Question

Which is correct about SN reaction? (a) Here reaction intermediate is carbocation (b) It is favoured by polar solvents (c) It is favoured by stability of carbocation (d) all of these

Short answer

Option (d) is correct; all statements describe SN1 reactions.

Step-by-step solution

Understand SN Reactions

SN reactions stand for nucleophilic substitution reactions, where a nucleophile replaces a leaving group attached to a carbon atom. There are two main types: SN1 and SN2, each with different mechanisms and characteristics.

Analyze SN1 Reaction Characteristics

SN1 reactions proceed through two steps, with the formation of a carbocation intermediate in the first step. SN1 is favored by polar solvents and by the stability of the carbocation formed, as a stable carbocation will more readily form and react with the nucleophile.

Examine SN2 Reaction Characteristics

SN2 reactions occur in one concerted step. There is no carbocation intermediate formed as the nucleophile attacks the carbon atom at the same time as the leaving group departs. SN2 reactions are favoured by polar aprotic solvents rather than heavily polar ones.

Determine Which Answer Choice Matches SN1

Assess which of the provided options aligns with the SN1 reaction mechanism. Since SN1 involves a carbocation intermediate and is favored by polar solvents as well as the stability of the carbocation, it aligns with all the points in options (a), (b), and (c).

Conclude with the Correct Answer

Confirm that since all statements (a), (b), and (c) correctly describe characteristics of the SN1 reaction, option (d) is the correct answer, as it encompasses all these elements.

Key concepts

Nucleophilic Substitution Reactions

Nucleophilic substitution reactions are central to understanding organic chemistry. During these reactions, a nucleophile, which is an electron-rich species, substitutes a leaving group attached to a carbon atom. The carbon atom is often part of a larger organic molecule. These reactions are critical because they allow the transformation of molecules by altering functional groups. There are two main types:

• SN1 (Substitution Nucleophilic Unimolecular)
• SN2 (Substitution Nucleophilic Bimolecular)

These pathways differ in their mechanisms and conditions, influencing the course and outcome of the reaction.
For students, understanding these differences is crucial for predicting reaction products and designing chemical pathways.

SN1 Mechanism

The SN1 mechanism is a two-step process that involves the generation of a carbocation intermediate. Here’s how it works:

• Step 1: The leaving group departs, creating a positively charged carbocation.
• Step 2: The nucleophile then attacks the carbocation, forming a new product.

The SN1 reactions are distinguished by their rate-determining step, which depends solely on the concentration of the substrate, not the nucleophile. This means the reaction rate is first order concerning the substrate. SN1 is especially effective for substrates that can form stable carbocations.

Carbocation Stability

In SN1 reactions, the stability of the intermediate carbocation significantly influences the reaction's progress. More stable carbocations form more readily and support the reaction better. The stability of a carbocation is largely determined by:

• The number of alkyl groups attached, as these can donate electron density.
• The presence of resonance structures that allow the positive charge to be delocalized.

A tertiary carbocation, for instance, is more stable than a primary or secondary carbocation, owing to more extensive electron-donating effects and potential resonance delocalization. Understanding carbocation stability helps predict the likelihood and speed of SN1 reactions.

Polar Solvents

The choice of solvent is critical in SN1 reactions. Polar solvents facilitate the reaction by stabilizing the charged species—both intermediates and transition states. Polar protic solvents, in particular, are common in SN1 reactions as they help solvate the leaving group and carbocations effectively. This is because:

• They can hydrogen-bond, aiding in stabilizing the positive and negative charges.
• They provide a medium that supports the dissociation of the leaving group.

Common polar protic solvents include water and alcohols, and their presence can enhance the rate of SN1 reactions due to better solvation.

Reaction Intermediates

In chemical reactions, intermediates are species that form during the conversion of reactants to products and are not seen in the final products. In the case of SN1 reactions, the key intermediate is the carbocation. These intermediates are crucial as they dictate the pathway and outcome of the reaction. Other intermediates can form during complicated reactions, helping control the rate and direction of the transformation.
Recognizing and understanding these intermediates allows chemists to manipulate conditions to favor desired products, making knowledge of intermediates essential in synthetic chemistry.