Problem 10 - A Complete Resource Book in Chemistry for JEE Main

Question

Which among \(\mathrm{MeX}, \mathrm{RCH}_{2} \mathrm{X}, \mathrm{R}_{2} \mathrm{CHX}, \mathrm{R}_{3} \mathrm{CX}\) is most reactive towards \(\mathrm{S}_{\mathrm{N}}^{2}\) reaction? (a) \(\mathrm{R}_{3} \mathrm{CX}\) (b) \(\mathrm{RCH}_{2} \mathrm{X}\) (c) \(\mathrm{R}_{2} \mathrm{CHX}\) (d) \(\mathrm{MeX}\)

Step-by-Step Solution

Answer

\(\mathrm{MeX}\) is the most reactive towards SN2 reactions.

Step 1: Understanding the SN2 Mechanism

The SN2 reaction is a bimolecular nucleophilic substitution reaction where the nucleophile attacks the electrophile from the side opposite of the leaving group. This reaction requires a high degree of steric accessibility to allow for a concerted attack and displacement.

Step 2: Assessing Steric Hindrance

In the SN2 reaction, less sterically hindered substrates are more reactive. The order of steric hindrance among the given options is \[\text{R}_{3}\text{CX} > \text{R}_{2}\text{CHX} > \text{RCH}_{2}\text{X} > \text{MeX}\] where \(\text{R}_{3}\text{CX}\) has the most steric crowding and \(\text{MeX}\) (methyl halide) has the least.

Step 3: Determining Reactivity Based on Sterics

Since SN2 reactions proceed faster with less steric hindrance, the less hindered the substrate, the more reactive it is. \(\mathrm{MeX}\) has the least steric hindrance, followed by \(\mathrm{RCH}_{2} \mathrm{X}\), making \(\mathrm{MeX}\) the most reactive here.

Step 4: Conclusion

After evaluating the steric factors, \(\mathrm{MeX}\), with no branching around the reaction center, is the most reactive towards SN2 reactions.

Key Concepts

Nucleophilic Substitution

An SN2 reaction is a type of nucleophilic substitution where the nucleophile and the substrate simultaneously interact in a single step. This reaction is characterized by the nucleophile attacking the substrate from the side opposite the leaving group. The simultaneous bond formation and breaking make it a concerted process. Because it occurs in one step, the term "bimolecular" is often used to express that both the nucleophile and the substrate are involved in the rate-determining step.
SN2 reactions are vital in organic synthesis, allowing the transformation of one functional group into another with precision. Understanding this process is crucial in chemistry as it aids in predicting how different molecules will react under given conditions.
The substrate's structure significantly influences the reaction path and rate, which brings us to the importance of steric hindrance in these reactions.

Steric Hindrance

Steric hindrance refers to the slowing down of chemical reactions due to the physical presence of bulky groups around a reaction site. In SN2 reactions, steric hindrance plays a crucial role because the nucleophile must approach and attach itself to the electrophilic carbon atom. However, sizeable surrounding groups can block or impede this approach.
Here’s how steric hindrance affects SN2 reactions:

• Less steric hindrance: Faster SN2 reactions occur with substrates having minimal obstruction around the reactive center, allowing easy access for the nucleophile.
• More steric hindrance: Slower reaction rates are observed when bulky groups surround the electrophilic carbon, thus making nucleophile attack difficult.

In terms of reactivity order, steric hindrance is why methyl halides (MeX) react more rapidly than primary (RCH₂X), secondary (R₂CHX), or tertiary (R₃CX) halides. The absence of steric bulk around MeX ensures it is highly reactive in SN2 reactions. Understanding these factors can aid in predicting and explaining the reactivity of different substrates.

Reactivity Order

The reactivity order in SN2 reactions is primarily determined by the degree of steric hindrance around the electrophilic carbon in the substrate. The simpler and less hindered a compound is, the more reactive it will be to nucleophilic attack in an SN2 context.
The typical order is as follows:

• Methyl halides (MeX): These are the most reactive towards SN2 reactions due to the complete absence of steric hindrance, making nucleophile access seamless.
• Primary halides (RCH₂X): Slightly less reactive than methyl halides but still quite favorable for SN2 reactions because they have minimal steric hindrance from one R group.
• Secondary halides (R₂CHX): These are less reactive due to increased steric bulk from two R groups, thus offering more resistance to nucleophile approach.
• Tertiary halides (R₃CX): Typically unreactive in SN2 mechanisms due to significant steric hindrance from three R groups, impeding nucleophilic access completely.

For designing chemical syntheses or understanding how and why reactions proceed, recognizing the reactivity order in SN2 processes is indispensable.