Question

Which among the following compounds will undergo fastest \(S_N2\) reaction?

• A.

  

• B.

  

• C.

  

• D.

  

Answer 1

The Correct Option is C

To determine which compound undergoes the fastest \(S_N2\) reaction, we must consider the factors that influence \(S_N2\) reactions. The \(S_N2\) mechanism involves a one-step process where the nucleophile attacks the electrophilic carbon from the opposite side, causing the leaving group to leave simultaneously. Some key factors affecting \(S_N2\) reactions include:

• Substrate Structure: \(S_N2\) reactions favor less sterically hindered substrates. Primary halides react faster than secondary, which react faster than tertiary halides.
• Nucleophile Strength: A strong nucleophile will enhance the rate of \(S_N2\) reactions, but this factor is constant across similar groups in different substrates.
• Leaving Group Ability: A good leaving group increases the \(S_N2\) reaction rate. Typically, weak bases make better leaving groups.
• Solvent Effects: Polar aprotic solvents favor \(S_N2\) reactions by solvating cations well, thereby freeing anions to attack the substrate.

Let us analyze each of the given options based on steric hindrance:

This compound is a tertiary alkyl halide. Tertiary halides are highly hindered and react very slowly, if at all, via an \(S_N2\) mechanism.

This compound is a secondary alkyl halide. Although it can undergo \(S_N2\) reactions, it is more hindered than primary halides.

This compound is a primary alkyl halide. Primary halides are less hindered and usually the best candidates for \(S_N2\) reactions.

This compound is a primary halide too, but given the branching nearby, it could be slightly more hindered compared to the simplest primary halide.

Thus, the compound shown in the following image:

is the correct answer as it is a primary halide with minimal hindrance, allowing the fastest \(S_N2\) reaction.

Answer 2

The \(S_N2\) reaction rate depends on steric hindrance. Primary alkyl halides react faster than secondary or tertiary halides:
Rate of \(S_N2\) : methyl halide > primary > secondary > tertiary.
Among the given options: