The reaction depicts the acid-catalyzed dehydration of 1-phenylethanol (Ph-CH(OH)-CH₃) using H₂SO₄, yielding styrene (Ph-CH=CH₂) as the major product and 1-phenylpropene or a similar alkene as the minor product. This follows an E1 mechanism typical for secondary benzylic alcohols under concentrated acid conditions. The correct pathway is E1 followed by isomerization.

Reaction Mechanism

The dehydration begins with protonation of the hydroxyl group by H₂SO₄, forming a protonated alcohol (Ph-CH(OH₂⁺)-CH₃) where water becomes a good leaving group. Loss of H₂O generates a secondary benzylic carbocation (Ph-CH⁺-CH₃), the rate-determining step, stabilized by resonance with the phenyl ring. A base (e.g., HSO₄⁻ or H₂O) then abstracts a β-hydrogen from the methyl group, forming the more stable trans-styrene (major, trisubstituted-like due to conjugation) via Zaitsev’s rule.

Isomerization occurs as the initial carbocation loses a proton from the benzylic CH to form 1-phenylethene (styrene, conjugated and stable), but minor deprotonation from the phenyl-adjacent carbon yields a less stable alkene (e.g., Ph-C(CH₃)=CH₂, α-methylstyrene).

Option Analysis

(A) E2 followed by isomerization: Incorrect. E2 requires a strong base and anti-periplanar geometry; concentrated H₂SO₄ favors carbocation (E1) over concerted elimination for secondary alcohols.

(B) E1 followed by isomerization: Correct. Matches the stepwise carbocation formation, elimination to styrene (major), and carbocation rearrangement/isomerization yielding minor product.

(C) SN1 followed by isomerization: Incorrect. SN1 would form substitution products (e.g., ethers), not alkenes; dehydration prioritizes elimination.

(D) Isomerization through carbocation: Partially true but incomplete; lacks the initial E1 elimination step defining the pathway.

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Introduction to Acid Catalyzed Dehydration

Acid catalyzed dehydration of 1-phenylethanol to styrene using H₂SO₄ is a classic E1 elimination reaction forming alkenes from secondary alcohols. The major product styrene arises from stable benzylic carbocation, while minor products form via isomerization—key for CSIR NET organic chemistry.

Detailed E1 Mechanism Steps

Protonation converts OH to H₂O leaving group, followed by carbocation formation (Ph-CH⁺-CH₃). Elimination of β-H yields conjugated styrene (major); carbocation hydride shift enables minor alkene. No E2 due to weak base conditions.

• Stable benzylic carbocation drives E1 over E2.
• Zaitsev product (styrene) dominates.
• Rearrangement explains minor yields.

CSIR NET Exam Insights

For competitive exams, recognize secondary benzylic alcohols follow E1 with H₂SO₄, not E2 (primary) or SN1. Option B (E1 + isomerization) is correct per mechanism.