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

(d) MeX is most reactive towards SN2 reactions.

Step 1: Understanding the SN2 Reaction Mechanism

An SN2 (substitution nucleophilic bimolecular) reaction involves the nucleophile attacking the electrophilic carbon leading to the displacement of a leaving group. This is a single-step process with a transition state where the incoming nucleophile and the leaving group are both partially bonded to the carbon. Therefore, steric hindrance around the electrophilic carbon significantly affects the reaction rate.

Step 2: Identifying Steric Effects

For an SN2 reaction, less steric hindrance allows the nucleophile to attack more easily. The less crowded the carbon atom is, the more reactive it is towards SN2 reactions. This means that methyl halides ( MeX ) are the least hindered, followed by primary ( RCH2X ), then secondary ( R2CHX ), and finally, tertiary ( R3CX ) alkyl halides which are the most hindered.

Step 3: Comparing Degrees of Hindrance

Among the options: (a) R3CX is tertiary, so it has the most steric hindrance. (b) RCH2X is primary, having less steric hindrance. (c) R2CHX is secondary, with moderate steric hindrance. (d) MeX (Methyl halide) has the least steric hindrance since it has no substituents other than hydrogens around the carbon.

Step 4: Determining Reactivity Trend

Considering steric effects are the main factor for SN2 reaction rate, the reactivity trend from most to least reactive towards SN2 reactions is: MeX > RCH2X > R2CHX > R3CX . Therefore, methyl halides ( MeX ) are the most reactive.

Key Concepts

Nucleophilic Substitution

Nucleophilic substitution is a fundamental type of reaction in organic chemistry. It involves the replacement of a leaving group in a molecule by a nucleophile. In the context of the SN2 reaction, the nucleophile is a molecule or ion that is rich in electrons and is seeking to donate these electrons to the electrophilic carbon.

• The "nucleophile" is the donor of a pair of electrons, effectively attacking the carbon atom.
• The "leaving group" is the part of the molecule that will depart with a pair of electrons.

The SN2 reaction is typically favored in conditions where the electrophilic carbon is accessible, allowing the nucleophile to effectively attack and displace the leaving group. The efficiency and mechanisms of these reactions can be discussed mainly by considering the roles of steric hindrance and the nature of the nucleophile.

Reaction Mechanism

The mechanism of an SN2 reaction is characterized as a concerted, single-step action. This means that the bond-forming and bond-breaking events occur simultaneously. The nucleophile approaches the substrate from the opposite side of the leaving group, creating a transition state where it is partially bonded to the carbon atom, while the leaving group is also partially departing. This leads to an inversion of configuration at the carbon center, also known as the "back-side attack" mechanism.

• The net reaction happens in a one-step process without any intermediates.
• The transition state is a critical stage where old bonds are partially broken and new bonds are partially formed.

The speed of the SN2 reaction is highly dependent on the accessibility of the electrophilic carbon and is therefore impacted by steric factors. These factors directly influence the reactivity of a substrate in SN2 reactions.

Steric Hindrance

Steric hindrance is a major factor influencing the rate of SN2 reactions. It refers to the physical hindrance that the surrounding groups around a carbon atom present, impeding the approach of the nucleophile towards the carbon center.

• The more crowded the carbon center, the higher the steric hindrance, and thus, the slower the SN2 reaction proceeds.
• Steric hindrance increases from methyl halides to primary, secondary, and then tertiary alkyl halides.

In practice, the least hindered substrates, such as methyl halides, react the fastest in SN2 reactions. They offer little resistance to the nucleophile, whereas tertiary substrates are too crowded, making SN2 reactions unfavorable. Hence, analyzing steric hindrance is crucial to predicting the feasibility of these reactions.

Methyl Halides

Methyl halides (\( ext{MeX} \)) are the simplest type of halide, where a halogen is bound to a methyl group. These compounds serve as ideal substrates in SN2 reactions due to their minimal steric hindrance.

• Methyl halides have no substituents other than hydrogen atoms, allowing unhindered access by the nucleophile.
• They are highly reactive, undergoing rapid SN2 reactions when compared to larger alkyl halides.

The lack of bulky groups around the electrophilic carbon in methyl halides makes them the fastest to react in SN2 mechanisms. This property of methyl halides is highly beneficial in synthetic chemistry where efficiency and speed are paramount.

Electrophilic Carbon

In SN2 reactions, the electrophilic carbon is the central atom where the nucleophilic attack takes place. It is typically bonded to a leaving group and is electron-deficient, making it an attractive site for nucleophiles seeking to donate electrons.

• The electrophilic carbon is the target for the nucleophile's attack, essential for the substitution process.
• The carbon’s ability to be easily accessed by the nucleophile significantly influences the reaction rate and efficiency.

When the carbon is surrounded by less bulky groups, it is more exposed and thus more susceptible to nucleophilic attack. This is why methyl halides, with no additional carbon groups, result in a more efficient and rapid SN2 reaction.