Problem 17 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

In \(\mathrm{S}_{\mathrm{N}} 2\) reactions, the correct order of reactivity for the following compounds: \(\mathrm{CH}_{3} \mathrm{Cl}, \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl},\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCl}\) and \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCl}\) is: (a) \(\mathrm{CH}_{3} \mathrm{Cl}>\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCl}>\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl}>\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCl}\) (b) \(\mathrm{CH}_{3} \mathrm{Cl}>\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl}>\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCl}>\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCl}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl}>\mathrm{CH}_{3} \mathrm{Cl}>\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCl}>\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCl}\) (d) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCl}>\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl}>\mathrm{CH}_{3} \mathrm{Cl}>\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCl}\)

Step-by-Step Solution

Answer

The correct order of reactivity is option (b): \(\mathrm{CH}_{3} \mathrm{Cl} > \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl} > (\mathrm{CH}_{3})_{2} \mathrm{CHCl} > (\mathrm{CH}_{3})_{3} \mathrm{CCl}\).

Step 1: Understanding SN2 Reactions

In an SN2 reaction, a nucleophile attacks the carbon atom that is bonded to a halogen. The reaction leads to the substitution of the halogen atom. SN2 reactions are characterized by a backside attack, which is facilitated in less sterically hindered (less substituted) carbon centers.

Step 2: Analyze the Compounds

Analyze each compound provided: 1. \(\mathrm{CH}_{3} \mathrm{Cl}\) is a methyl chloride with no alkyl group substituents at the carbon bonded to chlorine, making it the least hindered. 2. \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl}\) is a primary chloride with one ethyl group, slightly more hindered than \(\mathrm{CH}_{3} \mathrm{Cl}\). 3. \((\mathrm{CH}_{3})_{2} \mathrm{CHCl}\) is a secondary chloride with two methyl groups, leading to increased steric hindrance. 4. \((\mathrm{CH}_{3})_{3} \mathrm{CCl}\) is a tertiary chloride, having three methyl groups, which causes significant steric hindrance.

Step 3: Determine Reactivity Order

SN2 reactions are fastest with less sterically hindered systems. The order of reactivity generally decreases with increasing substitution at the reaction center. Therefore, the reactivity order from most to least reactive for SN2 reactions is primarily based on steric hindrance: \(\mathrm{CH}_{3} \mathrm{Cl} > \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl} > (\mathrm{CH}_{3})_{2} \mathrm{CHCl} > (\mathrm{CH}_{3})_{3} \mathrm{CCl}\).

Step 4: Identify Correct Option

Compare the determined order of reactivity with the choices provided in the exercise.The correct order for SN2 reactivity as analyzed is:\[\mathrm{CH}_{3} \mathrm{Cl} > \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl} > (\mathrm{CH}_{3})_{2} \mathrm{CHCl} > (\mathrm{CH}_{3})_{3} \mathrm{CCl}\]This matches option (b) \(\mathrm{CH}_{3} \mathrm{Cl} > \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl} > (\mathrm{CH}_{3})_{2} \mathrm{CHCl} > (\mathrm{CH}_{3})_{3} \mathrm{CCl}\).

Key Concepts

Reactivity Order in SN2 Reactions

In the world of organic chemistry, SN2 reactions are a fascinating type of nucleophilic substitution characterized by their specific mechanism. The reactivity order of different compounds in SN2 reactions is crucially determined by the structure of the substrate. Generally, compounds with less steric hindrance are most reactive. The idea is simple: the less crowded the carbon atom bonded to the leaving group, the easier it is for the nucleophile to perform a backside attack. This makes methyl halides (like \( \mathrm{CH}_{3} \mathrm{Cl} \)), with no substituents around the reactive center, the most reactive in SN2 reactions. As we increase the number of substituents from primary (e.g., \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl} \)) to secondary (e.g., \( (\mathrm{CH}_{3})_{2} \mathrm{CHCl} \)) and finally to tertiary (e.g., \( (\mathrm{CH}_{3})_{3} \mathrm{CCl} \)), steric hindrance increases and reactivity decreases. Thus, the reactivity order for the compounds listed in SN2 reactions is \( \mathrm{CH}_{3} \mathrm{Cl} > \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl} > (\mathrm{CH}_{3})_{2} \mathrm{CHCl} > (\mathrm{CH}_{3})_{3} \mathrm{CCl} \). This prioritizes less substituted, and therefore, less hindered substrates for optimal reactivity.

Understanding Steric Hindrance

Steric hindrance is like a barrier to SN2 reactions, where the incoming nucleophile needs an obstacle-free path to attack the carbon atom bonded to the leaving group. Imagine trying to park a car in a tight spot; it's significantly harder with more cars around. Similarly, in molecules with bulky groups around the reactive center, such as tertiary structures like \( (\mathrm{CH}_{3})_{3} \mathrm{CCl} \), the nucleophile has a tough time reaching the reaction center.

This hindrance blocks or slows down the reaction, making these heavily substituted centers less reactive in SN2 processes. On the flip side, simpler compounds like \( \mathrm{CH}_{3} \mathrm{Cl} \) with no additional side groups offer an unobstructed path for backside attack by a nucleophile. Understanding steric effects is crucial since it explains why SN2 reactions are less favorable for tertiary carbon centers. By minimizing steric hindrance, reaction rates can be increased.

Nucleophilic Substitution and the SN2 Mechanism

Nucleophilic substitution, specifically the SN2 mechanism, is a crucial concept in organic chemistry. This type of reaction involves a nucleophile—the molecule that 'loves nuclei'—attacking an electrophilic site, usually a carbon atom attached to a leaving group like chlorine in our examples. The term SN2 stands for "substitution nucleophilic bimolecular," highlighting two molecules involved in the rate-determining step: the nucleophile and the substrate. This bimolecular aspect means the reaction rate depends on both the concentration of the nucleophile and the substrate.

In the SN2 mechanism, the nucleophile approaches the substrate from the opposite side of the leaving group, effectively "kicking out" the leaving group from the backside. This backside attack is an elegant, one-step process where a strong nucleophile needs to replace the leaving group instantaneously. Hence, a good nucleophile is usually negatively charged or neutral but highly electronegative, and a good leaving group is stable as a standalone molecule, like chloride ion \( \mathrm{Cl}^- \). The SN2 reaction is a perfect dance of chemistry, demonstrating how nucleophilic substitution is dependent on the delicate balance among nucleophilic strength, leaving group ability, and steric factors. Understanding this mechanism offers insights into designing better reactions by selecting suitable substrates and conditions to optimize reaction pathways.