Organic Compounds Containing Oxygen: Chemistry | JEE Main
The major product for the following reaction is:
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Hint 1 of 3
What fundamental reaction pathway is taking place between the substrate and the base?
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Step-by-step solutionView
Correct answer
The major product is formed by base-induced elimination that yields the most stable, conjugated diene system.
Option analysis
Why each option works or fails
A ·
None. This is the correct product. Elimination occurs toward the existing double bond to yield a fully conjugated, thermodynamically stable diene.
B ·
Believing that elimination occurs to give an isolated, non-conjugated diene or following an incorrect kinetic proton-abstraction pathway. Check whether alternative elimination pathways can establish conjugation; conjugated dienes are significantly more stable than isolated dienes.
C ·
Failing to recognize that allylic stabilization and conjugation dictate the major product over an isolated alkene. Evaluate the stability of the final π-system; forming a conjugated diene is strongly favored over an isolated double bond.
D ·
Selecting the less substituted or non-conjugated alkene isomer by assuming kinetic control dominates without considering thermodynamic conjugation. Compare the thermodynamic stability of potential elimination products; conjugation provides substantial resonance stabilization, directing regioselectivity.
Reviewed route
Solution
StepWorking
01identify
The reaction is an acid-catalyzed dehydration of a secondary allylic alcohol with concentrated H2SO4 under heating (E1 pathway).
02mechanism
Protonation of the hydroxyl group by H+ gives an alkyloxonium ion. This ion then loses a water molecule to form a secondary allylic carbocation adjacent to a cyclopentyl ring.
03mechanism
A 1,2-hydride shift occurs from the tertiary carbon of the cyclopentyl ring to the carbocation center. This yields a more stable tertiary carbocation directly attached to the conjugated double bond.
04mechanism
A proton is eliminated from the adjacent CH2 group of the cyclopentyl ring. This generates an endocyclic double bond. It is conjugated with the existing exocyclic/chain double bond, forming a conjugated diene system.
05product
The final major product is 1-(prop-1-en-1-yl)cyclopent-1-ene (Option 0).
✓verify
The conjugated diene system possesses extended resonance stabilization and more highly substituted alkene units compared to unconjugated or less substituted alternatives.
Hints that build this answer step by step
What fundamental reaction pathway is taking place between the substrate and the base?
Base-promoted β-elimination yielding a diene product
Which factor primarily determines the regiochemical outcome of the newly formed double bond?
Formation of a conjugated diene system rather than an isolated diene
Which structure corresponds to the thermodynamically favored conjugated diene?
The structure with alternating double bonds (conjugated diene) shown in option 1
Why does a 1,2-hydride shift occur instead of ring expansion?
The ring is already a relatively stable 5-membered ring (cyclopentyl). A 1,2-hydride shift immediately forms a very stable tertiary allylic carbocation. This pathway is kinetically and thermodynamically favorable.
