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Chemistry · 2025 · 3 marks
Haloalkanes and HaloarenesChemical Reactions of Haloalkanes and Haloarenes3 marksUnderstandshort answer
CBSE 2025 · Region 5 · Set 3 · Q25
Account for the following: (a) Haloarenes are less reactive towards nucleophilic substitution reactions. (b) p -dichlorobenzene has higher melting…
Account for the following :
(a)
Haloarenes are less reactive towards nucleophilic substitution reactions.
(b)
$\displaystyle p$-dichlorobenzene has higher melting point than ortho and meta isomers.
(c)
Tertiary alkyl halides are least reactive towards $\displaystyle \mathrm{S}_{\mathrm{N}} 2$ reaction.
Marking-scheme solution
(a)
Due to resonance in haloarene leading to partial double bond character of C-X bond
/sp2 hybridised carbon atom leading to shorter C-X bond length in haloarene /
instability of phenyl carbocation/Electron-rich benzene ring repels nucleophile.
(b)
Due to symmetry of p-dichlorobenzene it fits better in the crystal lattice.
(c)
Tertiary carbon atom is more sterically hindered for the attack of nucleophile.
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CBSE Class 12 Chemistry past-paper question from the 2025 board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.