Alkyl Halides - Practice Questions & MCQ

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Quick Facts

Reaction with PCl5, PCl3, SOCl2 and HX is considered one the most difficult concept.
Nature of C-X bond and Physical Properties is considered one of the most asked concept.
40 Questions around this concept.

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EASY

The given compound

Is an example of ________

Benzylic halide

Aryl Halide

Allylic halide

Vinylic halide

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Which of the following reaction(s) can be used for the preparation of alkyl halides?

(I) $CH_{3}CH_{2}OH+HCl\overset{anh.ZnCl_{2}}{\rightarrow}$

(II) $CH_{3}CH_{2}OH+HCl \rightarrow$

(III) $(CH_{3})_{3}COH+HCl \rightarrow$

(IV) $(CH_{3})_{2}CHOH+HCl \overset{anh.ZnCl_{2}}{\rightarrow}$

(I), (III) and (IV) only

(I) and (II) only

(IV) only

(III) and (IV) only

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Concepts Covered - 5

Nature of C-X bond and Physical Properties

Halogen atoms are more electronegative than carbon, therefore, carbon-halogen bond of alkyl halide is polarised; the carbon atom bears a partial positive charge whereas the halogen atom bears a partial negative charge. As we go down the group in the periodic table, the size of halogen atom increases. Fluorine atom is the smallest and iodine atom is the largest. Consequently, the carbon-halogen bond length also increases from $\mathrm{C-F}$ to $\mathrm{C-I}$ .

Huns-dicker Reaction

The silver(I) salts of carboxylic acids react with halogens to give unstable intermediates which readily decarboxylate thermally to yield alkyl halides. The reaction is believed to involve homolysis of the C-C bond and a radical chain mechanism. In this reaction, ester is formed as a by-product. The reaction occurs as follows:

$\mathrm{RCOOAg\: +\: X_{2}\: \overset{CCl_{4}}{\longrightarrow}\: R-X\: +\: CO_{2}\: +\: AgX}$

Mechanism

For example:

Reaction with NaCN, AgCN, NaNO2 and AgNO2

Reaction with NaCN: NaCN or KCN is ionic in nature i.e, Na⁺CN^-. Thus, the nucleophile in this case is CN^-. The reaction occurs as follows:
$\mathrm{R-CH_{2}-X\: \overset{NaCN}{\longrightarrow}\: R-CH_{2}-CN\: +\: NaX}$
For example:
$\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Br}+\mathrm{NaCN} \rightarrow \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CN}+\mathrm{NaBr}$
Reaction with NaNO₂: NaNO₂ or KNO₂ is ionic in nature and it breaks into Na⁺ and NO^-₂. Nitrite ion NO^-₂ with structure O=N-O- is an ambient nucleophile with electron pairs on both N & O. NO^-₂ ion having an excess of electrons on O thus, allows it to act as a nucleophile in preference to N. The reaction occurs as follows:
$\mathrm{CH}_{3} \mathrm{Br}+\mathrm{NaNO}_{2} \rightarrow \mathrm{CH}_{3}-\mathrm{NO}_{2}+\mathrm{NaBr}$
For example:
$\mathrm{CH}_{3} \mathrm{Br}+\mathrm{NaNO}_{2}\: \rightarrow \: \mathrm{CH}_{3}-\mathrm{O}-\mathrm{NO}+\mathrm{NaBr}$

Finkelstein and Swartz Reaction

Finkelstein Reaction
Finkelstein's reaction is a method of preparation of alkyl iodides from alkyl chlorides or alkyl bromides. In this reaction, alkyl chlorides or bromides are treated with NaI in the presence of acetone to form alkyl iodides. The reaction occurs as follows:

$\mathrm{R}-\mathrm{X}+\mathrm{Nal}\rightarrow \mathrm{R}-1+\mathrm{NaX}$

We use NaI because it is soluble in acetone as it is covalent in nature. All other sodium halides are ionic in nature and thus not soluble.

For example:

$\mathrm{CH_{2}=CH-CH_{2}-Cl\: \overset{NaI/Acetone}{\longrightarrow}\: CH_{2}=CH-CH_{2}-I}$

Reaction with PCl5, PCl3, SOCl2 and HX

The reaction of alcohols ROH with PCl₅ and PCl₃ yields an alkyl halide RCl. The reactions of alcohols with PCl₅, PCl₃ and SOCl₂ occurs as follows:

$\\\mathrm{PCl_{5}\: \rightarrow \: POCl_{3}\: +\: HCl\: +\: RCl}\\\\\mathrm{PCl_{3}\: \rightarrow \: H_{3}PO_{3}\: +\: HCl\: +\: RCl}\\\\\mathrm{SOCl_{2}\: \rightarrow \: SO_{2}\: +\: HCl\: +\: RCl}$

POCl₃ and H₃PO₃ are generated in liquid phase and hence they are very hard to separate while SO₂ and HCl are gases and thus they are easy to remove. Hence, for chlorination, we always use SOCl₂ as the best option among the given reagents.

Mechanism

The reactions occurs as follows:

$\\\mathrm{RCOOH\: +\: PCl_{3}/PCl_{5}/SOCl_{2}\: \rightarrow \: RCOCl}\\\\\mathrm{R-OH\: +\:PCl_{3}/PCl_{5}/SOCl_{2}\: \rightarrow \: R-Cl}$