Organic Compounds Containing Oxygen: Chemistry | JEE Main
Major product 'P' formed in the following reaction is :-
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Hint 1 of 2
What type of reaction occurs when the haloalkane is treated with the base under heating?
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Step-by-step solutionView
Correct answer
The reaction undergoes an E2 elimination where the base abstracts a proton to form the more substituted, thermodynamically stable alkene (Zaitsev product).
Option analysis
Why each option works or fails
A ·
Believing the reaction selectively yields the less substituted Hofmann elimination product due to steric hindrance. Identify the base and leaving group; with an unhindered base and a standard halogen leaving group, elimination follows Zaitsev's rule to give the more substituted alkene.
B ·
None. This correctly identifies the most stable, highly substituted alkene formed via Zaitsev E2 elimination. Correctly identify the most substituted alkene having the maximal number of hyperconjugative α-hydrogens.
C ·
Assuming an alternative, less-substituted double bond isomer forms as the major product. Compare the number of alkyl substituents on the double bond; the major product is the one with higher substitution and greater thermodynamic stability.
D ·
Selecting a regioisomer resulting from elimination toward the less substituted carbon instead of the more substituted internal position. Check all adjacent β-hydrogens and select the β-hydrogen whose removal generates the alkene with the greatest thermodynamic stability.
Reviewed route
Solution
StepWorking
01identify
The reaction is an acid-catalyzed dehydration of an alcohol via an E1 mechanism. First, the hydroxyl group is protonated. Next, a water molecule leaves to form a carbocation. This carbocation can undergo rearrangement. Finally, elimination (deprotonation) gives the most stable thermodynamic product.
02mechanism
Step 1: The −OH group at the benzylic/allylic position is protonated by H+ from H2SO4 to form −OH2+.
Step 2: Loss of H2O creates a carbocation intermediate directly adjacent to the phenyl group and the cyclopentene ring.
Step 3: Loss of a β-proton (from the adjacent CH2 of the cyclopentene ring) forms a conjugated diene system. This diene is directly conjugated with the benzene ring. It extends the resonance conjugation throughout the entire π-system.
03product
The double bond forms in the cyclopentenyl ring. It is in conjugation with both the phenyl ring and the existing endocyclic double bond. This gives the fully conjugated, most stable diene (Option 1).
✓verify
Verify that the double bonds form a continuous conjugated π-system with the phenyl ring. Option 1 features full conjugation between the benzene ring, the bridgehead exocyclic-endocyclic diene system, providing maximum thermodynamic stability.
Hints that build this answer step by step
What type of reaction occurs when the haloalkane is treated with the base under heating?
An E2 elimination reaction yielding an alkene
Which regiochemical rule dictates the major alkene product under these standard elimination conditions?
Zaitsev's rule, producing the more substituted and thermodynamically stable alkene
Why doesn't elimination happen to form an isolated double bond?
Under heating with sulfuric acid (extH2extSO4,Δ), the reaction is thermodynamically controlled. Fully conjugated polyene systems have significantly lower energy due to extended resonance delocalization.
