Chemistry Class 12 NCERT Solutions: Chapter 10 Haloalkanes and Haloarenes Part 12

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Q: 20. The treatment of alkyl chlorides with aqueous leads to the formation of alcohols but in the presence of alcoholic , alkenes are major products. Explain.

Answer:

In an aqueous solution, almost completely ionizes to give ions. ion is a strong nucleophile, which leads the alkyl chloride to undergo a substitution reaction to form alcohol.

On the other hand, an alcoholic solution of contains alkoxide ion, which is a strong base. Thus, it can abstract a hydrogen from the β-carbon of the alkyl chloride and form an alkene by eliminating a molecule of .

ion is a much weaker base than ion. Also, ion is highly solvated in an aqueous solution and as a result, the basic character of ion decreases. Therefore, it cannot abstract a hydrogen from the carbon.

Q: 21. Primary alkyl halide (a) reacted with alcoholic to give compound (b).Compound (b) is reacted with to give (c) which is an isomer of (a). When (a) is reacted with sodium metal it gives compound (d), which is different from the compound formed when n-butyl bromide is reacted with sodium. Give the structural formula of (a) and write the equations for all the reactions.

Answer:

There are two primary alkyl halides having the formula, . They are bulyl bromide and isobutyl bromide.

Therefore, compound (a) is either n-butyl bromide or isobutyl bromide. Now, compound (a) reacts with metal to give compound (b) of molecular formula, , which is different from the compound formed when bromide reacts with Na metal. Hence, compound (a) must be isobutyl bromide.

Thus, compound (d) is dimethylhexane.

It is given that compound (a) reacts with alcoholic KOH to give compound (b). Hence, compound (b) is 2−methylpropene.

Also, compound (b) reacts with to give compound (c) which is an isomer of (a).

Hence, compound (c) is bromo methylpropane.