Question

Which is correct about \(\mathrm{SN}^{1}\) 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.

Step-by-step solution

Understanding SN1 Reaction

In an SN1 reaction, the rate-determining step involves the formation of a carbocation intermediate. The leaving group departs, leading to the formation of this positively charged intermediate.

Evaluating Option (a)

Option (a) states that the reaction intermediate is a carbocation. This is indeed correct for an SN1 reaction, as the defining feature of SN1 is the formation of a carbocation intermediate.

Evaluating Option (b)

Option (b) asserts that SN1 reactions are favored by polar solvents. Polar solvents stabilize the carbocation intermediate and/or the transition state through solvation, making these solvents indeed favorable for SN1 reactions.

Evaluating Option (c)

Option (c) states that SN1 is favored by the stability of the carbocation. The more stable the carbocation, the more likely it is to form, which directly increases the rate of the SN1 reaction. Thus, this option is correct.

Final Evaluation of Option (d)

Option (d) suggests that all of the previous options are correct. Since options (a), (b), and (c) are all true statements regarding SN1 reactions, this means option (d) is correct as well.

Key concepts

Carbocation Intermediate

One of the key features of an \(\text{SN}^{1}\) reaction is the formation of a carbocation intermediate. This occurs when the leaving group detaches from the molecule, leaving behind a positively charged ion known as a carbocation. Carbocations are crucial because they dictate the progression of the reaction. Once the carbocation forms, the nucleophile can attack, leading to the final product. If the carbocation is unstable, the reaction might not proceed efficiently. However, if it forms easily and is stable, the reaction can continue smoothly. Understanding carbocations is fundamental in grasping how \(\text{SN}^{1}\) reactions occur.

Polar Solvents

Polar solvents play a significant role in \(\text{SN}^{1}\) reactions. These solvents stabilize the carbocation intermediate through a process called solvation. This means that the solvent molecules surround and interact with the carbocation, helping to lower its energy. By stabilizing the intermediate, polar solvents facilitate the reaction's progression. Polar solvents like water, methanol, or acetone are often seen in \(\text{SN}^{1}\) reactions due to their ability to dissolve ions and stabilize charges effectively. Using such solvents enhances the overall efficiency of the reaction.

Carbocation Stability

The stability of the carbocation is a key factor influencing \(\text{SN}^{1}\) reactions. Carbocations classify into different types based on their substituents: primary, secondary, and tertiary. Generally, tertiary carbocations, having three alkyl groups attached, are more stable than secondary or primary ones. This stability occurs because the alkyl groups can donate electron density to the positively charged carbon through hyperconjugation and inductive effects. These interactions help distribute the positive charge over a larger area, stabilizing the carbocation. A more stable carbocation will be more likely to form and facilitate a faster reaction rate in an \(\text{SN}^{1}\) mechanism. Knowing how different factors affect carbocation stability can provide insights into the viability and speed of a reaction.