Question

Which isomer of C\(_4\)H\(_9\)Br is most reactive towards \(S_N2\) reaction?

Hint:

SN2 reactivity: Methyl>1°>2° >> 3°.

Answer 1

Step 1: Understanding the \(S_N2\) reaction.
The \(S_N2\) (bimolecular nucleophilic substitution) reaction is a type of nucleophilic substitution where the rate-determining step involves the simultaneous attack of a nucleophile and the departure of the leaving group. The reaction occurs in a single step and involves a backside attack on the electrophilic carbon. The reactivity of a compound in \(S_N2\) is influenced by the steric hindrance around the electrophilic carbon.

Step 2: Analyzing the structure of C\(_4\)H\(_9\)Br isomers.
C\(_4\)H\(_9\)Br can have multiple isomers, including:
- 1-Bromobutane (a primary alkyl halide)
- 2-Bromobutane (a secondary alkyl halide)
- Isobutyl bromide (a primary alkyl halide with branching)
- Tert-butyl bromide (a tertiary alkyl halide)

Step 3: Reactivity in \(S_N2\) reaction.
The reactivity of an alkyl halide in \(S_N2\) reactions decreases with increasing steric hindrance around the electrophilic carbon. In primary alkyl halides, the carbon undergoing substitution is less hindered and more accessible to the nucleophile, making them more reactive in \(S_N2\) reactions. However, branching near the reactive center can reduce accessibility and reactivity.

- 1-Bromobutane: This is a primary halide with minimal steric hindrance and is most reactive in \(S_N2\).
- 2-Bromobutane: This is a secondary halide and is less reactive than 1-bromobutane due to greater steric hindrance.
- Isobutyl bromide: This is a primary halide but has some branching near the reactive center, making it slightly less reactive than 1-bromobutane.
- Tert-butyl bromide: This is a tertiary halide with significant steric hindrance, making it very poor for \(S_N2\) reactions, as the nucleophile cannot effectively attack the carbon center.

Step 4: Conclusion.
The isomer 1-bromobutane is the most reactive towards \(S_N2\) reactions due to its primary structure and minimal steric hindrance, which allows for easier nucleophilic attack.

Final Answer: The isomer 1-bromobutane is most reactive towards \(S_N2\) reactions.