Question

Which compound in each of the following pairs will react faster in \(\mathrm{S}_{\mathrm{N}} 2\) reaction with - OH? (i) \(\mathrm{CH}_{3} \mathrm{Br}\) or \(\mathrm{CH}_{3} \mathrm{I}\) (ii) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCl}\) or \(\mathrm{CH}_{3} \mathrm{Cl}\)

Short answer

(i) \(\mathrm{CH}_3\mathrm{I}\) (ii) \(\mathrm{CH}_3\mathrm{Cl}\)

Step-by-step solution

Understanding SN2 reaction mechanism

In an \( \mathrm{S_N2} \) reaction, the nucleophile attacks the electrophilic carbon from the side opposite to the leaving group, resulting in the inversion of configuration. The rate of \( \mathrm{S_N2} \) reactions is influenced by steric hindrance and the strength of the leaving group.

Compare leaving group abilities for (i)

In the case of \(\mathrm{CH}_3\mathrm{Br}\) and \(\mathrm{CH}_3\mathrm{I}\), the rate of reaction depends on the leaving group's ability. Iodide (I-) is a better leaving group than bromide (Br-) because iodide is larger and can stabilize the negative charge better when it leaves. Therefore, \(\mathrm{CH}_3\mathrm{I}\) will react faster in an \(\mathrm{S_N2}\) mechanism.

Analyze steric hindrance for (ii)

\(\left(\mathrm{CH}_3\right)_3\mathrm{CCl}\) has three methyl groups attached to the central carbon, which creates significant steric hindrance. This makes it difficult for the nucleophile to attack the central carbon. In contrast, \(\mathrm{CH}_3\mathrm{Cl}\) has less steric hindrance as it only has one methyl group. Thus, \(\mathrm{CH}_3\mathrm{Cl}\) will react faster in an \(\mathrm{S_N2}\) mechanism.

Key concepts

Nucleophilic Substitution

Nucleophilic substitution is a fundamental chemical reaction, especially in organic chemistry. It involves the replacement of a leaving group by a nucleophile. In the classic SN2 mechanism, this occurs in one concerted step. The nucleophile attacks the substrate, usually a carbon atom, from the back side. This approach contrasts the leaving group's position, causing a polar reaction where bonds break and form simultaneously.
This reaction type results in the inversion of stereochemistry at the center under attack, resembling an umbrella turning inside out. The efficiency and speed of an SN2 reaction depend heavily on factors such as the strength and stability of the leaving group, the substrate's structure, and the nucleophile's strength. Let's break these down further.

Steric Hindrance

Steric hindrance is a crucial concept in understanding the reactivity trends in SN2 reactions. It refers to the spatial crowding around the reaction site, usually caused by bulky groups that prevent the nucleophile's attack. When too many groups surround a reactive center, they block the nucleophile, slowing or even preventing the reaction.

• For example, in the pair \((\mathrm{CH}_3)_3\mathrm{CCl}\) versus \(\mathrm{CH}_3\mathrm{Cl}\), the former has a bulky tert-butyl group that creates significant steric hindrance.
• This hindrance obstructs the nucleophile's path, making it challenging for an efficient attack.
• Conversely, \(\mathrm{CH}_3\mathrm{Cl}\) is less hindered, as it features only one methyl group, facilitating a smoother nucleophilic attack.
  

Thus, substrates with less steric congestion tend to react faster in SN2 reactions due to the reduced physical blockage of the nucleophile.

Leaving Group Ability

Leaving group ability is a vital factor that affects the rate of nucleophilic substitution reactions. A good leaving group easily departs from the substrate and can stabilize the negative charge left behind. This quality is imperative in determining the speed of the SN2 reaction.

• For example, comparing \(\mathrm{CH}_3\mathrm{Br}\) and \(\mathrm{CH}_3\mathrm{I}\), iodide (I-) is a superior leaving group compared to bromide (Br-).
• This superiority is due to iodide's larger size, which allows it to better accommodate and stabilize the negative charge when it leaves.
• Therefore, the iodide-containing compound reacts faster in an SN2 mechanism.
  

Essentially, the more capable a leaving group is at dispersing the resulting negative charge, the quicker the reaction proceeds. This ability is as significant as steric hindrance in evaluating SN2 reactions speed.