CBSE 2023 · Region 3 · Set 1 · Q31 · 4 marks

The polarity of C - X bond of alkyl halides is responsible for their nucleophilic substitution, elimination and their reaction with metal atoms to form organometallic compounds. Alkyl halides are prepared by the free radical halogenation of alkanes, addition of halogen acids to alkenes, replacement of - OH group of alcohols with halogens using phosphorus halides, thionyl chloride or halogen acids. Aryl halides are prepared by electrophilic substitution of arenes. Nucleophilic substitution reactions are categorised into $\displaystyle \mathrm{S}_{\mathrm{N}}{ }^{1}$ and $\displaystyle \mathrm{S}_{\mathrm{N}}{ }^{2}$ on the basis of their kinetic properties. Chirality has a profound role in understanding the $\displaystyle \mathrm{S}_{\mathrm{N}}{ }^{1}$ and $\displaystyle \mathrm{S}_{\mathrm{N}}{ }^{2}$ mechanism. Answer the following questions:

(i)

What happens when bromobenzene is treated with Mg in the presence of dry ether?

(ii)

Which compound in each of the following pairs will react faster in $\displaystyle \mathrm{S}_{\mathrm{N}}{ }^{1}$ reaction with $\displaystyle \mathrm{OH}^{-}$?

(1)

$\displaystyle \mathrm{CH}_{2}=\mathrm{CH}-\mathrm{CH}_{2}-\mathrm{Cl}$ or $\displaystyle \mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{Cl}$

(2)

$\displaystyle \left(\mathrm{CH}_{3}\right)_{3} \mathrm{C}-\mathrm{Cl}$ or $\displaystyle \mathrm{CH}_{3} \mathrm{Cl}$

(iii)OR(iii)

Write the equations for the preparation of $\displaystyle 1$-iodobutane from

(1)

$\displaystyle 1$-chlorobutane

(2)

but-$\displaystyle 1$-ene.

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CBSE Class 12 Chemistry past-paper question from the 2023board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.