Question Details

Consider the following reaction scheme and choose the correct option(s) for the major products Q, R, and S.


Options

A

B

C

D

Show Answer

Correct Answer :

Option B

Solution :

The correct option showing the major products Q, R, and S is:


Step-by-Step Reaction Mechanism:


1. Formation of Intermediate P (Hydroboration-Oxidation of Styrene):

Styrene, C6H5CH=CH2, undergoes hydroboration-oxidation using (i) B2H6 followed by (ii) NaOH,H2O2,H2O. This reaction follows anti-Markovnikov addition of water across the double bond to yield 2-phenylethan-1-ol (or 2-phenylethanol):

P=C6H5-CH2-CH2OH


2. Formation of Product Q:

Intermediate P (C6H5CH2CH2OH) undergoes a sequence of three reactions:

(i) Oxidation with Jones reagent (CrO3/H2SO4): Primary alcohol P is oxidized to 2-phenylacetic acid (C6H5CH2COOH).

(ii) Hell-Volhard-Zelinsky (HVZ) Reaction (Cl2, Red phosphorus): Chlorination takes place specifically at the α-position of the carboxylic acid to form 2-chloro-2-phenylacetyl chloride intermediate.

(iii) Hydrolysis (H2O): The acid chloride group hydrolyzes back to the carboxylic acid, producing 2-chloro-2-phenylacetic acid:

Q=C6H5-CH(Cl)-COOH


3. Formation of Product R:

Starting again from P (C6H5CH2CH2OH):

(i) Treatment with thionyl chloride (SOCl2): Replaces the -OH group with a chlorine atom to form (2-chloroethyl)benzene (C6H5CH2CH2Cl).

(ii) Nucleophilic substitution with NaCN: Cyanide replaces the chloride ion to yield 3-phenylpropanenitrile (C6H5CH2CH2CN).

(iii) Acidic hydrolysis with heating (H3O+,Δ): Hydrolyzes the nitrile group completely to a carboxylic acid, forming 3-phenylpropanoic acid:

R=C6H5-CH2-CH2-COOH


4. Formation of Product S:

Compound R (C6H5CH2CH2COOH) is treated with concentrated sulfuric acid (conc. H2SO4). Under these strong acid conditions, intramolecular Friedel-Crafts acylation takes place, where the acylium ion attacks the benzene ring to form a five-membered cyclic ketone ring (1-indanone):

S=1-indanone


Therefore, the correct major products Q, R, and S match the second option provided in the question image.

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