Question

Which of the following compounds is most reactive in S$_N$1 reaction?

• A.
• B.
• C.
• D.

Hint:

Using a simple frame or just bolding for the box Key Points:
S$_N$1 reactivity is determined by carbocation stability. Carbocation stability order: 3$\textdegree$>2$\textdegree$>1$\textdegree$>methyl.
Resonance significantly stabilizes carbocations (Allylic, Benzylic).
More extensive resonance leads to greater stability.
Antiaromatic carbocations are extremely unstable.
The heptatrienyl cation (from B) is the most stable due to extensive conjugation.

Answer 1

The Correct Option is B

The rate of S_N1 reactions is primarily determined by the stability of the carbocation intermediate formed after the leaving group (Br^-) departs. A more stable carbocation leads to a faster S_N1 reaction. Let's analyze the carbocation formed from each compound:

• (A) 5-Bromocyclopenta-1,3-diene: Loss of Br^- yields the cyclopentadienyl cation. This cation contains 4 $\pi$ electrons in a cyclic conjugated system, making it antiaromatic according to Hückel's rule (4n $\pi$ electrons). Antiaromatic systems are highly unstable, rendering this compound very unreactive in S_N1.
• (B) 7-Bromohepta-1,3,5-triene: Loss of Br^- forms the heptatrienyl cation (CH₂=CH-CH=CH-CH=CH-CH₂⁺). This is a primary allylic carbocation, but the positive charge is delocalized through resonance across the entire conjugated system of three double bonds (7 carbon atoms). This extensive resonance stabilization significantly stabilizes the carbocation.
• (C) 3-Bromoprop-1-ene (Allyl bromide): Loss of Br^- results in the allyl cation (CH₂=CH-CH₂⁺). This primary allylic carbocation is stabilized by resonance across three carbon atoms and is relatively stable.
• (D) 3-Bromobut-1-ene: Loss of Br^- produces the 1-methylallyl cation (CH₂=CH-CH⁺-CH₃). This secondary allylic carbocation is also stabilized by resonance across three carbon atoms. Secondary allylic cations are generally more stable than primary allylic cations due to the inductive effect of the methyl group.

Comparing carbocation stabilities: The stability generally follows this order: Extended conjugation>Secondary allylic>Primary allylic >> Antiaromatic / Vinylic.

• Carbocation from (B) is highly stabilized by extended resonance across 7 carbons.
• Carbocation from (D) is secondary allylic (resonance across 3 carbons + inductive effect).
• Carbocation from (C) is primary allylic (resonance across 3 carbons).
• Carbocation from (A) is antiaromatic (highly unstable).

Compound (B) forms the most stable carbocation due to the most extensive resonance delocalization, making it the most reactive in an S_N1 reaction.