Q10 / 180 · COMEDK 2026 (9 May Shift 2) · Chemistry
With reference to the two statements Assertion and Reason, choose the correct option.
Assertion: The order of reactivity towards S$_N$1 reaction is: C$_6$H$_5$$-$CH$_2$Br > (CH$_3$)$_3$$-$C$-$Br > (CH$_3$)$_2$$-$CH$-$Br.
Reason: Among the given 3 compounds the Benzyl carbocation formed is the most stable while Isopropyl carbocation is the least stable one.
Assertion: The order of reactivity towards S$_N$1 reaction is: C$_6$H$_5$$-$CH$_2$Br > (CH$_3$)$_3$$-$C$-$Br > (CH$_3$)$_2$$-$CH$-$Br.
Reason: Among the given 3 compounds the Benzyl carbocation formed is the most stable while Isopropyl carbocation is the least stable one.
- Correct answerABoth Assertion and Reason are correct and Reason is the correct explanation of Assertion
- BBoth Assertion and Reason are correct but Reason is not the correct explanation of Assertion
- CReason is correct but Assertion is wrong
- DAssertion is correct but Reason is wrong
Solution
The reactivity of alkyl halides towards $\text{S}_{\text{N}}1$ reactions depends on the stability of the intermediate carbocation formed during the rate-determining step.
The carbocations formed from the given alkyl halides are:
1. Benzyl carbocation ($\text{C}_6\text{H}_5\text{CH}_2^+$) from $\text{C}_6\text{H}_5\text{CH}_2\text{Br}$
2. tert-Butyl carbocation ($(\text{CH}_3)_3\text{C}^+$) from $(\text{CH}_3)_3\text{CBr}$
3. Isopropyl carbocation ($(\text{CH}_3)_2\text{CH}^+$) from $(\text{CH}_3)_2\text{CHBr}$
The benzyl carbocation is highly stabilized by the delocalization of the positive charge over the benzene ring through resonance. The tert-butyl carbocation is stabilized by hyperconjugation ($9$ $\alpha$-hydrogens) and the +I effect of three methyl groups. The isopropyl carbocation is stabilized by hyperconjugation ($6$ $\alpha$-hydrogens) and the +I effect of two methyl groups.
The resonance stabilization in the benzyl carbocation makes it the most stable among the three, while the isopropyl carbocation is the least stable.
Since the order of carbocation stability is Benzyl > tert-Butyl > Isopropyl, the order of reactivity towards $\text{S}_{\text{N}}1$ reaction follows the same trend: $\text{C}_6\text{H}_5\text{CH}_2\text{Br} > (\text{CH}_3)_3\text{CBr} > (\text{CH}_3)_2\text{CHBr}$.
Both the Assertion and the Reason are correct, and the Reason correctly explains the Assertion.
Answer: Both Assertion and Reason are correct and Reason is the correct explanation of Assertion
The carbocations formed from the given alkyl halides are:
1. Benzyl carbocation ($\text{C}_6\text{H}_5\text{CH}_2^+$) from $\text{C}_6\text{H}_5\text{CH}_2\text{Br}$
2. tert-Butyl carbocation ($(\text{CH}_3)_3\text{C}^+$) from $(\text{CH}_3)_3\text{CBr}$
3. Isopropyl carbocation ($(\text{CH}_3)_2\text{CH}^+$) from $(\text{CH}_3)_2\text{CHBr}$
The benzyl carbocation is highly stabilized by the delocalization of the positive charge over the benzene ring through resonance. The tert-butyl carbocation is stabilized by hyperconjugation ($9$ $\alpha$-hydrogens) and the +I effect of three methyl groups. The isopropyl carbocation is stabilized by hyperconjugation ($6$ $\alpha$-hydrogens) and the +I effect of two methyl groups.
The resonance stabilization in the benzyl carbocation makes it the most stable among the three, while the isopropyl carbocation is the least stable.
Since the order of carbocation stability is Benzyl > tert-Butyl > Isopropyl, the order of reactivity towards $\text{S}_{\text{N}}1$ reaction follows the same trend: $\text{C}_6\text{H}_5\text{CH}_2\text{Br} > (\text{CH}_3)_3\text{CBr} > (\text{CH}_3)_2\text{CHBr}$.
Both the Assertion and the Reason are correct, and the Reason correctly explains the Assertion.
Answer: Both Assertion and Reason are correct and Reason is the correct explanation of Assertion
Hence, the correct answer is option: 'A'.