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 reactivity in S_N1 reactions depends primarily on the stability of the carbocation intermediate formed after the leaving group (Br^-) departs. The more stable the carbocation, the faster the S_N1 reaction. Let's examine the carbocation formed from each compound:

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

Comparing the stability of the carbocations: The stability generally follows the order: Extended conjugation>Secondary allylic>Primary allylic >> Antiaromatic / Vinylic.

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

The most stable carbocation is formed from compound (B) due to the most extensive resonance delocalization. Therefore, compound (B) is the most reactive in S_N1 reaction.