AIIMS 2006 Chemistry Alkene Synthesis MCQ Question

To convert the compound C₆H₅CH₂CH₃ (which is phenylpropane) into C₆H₅CH=CH₂ (which is styrene), we need to eliminate one hydrogen atom from the carbon chain adjacent to the phenyl group and form a double bond. The correct answer to the question is Option B: SO₂Cl₂; alc. KOH. Let's break down why this is the case and clarify the other options.

Explanation of the Correct Answer (Option B)

1. Reagents Used:

   • SO₂Cl₂ (Thionyl chloride): This reagent will convert the alcohol (if any) or alkyl group into a chlorinated product, facilitating the formation of a double bond via elimination.
   • alc. KOH (Alcoholic Potassium Hydroxide): This is a strong base that promotes elimination reactions, specifically dehydrohalogenation, where a hydrogen halide is removed from the molecule to form a double bond.

2. Reaction Mechanism:

   • The thionyl chloride will first convert the primary alcohol (if it were present) or a suitable precursor into a corresponding alkyl chloride. In our case, if we start with C₆H₅CH₂CH₃ and treat it with SO₂Cl₂, we can obtain C₆H₅CHClCH₃.
   • Next, the alcoholic KOH acts as a base, eliminating the HCl formed during the reaction. The elimination would yield C₆H₅CH=CH₂ (styrene) through a process known as dehydrohalogenation.

Clarification of Incorrect Options

Option A: SOCl₂; H₂O

• Why Incorrect: SOCl₂ will also convert an alcohol to a chloride. However, the presence of water suggests that we are not performing an elimination reaction but rather a substitution reaction. The water will tend to hydrolyze any chlorinated intermediate, leading to the formation of alcohol rather than a double bond.

Option C: Cl₂/hν; H₂O

• Why Incorrect: The use of Cl₂ under UV light (hν) indicates a free radical halogenation process, which would add a chlorine atom to the alkane chain. The presence of water would further lead to hydrolysis and substitution reactions, not forming a double bond. Thus, it does not facilitate the desired conversion to styrene.

Option D: SOCl₂; alc. KOH

• Why Incorrect: While SOCl₂ can convert the alcohol to a chloride, the use of aqueous KOH would lead to hydrolysis, promoting substitution reactions rather than elimination. The alcoholic KOH is crucial for dehydrohalogenation to form the alkene, but in this option, the presence of SOCl₂ with an aqueous base does not favor the formation of the double bond.

Summary

To summarize, the conversion of C₆H₅CH₂CH₃ to C₆H₅CH=CH₂ can effectively be achieved using thionyl chloride followed by alcoholic KOH. The correct sequence of reactions utilizes elimination mechanisms to generate the desired alkene. The other options fail due to substitution or hydrolysis pathways that do not lead to the formation of the double bond necessary for styrene synthesis.

Thus, the correct answer is Option B: SO₂Cl₂; alc. KOH.