JEE Challenger
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Statements on Hydrohalogenation and Preparation of Aryl Compounds

Given below are two statements:

Statement I: 3-phenylpropene reacts with HBr\text{HBr} and gives secondary alkyl bromide having a chiral carbon atom as the major product.

Statement II: Aryl chlorides and aryl cyanides can be prepared by Sandmeyer reaction as well as Gattermann reaction.

In the light of the above statements, choose the correct answer from the options given below

Options

A

Both Statement I and Statement II are true

B

Both Statement I and Statement II are false

C

Statement I is true but Statement II is false

Correct
D

Statement I is false but Statement II is true

Topics & Concepts

Step-by-Step Solution

To determine the correctness of both statements, let us analyze them step-by-step:


Evaluation of Statement I:

  1. Structure of Reactant: 3-phenylpropene has the molecular formula C6H5-CH2-CH=CH2\text{C}_6\text{H}_5\text{-CH}_2\text{-CH=CH}_2.

  2. Electrophilic Addition Mechanism (HBr\text{HBr}):

    • Step 1: Protonation The double bond undergoes electrophilic addition with H+\text{H}^+ to form a secondary carbocation: C6H5-CH2-CH=CH2+H+C6H5-CH2-C+H-CH3\text{C}_6\text{H}_5\text{-CH}_2\text{-CH=CH}_2 + \text{H}^+ \longrightarrow \text{C}_6\text{H}_5\text{-CH}_2\text{-}\overset{+}{\text{C}}\text{H-CH}_3

    • Step 2: Rearrangement via 1,2-Hydride Shift The secondary carbocation rearranges via a 1,21,2-hydride shift to form a more stable benzylic carbocation, which is stabilized by resonance with the aromatic ring: C6H5-CH2-C+H-CH31,2-H shiftC6H5-C+H-CH2-CH3\text{C}_6\text{H}_5\text{-CH}_2\text{-}\overset{+}{\text{C}}\text{H-CH}_3 \xrightarrow{1,2\text{-H shift}} \text{C}_6\text{H}_5\text{-}\overset{+}{\text{C}}\text{H-CH}_2\text{-CH}_3

    • Step 3: Nucleophilic Attack by Br\text{Br}^- The bromide ion attacks the benzylic carbocation to form the major product: C6H5-C+H-CH2-CH3+BrC6H5-CH(Br)-CH2-CH3\text{C}_6\text{H}_5\text{-}\overset{+}{\text{C}}\text{H-CH}_2\text{-CH}_3 + \text{Br}^- \longrightarrow \text{C}_6\text{H}_5\text{-CH(Br)-CH}_2\text{-CH}_3

  3. Product Characteristics:

    • The major product is 1-bromo-1-phenylpropane.
    • The carbon attached to the bromine atom is bonded to two other carbon atoms (the benzylic ring carbon and the ethyl group carbon), making it a secondary alkyl bromide.
    • The carbon atom (C\text{C}^*) in C6H5-CH(Br)-CH2CH3\text{C}_6\text{H}_5\text{-}\text{C}^*\text{H(Br)-CH}_2\text{CH}_3 is bonded to four distinctly different groups: H-\text{H}, Br-\text{Br}, C6H5-\text{C}_6\text{H}_5, and CH2CH3-\text{CH}_2\text{CH}_3. Hence, it is a chiral carbon atom.

Therefore, Statement I is true.


Evaluation of Statement II:

  1. Sandmeyer Reaction: Benzenediazonium chloride reacts with cuprous salts like Cu2Cl2/HCl\text{Cu}_2\text{Cl}_2/\text{HCl}, Cu2Br2/HBr\text{Cu}_2\text{Br}_2/\text{HBr}, and CuCN/KCN\text{CuCN}/\text{KCN} to give aryl chlorides (Ar-Cl\text{Ar-Cl}), aryl bromides (Ar-Br\text{Ar-Br}), and aryl cyanides (Ar-CN\text{Ar-CN}), respectively.

  2. Gattermann Reaction: In the Gattermann reaction, copper powder in the presence of the corresponding halogen acid (Cu/HCl\text{Cu}/\text{HCl} or Cu/HBr\text{Cu}/\text{HBr}) is used to prepare aryl chlorides and aryl bromides: Ar-N2+XCu/HClAr-Cl+N2+CuX\text{Ar-N}_2^+\text{X}^- \xrightarrow{\text{Cu/HCl}} \text{Ar-Cl} + \text{N}_2 + \text{CuX} However, aryl cyanides (Ar-CN\text{Ar-CN}) cannot be prepared by the Gattermann reaction.

Therefore, Statement II is false.


Conclusion:

  • Statement I is true
  • Statement II is false

This corresponds to Option C.

Statements on Hydrohalogenation and Preparation of Aryl Compounds | Chemistry PYQ Solution - JEE Challenger