridized while in haloarenes it is ${\text{s}}{{\text{p}}^2}$ hybridized.
C. ${\text{C - X}}$ bond in haloarenes acquire a double bond character due to the higher electronegativity of ${\text{X}}$ than haloalkanes.
D. Haloalkanes are less reactive than haloarenes due to difficulty in ${\text{C - X}}$ cleavage in haloalkanes.

Answer 1

Hint: Haloalkanes are the derivatives of alkanes where one or more hydrogen atoms are substituted with halogen atoms. Homologous series of haloalkanes is ${{\text{C}}_{\text{n}}}{{\text{H}}_{2{\text{n}} + 1}}{\text{X}}$, where ${\text{X}}$ is any halogen. Haloarenes are also known as aryl halides.

Complete step by step solution:
Most of the reactions of halides involve the cleavage of ${\text{C - X}}$ bond. Larger the bond dissociation energy of the ${\text{C - X}}$ bond, smaller is the reactivity. The bond dissociation energy decreases with the increase in the size of halogen atom, i.e. ${\text{C - Cl > C - Br > C - I}}$. Hence the reactivity increases from alkyl chlorides to bromides and from bromides to iodides. In the halo derivatives, the halogen atom is more electronegative than carbon, the bond between carbon and halogen is polar in character.
But in haloarenes, polarity is very low between ${\text{C - X}}$ bond than that in haloalkanes. Thus the bond is shorter and harder to remove. This is because when an unhybridized p orbital in chlorine interacts with the p orbital in a benzene ring, it will cause a drift of electrons toward the carbon atom in the benzene ring. Moreover, the carbon in haloalkanes is ${\text{s}}{{\text{p}}^3}$ hybridized while that in haloarenes is ${\text{s}}{{\text{p}}^2}$ hybridized. This halogen is more tight in haloarenes than that in haloalkanes.

Hence the correct option is B.

Note: Haloarenes are less reactive than haloalkanes. Also they are more stable. Haloarenes have partial double bond character. It is also resonance stabilized. In aryl halides, due to the lone pair of halogens, ${\text{C - X}}$ bond has double bond character while in alkyl halides, ${\text{C - X}}$ is weak. Also the aromaticity of benzene is a primary reason.