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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.

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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).

Due to high electronegativity of halogen atom (say chlorine atom), the —C—X (or Cl—Cl) bond in alkyl halide (or methyl chloride) is slightly polar and carbon atom of—C—X (or —C—Cl) bond acquires a slight positive charge. Thus

The C^δ+ atom is easily attacked by nucleophile (electron-rich species), such as OH^–. Thus hydrolysis takes place easily.

On the other hand, aryl halide (or chlorobenzene) is resonance stabilized, so the C—Cl bond has a partial double character. Consequently, the rupture of this partial double bond becomes rather difficult. Hence, aryl halide (or chlorobenzene) is less reactive than alkyl halide (or chloromethane). Moreover, the C—Cl bond in aryl halide has a sp²hybridized C-atom, while it is sp³ hybridized in alkyl halide (or chloromethane). Since sp³ hybrid C-atom is more electronegative than the sp³ hybrid C-atom, so C–Cl bond in aryl halide (or chlorobenzene) has less tendency to release electrons to Cl-atom. In other words, the C—Cl bond in aryl halide (or chlorobenzene) is less polar than in alkyl halide (or chloromethane). This makes nucleophilic substitution (say by OH^– group) more difficult. Hence, 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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