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Alkyl Halides - Practice Questions & MCQ

Edited By admin | Updated on Sep 18, 2023 18:35 AM | #JEE Main

Quick Facts

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

  • 47 Questions around this concept.

Solve by difficulty

For the reaction

\mathrm{RCH}_2 \mathrm{Br}+\mathrm{I}^{-} \stackrel{\text { Acetone }}{\longrightarrow} \underset{\text { major }}{\mathrm{RCH}_2 \mathrm{l}}+\mathrm{Br}^{-}
The correct statement is

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}.

                        

Physical properties of Haloalkanes and Haloarenes:

(1) These are less soluble in \mathrm{H_2O} but more soluble in Organic solvents

(2) Their density follows the order: 

    Iodide > Bromide > Fluoride > Chloride

(3) Their boiling point follows the order: 

    Iodide > Bromide > Fluoride > Chloride

(4) Boiling point of Isomeric haloalkanes decreases with the increase in branching

(5) Boiling point of isomeric dihalobenzene is nearly the same and follows the order

    para > ortho > meta

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}

For example: 

\mathrm{CH _{3} COOAg + Br _{2} \stackrel {CCl_4}\longrightarrow CH _{3} COOBr + AgBr}

 

Mechanism

Chain Initiation

\mathrm{CH _{3} COOBr \stackrel{ RDS }{\longrightarrow} CH _{3} COO ^{\bullet}+ Br^{\bullet}}

Chain Propagation

\mathrm{CH _{3} COO^{\bullet} \longrightarrow \ ^{\bullet}CH _{3}+ CO _{2}}

\mathrm{^{\bullet}\text {CH}_{3}+ CH_{3} COOBr \longrightarrow CH _{3} Br + CH_3 COO ^{\bullet}}

Chain Termination

\mathrm{^{\bullet}CH _{3} + Br^{\bullet} \longrightarrow CH_3Br} (major)

\mathrm{CH _{3} COO ^{\bullet} + ^{\bullet}CH_3 \longrightarrow CH_3COOCH_3} (minor)

\mathrm{CH _{3} COO ^{\bullet} + Br^{\bullet} \longrightarrow CH_3COOBr}(minor) 

 

For example:

The rate of reaction with changing the alkyl group (R) in the above reaction varies as 

1 \degree > 2 \degree > 3\degree

It is to be noted that with \mathrm{I_2}, silver salt of carboxylic acid gives ester as the main product instead of alkyl iodide.

\mathrm{2 RCOOAg + I _{2} \stackrel{ CCl _{4}}{\longrightarrow} RCOOR + CO _{2} +2 AgI}

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 AgCN: AgCN is covalent in nature and as a result, the C atom is covalently bonded to Ag. Thus, the attack of the nucleophile in this case will take place from the Lone pairs over N atom

            \mathrm{R-CH_{2}-X\: \overset{AgCN}{\longrightarrow}\: R-CH_{2}-NC\: +\: AgX}

For example

        \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Br}+\mathrm{AgCN} \rightarrow \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{NC}+\mathrm{AgBr}

 

  • 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{O}-\mathrm{NO}+\mathrm{NaBr}

  • Reaction with AgNO2: AgNO2 is covalent in nature and it does breaks into Ag+ and NO-2 ions. The O atom remains bonded to the Ag and hence the attack of the nucleophile takes place by the lone pairs over Nitrogen.

The reaction occurs as follows

\mathrm{CH}_{3} \mathrm{Br}+\mathrm{AgNO}_{2}\: \rightarrow \: \mathrm{CH}_{3}-\mathrm{NO_2}+\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{\mathrm{R}-\mathrm{X}+\mathrm{NaI}\rightarrow \mathrm{R}-I+\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}

Swarts Reaction

Halide exchange is also used for the preparation of alkyl fluorides by Swarts Reaction. Alkyl chloride/bromide is heated in presence of AgF, Hg2F2, CoF2 or SbF3 to give alkyl fluoride.

For example:

    \mathrm{CH _{3}- Br + AgF \longrightarrow CH _{3}- F + AgBr}

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}

Study it with Videos

Nature of C-X bond and Physical Properties
Huns-dicker Reaction
Reaction with NaCN, AgCN, NaNO2 and AgNO2

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