Question

In the following pairs of halogen compounds, which compound undergoes SN¹ reaction?

• A.
• B.
• C.
• D.

Answer 1

The Correct Option is D

Correct answer: The compound that undergoes an SN¹ reaction is the one that can form the most stable carbocation intermediate.

The SN¹ reaction mechanism involves two key steps:

1. Formation of a carbocation intermediate by the departure of the leaving group.
2. Nucleophilic attack on the carbocation to form the final product.

The rate-determining step is the formation of the carbocation; therefore, the stability of this intermediate is crucial. The order of carbocation stability is:

Benzylic ≈ Allylic > Tertiary > Secondary > Primary > Methyl

In the given pairs of halogen compounds, the compound that can form the most stable carbocation will undergo the SN¹ reaction more readily. Specifically:

- Benzylic halides (where the halogen is attached to a carbon adjacent to a benzene ring) can form a **benzylic carbocation**, which is highly stabilized by resonance with the aromatic ring.

- Allylic halides (where the halogen is attached to a carbon adjacent to a double bond) can form an **allylic carbocation**, also stabilized by resonance.

- Tertiary alkyl halides form **tertiary carbocations**, stabilized by inductive and hyperconjugative effects from surrounding alkyl groups.

Therefore, when comparing pairs:

- A benzylic halide will undergo SN¹ more readily than an allylic or tertiary halide due to the exceptional stability of the benzylic carbocation.

- An allylic halide will undergo SN¹ more readily than a tertiary halide, but less readily than a benzylic halide.

- A tertiary alkyl halide will undergo SN¹ more readily than secondary or primary alkyl halides.

In summary, the compound in each pair that can form the most stable carbocation will be the one that undergoes the SN¹ reaction.

Answer 2

SN¹ reactions are favored by the formation of stable carbocations. The stability of carbocations follows the order: tertiary > secondary > primary > methyl.

Let's analyze each pair:

Option 1:

The first compound would form a primary carbocation, while the second would form a secondary carbocation. The secondary carbocation is more stable, so the second compound undergoes SN¹ reaction more readily.

Option 2:

The first compound would form a primary carbocation, while the second cannot undergo SN1 at all (bridgehead carbon). Therefore, the first compound may undergo SN1 but is highly unfavorable.

Option 3:

The first compound is a vinyl halide, and the second compound is an aryl halide. Both are unlikely to undergo SN1 reactions due to the instability of the resulting carbocations and the difficulty of breaking the C-X bond. However, neither would readily undergo SN1.

Option 4:

The first compound can form a primary carbocation, and the second compound can form a tertiary carbocation. The second compound will undergo SN¹ more readily because tertiary carbocations are more stable.

Comparing the pairs, the second compound in both option 1 and 4 are viable candidates, but the tertiary carbocation of option 4 will be significantly favored.