Question

Statement I: High concentration of strong nucleophilic reagent with secondary alkyl halides which do not have bulky substituents will follow \(S_N2\) mechanism.

Statement II: A secondary alkyl halide when treated with a large excess of ethanol follows \(S_N1\) mechanism.

In the light of the above statements, choose the most appropriate from the questions given below:

• A. Both statements are true
• B. Statement I is true, II is false
• C. Both statements are false
• D. Statement I is false, Statement II is true

Answer 1

The Correct Option is A

Analysis of Statement I: S_N2 reactions are characterized by a single concerted step where the nucleophile attacks the substrate from the opposite side of the leaving group. High concentration of a strong nucleophile enhances the rate of S_N2 reactions, especially with secondary alkyl halides that are not hindered by bulky substituents. Thus, Statement I is true.

Analysis of Statement II: S_N1 reactions occur in two steps: formation of a carbocation intermediate followed by nucleophilic attack. Secondary alkyl halides can undergo S_N1 reactions in the presence of polar protic solvents like ethanol. A large excess of ethanol serves as a nucleophile and stabilizes the carbocation, favoring the S_N1 mechanism. Thus, Statement II is also true.

Therefore, both statements are correct: Both Statement I and Statement II are true.

Answer 2

Analyze the two given statements regarding the reaction mechanisms of secondary alkyl halides and determine their correctness and relationship.

Concept Used:

The mechanism followed by an alkyl halide (S_N1 or S_N2) depends on multiple factors:

• S_N2: Favored by strong nucleophiles, low steric hindrance, and polar aprotic solvents. It is a concerted, bimolecular process.
• S_N1: Favored by weak nucleophiles, solvents that stabilize carbocations (polar protic), and the ability to form a stable carbocation. It is a two-step, unimolecular process.

Step-by-Step Solution:

Step 1: Analyze Statement I. It states that a high concentration of a strong nucleophile with a secondary alkyl halide lacking bulky substituents will follow the S_N2 mechanism. This is correct. A strong nucleophile promotes the S_N2 pathway, and the absence of bulky groups reduces steric hindrance, which is a major deterrent for the S_N2 mechanism, especially at a secondary carbon.

Step 2: Analyze Statement II. It states that a secondary alkyl halide treated with a large excess of ethanol follows the S_N1 mechanism. Ethanol is a polar protic solvent and a weak nucleophile. These conditions favor the ionization of the alkyl halide to form a carbocation, which is the rate-determining step of the S_N1 mechanism. Therefore, Statement II is also correct.

Step 3: Determine the relationship between the statements. Statement I describes conditions leading to S_N2 (strong nucleophile, low steric hindrance). Statement II describes different conditions leading to S_N1 (weak nucleophile/solvent, polar protic medium). Both statements are independently correct, but Statement II is not a correct explanation for Statement I, as it describes a different and opposite set of conditions.

Thus, both Statement I and Statement II are correct.