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An organic compound ‘A’ having the molecular formula C₄H₈ on treatment with dilute sulphuric acid give another compound ‘B’. B on treatment with conc. HCl and anhydrous zinc chloride gives ‘C’. C on treatment with sodium ethoxide gives back ‘A’. Identify the compound. Write the equations involved.

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292.
The C—X bond in aryl halides is shorter in comparison to alkyl halides. Explain.

The electron pair present on the halogen atom resonates with the benzene ring and the following contributing structures for aryl halide are possible.

Although the charged structures (I), (II) and (III) have only little contribution towards the hybrid, yet carbon atom joined to halogen atom (X) acquires partial double bond (or sp²hybrid) character. Consequently, there is some shortening of the C—X bond length (1.69 Å) in aryl halide than in alkyl halide (1.82 Å), where there is only sp³-hybridisation and no resonance.

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Long Answer Type

293. Explain why an alkyl halide (or methyl chloride) is hydrolysed more readily than aryl halide (or chlorobenzene).

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Short Answer Type

294.

Explain giving reason although haloalkanes are polar in character, yet they are insoluble in water.

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295.

Predict all the alkenes that would be formed by dehydrohalogenation of the following halides with sodium ethoxide in ethanol and identify the major alkene.
(i)    1-Bromo-1-methyl cyclohexane.
(ii)    2-Chloro-2-methyl butane.
(iii)    2, 2, 3-Trimethyl-3-bromopentane.

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296. Write the mechanism of the following reaction:

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297. Arrange the compounds of each set in order of reactivity towards S_N2 displacement.

2-Bromo-2-methylbutane, 1-Bromopentane, 2-Bromopentane.

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298. Arrange the compounds of each set in order of reactivity towards S_N2 displacement.

1-Bromo-3-methyl butane, 2-Bromo-2-methylbutane, 2-Bromo-3-methylbutane.

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299. Arrange the compounds of each set in order of reactivity towards S_N2 displacement.
1-Bromobutane, 1-Bromo-2, 2-dimethylpropane, 1-Bromo-2-methylbutane, 1-Bromo-3-methylbutane.

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Long Answer Type

300. Primary alkyl halide C₄H₉Br (a) reacted with alcoholic KOH to give compound (b). Compound (b) is treated with HBr to give (c) which is an isomer of (a). When (a) is reacted with sodium metal it gives a compound (d), C₈H₁₈ that was different from the compound when n-butyl bromide was reacted with sodium. Give the structural formula of (a) and write the equations for all the reactions.

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