Question

Directions: This section contains 1 paragraph. Based upon the paragraph, 3 multiple choice questions have to be answered. Each question has 4 choices \((a),(b),(c)\) and \((d)\), out of which ONLY ONE is correct. Passage I Mechanism of nucleophilic substitutions Aliphatic nucleophilic substitutions occur by two different mechanisms namely \(\mathrm{S}_{\mathrm{N}} 1\) and \(\mathrm{S}_{\mathrm{N}} 2 .\) The various factors like degree of substitution, polarity of the medium, steric factors, nucleophilicity, concentration of the reacting substances etc, determine whether a reaction proceeds by \(\mathrm{S}_{\mathrm{N}} 1\) or \(\mathrm{S}_{\mathrm{N}} 2\) mechanism. Four pairs of reactions (A) - (D) are given below.

Short answer

A) A highly substituted substrate B) A polar protic solvent C) A high concentration of a strong nucleophile D) the presence of a carbocation intermediate Answer: To answer this question, consider the factors that favor the \(\mathrm{S}_{\mathrm{N}}2\) mechanism over the \(\mathrm{S}_{\mathrm{N}}1\) mechanism. A) A highly substituted substrate favors \(\mathrm{S}_{\mathrm{N}}1\), as it generates a more stable carbocation intermediate. This option does not favor \(\mathrm{S}_{\mathrm{N}}2\). B) A polar protic solvent favors the \(\mathrm{S}_{\mathrm{N}}1\) mechanism as it stabilizes the carbocation intermediate. This option does not favor \(\mathrm{S}_{\mathrm{N}}2\). C) A high concentration of a strong nucleophile favors the \(\mathrm{S}_{\mathrm{N}}2\) mechanism, as the reaction rate depends on both the substrate and nucleophile concentrations. This option favors \(\mathrm{S}_{\mathrm{N}}2\). D) The presence of a carbocation intermediate is a characteristic of the \(\mathrm{S}_{\mathrm{N}}1\) mechanism, not the \(\mathrm{S}_{\mathrm{N}}2\) mechanism. This option does not favor \(\mathrm{S}_{\mathrm{N}}2\). Therefore, the correct answer is C) A high concentration of a strong nucleophile.

Step-by-step solution

Understanding the two mechanisms

The two types of aliphatic nucleophilic substitutions, \(\mathrm{S}_{\mathrm{N}}1\) and \(\mathrm{S}_{\mathrm{N}}2\), have distinct mechanisms. The \(\mathrm{S}_{\mathrm{N}}1\) mechanism is characterized by a two-step reaction involving a carbocation intermediate, with the rate of the reaction determined by the substrate concentration. In contrast, the \(\mathrm{S}_{\mathrm{N}}2\) mechanism involves a concerted reaction, in which the nucleophile attacks the substrate while the leaving group departs. The rate of this reaction depends on both the substrate and nucleophile concentrations.

Identifying factors affecting the mechanisms

Determine which factors influence whether a reaction proceeds through the \(\mathrm{S}_{\mathrm{N}}1\) or \(\mathrm{S}_{\mathrm{N}}2\) mechanism. According to the passage, these factors include the degree of substitution, polarity of the medium, steric factors, nucleophilicity, and concentration of the reacting substances.

Analyzing each multiple-choice question

For each of the given multiple-choice questions, analyze the options considering the factors mentioned above. By recognizing the characteristics of a reaction favoring either the \(\mathrm{S}_{\mathrm{N}}1\) or \(\mathrm{S}_{\mathrm{N}}2\) mechanism, you can then eliminate incorrect options and choose the correct answer. Remember, the passage stated that each question has only one correct answer. Take your time in analyzing each question, carefully evaluating the given options, and selecting the appropriate answer based on the factors affecting the \(\mathrm{S}_{\mathrm{N}}1\) and \(\mathrm{S}_{\mathrm{N}}2\) mechanisms.

Key concepts

SN1 Reaction

An SN1 reaction, which stands for nucleophilic substitution first order, is a multi-step pathway characterized by the formation of a carbocation intermediate. In an SN1 reaction, the leaving group first detaches from the substrate to form a carbocation, making this process the rate-determining step. This implies that the overall reaction rate is dependent solely on the concentration of the substrate, not the nucleophile.

After the carbocation forms, the nucleophile can then attack from either side, leading to a racemic mixture of products if the reacting center is chiral. SN1 reactions are typically favored in tertiary substrates due to their ability to stabilize the resulting carbocation through hyperconjugation and inductive effects. Polar, protic solvents also support SN1 reactions as they stabilize the carbocation and leaving group through solvation.

SN2 Reaction

In contrast to SN1 reactions, an SN2 mechanism, denoting nucleophilic substitution second order, encompasses a single concerted step. Here, the nucleophile attacks the substrate simultaneously as the leaving group departs. This attack typically occurs from the backside, leading to an inversion of configuration, which is significant in compounds with chiral centers.

The reaction rate for SN2 mechanisms depends on both the substrate and the nucleophile concentrations, hence the second order ('bimolecular') designation. SN2 reactions are promoted in primary substrates that offer less steric hindrance for the incoming nucleophile. Aprotic solvents are typically preferred as they do not solvate the nucleophile as strongly, allowing it to remain more reactive.

Factors Affecting Nucleophilic Substitution

Several factors determine whether a reaction will proceed via SN1 or SN2 mechanism. These include:

• Degree of Substitution: SN1 is supported by tertiary substrates while SN2 favors primary or methyl substrates.
• Steric Hindrance: Increased steric bulk near the reactive center impedes SN2 reactions but doesn't significantly affect SN1.
• Nucleophilicity: Strong, unhindered nucleophiles are ideal for SN2 reactions; weak nucleophiles may still participate in SN1.
• Solvent Effects: Protic solvents stabilize intermediates and reactants in SN1, while aprotic solvents increase nucleophilicity for SN2.
• Leaving Group Ability: A good leaving group is necessary for both SN1 and SN2; better leaving groups can accelerate these reactions.

These considerations must be taken into account when predicting or influencing the outcome of nucleophilic substitution reactions.

Aliphatic Nucleophilic Substitution

Aliphatic nucleophilic substitution refers to the reaction types where a nucleophile targets an aliphatic carbon, one that is neither aromatic nor part of a carbonyl group, replacing a leaving group. Both SN1 and SN2 mechanisms can occur with aliphatic substrates, but the structure of the substrate, as well as the conditions outlined in the factors above, ultimately dictate the specific mechanism that predominates.

Understanding and predicting the course of aliphatic nucleophilic substitution reactions are fundamental in synthesizing a vast array of chemical compounds. For students grappling with these concepts, it’s essential to form a clear picture of how the structure and environment influence the reaction pathways, thereby steering the ultimate formation of the products.