Give reasons:
i) The C–Cl bond length in chlorobenzene is shorter than the C–Cl bond length in CH–Cl.
ii) The dipole moment of chlorobenzene is lower than that of cyclohexyl chloride.
iii) S1 reactions are accompanied by racemisation in optically active alkyl halides.
i) The C–Cl bond length in chlorobenzene is shorter than the C–Cl bond length in CH–Cl.
ii) The dipole moment of chlorobenzene is lower than that of cyclohexyl chloride.
iii) S1 reactions are accompanied by racemisation in optically active alkyl halides.
Solution
(i) In chlorobenzene the Cl lone pair conjugates with the ring, giving the C–Cl bond partial double-bond character; moreover the ring carbon is sp (more s-character, more electronegative), pulling Cl in — so the bond is shorter than the pure single C(sp)–Cl bond of CHCl.
(ii) Two effects cut the moment of chlorobenzene: the sp carbon's higher electronegativity reduces the C–Cl polarity, and resonance pushes electron density from Cl back into the ring, partly opposing the σ-polarisation. Cyclohexyl chloride has the full C(sp)–Cl polarity, hence the larger dipole moment.
(iii) In S1 the chiral carbon becomes a \textbf{planar} sp carbocation; the nucleophile then bonds from either face with nearly equal probability, producing both enantiomers — the product is (largely) racemised.
(ii) Two effects cut the moment of chlorobenzene: the sp carbon's higher electronegativity reduces the C–Cl polarity, and resonance pushes electron density from Cl back into the ring, partly opposing the σ-polarisation. Cyclohexyl chloride has the full C(sp)–Cl polarity, hence the larger dipole moment.
(iii) In S1 the chiral carbon becomes a \textbf{planar} sp carbocation; the nucleophile then bonds from either face with nearly equal probability, producing both enantiomers — the product is (largely) racemised.
CBSE Class 12 Board · 2016 · Delhi · Subjective