Haloalkanes and Haloarenes

Haloalkanes and Haloarenes

Question

Given reasons:
(i)C–Cl bond length in chlorobenzene is shorter than C–Cl bond length in CH3–Cl.
(ii)The dipole moment of chlorobenzene is lower than that of cyclohexyl chloride.
(iii)SN1 reactions are accompanied by racemization in optically active alkyl halides.

Answer

(i) The carbon in C-Cl bond in chlorobenzene is sp2 hybridised, while in CH3-Cl is sp3 hybridised. In sp2 hybrid orbitals have more of s -character and hence the carbon if chlorobenzene withdraws the electron pair between C-Cl with greater force. As a result, C-Cl bond is shorter than CH3-Cl.

(ii)Reasons behind the dipole moment of chlorobenzene being lower than that of cyclohexyl chloride are followed:

(a)In chlorobenzene, the C−Cl bond is shorter due to the partial double bond character.
(b)Hybridisations of C in C−Cl in chlorobenzene and cyclohexyl chloride are sp2 and sp3, respectively (sp2 C being more electronegative than sp3 C).
As dipole moment is the product of charge and bond distance, both the factors in chlorobenzene are lower as compared to cyclohexyl chloride. Therefore, the dipole moment of chlorobenzene is lower than that of cyclohexyl chloride.

(iii)In SN1 reaction, the formation of carbocation as an intermediate takes place. This carbocation has sp2-hybridised and planar structure. This planar carbocation is attacked by nucleophile from both the sides equally to form d and l isomers in equal proportion. Such products are called racemic mixture. Hence, SN1 reactions are accompanied by racemisation in optically active alkyl halides.

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