Give reasons:
a) n-Butyl bromide has a higher boiling point than t-butyl bromide.
b) A racemic mixture is optically inactive.
c) The presence of a nitro group (–NO) at the o/p positions increases the reactivity of haloarenes towards nucleophilic substitution reactions.
a) n-Butyl bromide has a higher boiling point than t-butyl bromide.
b) A racemic mixture is optically inactive.
c) The presence of a nitro group (–NO) at the o/p positions increases the reactivity of haloarenes towards nucleophilic substitution reactions.
Solution
(a) n-Butyl bromide is unbranched: greater surface contact gives stronger van der Waals attractions, hence the higher boiling point; branching in t-butyl bromide makes the molecule compact and lowers these forces.
(b) A racemic mixture holds the d- and l-enantiomers in equal amounts; their rotations are equal in magnitude and opposite in sign, cancelling exactly (external compensation) — net optical rotation zero.
(c) The –NO group at ortho/para positions withdraws electron density from the ring (−I, −M), making the halogen-bearing carbon electrophilic, and — decisively — delocalises the negative charge of the carbanion (Meisenheimer) intermediate onto its own oxygens. The intermediate is stabilised, so nucleophilic substitution becomes easy (e.g. 2,4-dinitrochlorobenzene hydrolyses readily).
(b) A racemic mixture holds the d- and l-enantiomers in equal amounts; their rotations are equal in magnitude and opposite in sign, cancelling exactly (external compensation) — net optical rotation zero.
(c) The –NO group at ortho/para positions withdraws electron density from the ring (−I, −M), making the halogen-bearing carbon electrophilic, and — decisively — delocalises the negative charge of the carbanion (Meisenheimer) intermediate onto its own oxygens. The intermediate is stabilised, so nucleophilic substitution becomes easy (e.g. 2,4-dinitrochlorobenzene hydrolyses readily).
CBSE Class 12 Board · 2015 · Delhi · Subjective