Question

Which of the following is more reactive towards \(S_N1\) reaction: 2-bromo-2-methylbutane or 1-bromopentane?

Hint:

SN1 reactivity: 3°>2°>1° due to carbocation stability.

Answer 1

Step 1: Understanding the Mechanism of \(S_N1\) Reaction.
The \(S_N1\) (Substitution Nucleophilic Unimolecular) reaction occurs in two steps: first, the leaving group departs, forming a carbocation, and then the nucleophile attacks the carbocation. The rate-determining step is the formation of the carbocation, so the stability of the carbocation plays a crucial role in the reactivity of the substrate.

Step 2: The Role of Carbocation Stability in \(S_N1\) Reactions.
For an \(S_N1\) reaction to occur efficiently, the intermediate carbocation must be stable. The more stable the carbocation, the more likely it is to form, and therefore, the more reactive the substrate is. Carbocation stability increases in the following order: primary < secondary < tertiary. Additionally, the presence of alkyl groups or electron-donating groups near the carbocation center can further stabilize it.

Step 3: Comparing 2-bromo-2-methylbutane and 1-bromopentane.
- **2-bromo-2-methylbutane**: This compound features a tertiary carbocation. Upon departure of the bromide ion, a tertiary carbocation (C⁺) is formed, which is highly stabilized by the inductive and hyperconjugation effects of the surrounding alkyl groups (3 methyl groups attached to the central carbon). This makes the carbocation intermediate very stable and hence more likely to form, making 2-bromo-2-methylbutane highly reactive towards \(S_N1\) reactions.
- **1-bromopentane**: This compound forms a primary carbocation upon leaving group departure, which is relatively unstable due to its limited alkyl stabilization. As a result, the \(S_N1\) reaction is much less favorable for 1-bromopentane compared to 2-bromo-2-methylbutane.

Step 4: Conclusion.
Therefore, **2-bromo-2-methylbutane** is more reactive towards an \(S_N1\) reaction compared to 1-bromopentane, because the former forms a more stable tertiary carbocation, while the latter forms a less stable primary carbocation.