Question

Which of the following statements are correct? 1\. \(\mathrm{SN}^{1}\) mechanism is most commonly given by tertiary alkyl halides. 2\. \(\mathrm{SN}^{1}\) mechanism proceeds through the formation of a carbocation. 3\. \(\mathrm{SN}^{2}\) mechanism involves retention of configuration. 4\. \(\mathrm{SN}^{2}\) mechanism proceeds through the formation of a transition state. (a) 1,2 and 4 (b) 1,3 and 4 (c) 2,3 and 4 (d) \(1,2,3\) and 4

Short answer

Option (a) 1, 2, and 4 are correct.

Step-by-step solution

Analyze Statement 1

The statement says that SN1 mechanisms are most commonly given by tertiary alkyl halides. This is correct because tertiary alkyl halides stabilize the carbocation intermediate that is formed during an SN1 reaction due to hyperconjugation and inductive effect.

Analyze Statement 2

The statement claims that the SN1 mechanism proceeds through the formation of a carbocation. This is also correct, as the SN1 mechanism involves the formation of a carbocation intermediate as the rate-determining step.

Analyze Statement 3

The statement posits that the SN2 mechanism involves retention of configuration. This is incorrect as the SN2 mechanism involves an inversion of configuration at the carbon atom where the substitution occurs, due to a backside attack by the nucleophile.

Analyze Statement 4

The statement mentions that the SN2 mechanism proceeds through the formation of a transition state. This is correct because SN2 reactions are characterized by a single concerted step where a transition state is formed as the nucleophile approaches the substrate and the leaving group departs simultaneously.

Conclude

Based on the analysis, statements 1, 2, and 4 are correct, while statement 3 is incorrect.

Key concepts

Tertiary Alkyl Halides

Tertiary alkyl halides are a fascinating part of organic chemistry due to their unique behavior in reactions. These alkyl halides have a central carbon atom bonded to three other carbon atoms. This configuration makes them bulkier and less susceptible to bimolecular substitution reactions like the SN2 mechanism. This bulkiness hinders the backside attack required in SN2, making tertiary alkyl halides ideal candidates for SN1 reactions instead.

An important feature of tertiary alkyl halides is their ability to stabilize carbocations, which are positively charged intermediates. This stability is due to the hyperconjugation and inductive effects provided by the three alkyl groups. Hyperconjugation involves the delocalization of electrons in the overlapping orbitals of the neighboring C-H bonds, and the inductive effect involves the electron-donating nature of alkyl groups.

• Stability of carbocation intermediates is crucial in SN1 reactions.
• Hyperconjugation and inductive effects aid this stability, making tertiary alkyl halides favorable for SN1 reactions.

Understanding these properties helps clarify why tertiary alkyl halides predominantly follow the SN1 mechanism, where the rate of reaction is not dependent on the concentration of the nucleophile but instead on the formation of the stable carbocation.

Carbocation Formation

Carbocation formation is a pivotal step in the SN1 reaction mechanism. During the SN1 process, the alkyl halide initially undergoes heterolytic cleavage, where the leaving group departs, resulting in a carbocation. This step is the rate-determining step of the reaction.

Carbocations are electron-deficient species with a positive charge, making them highly reactive intermediates. In the case of tertiary carbocations, they are more stable due to:

• Hyperconjugation: The delocalization of electrons from adjacent sigma bonds helps delocalize the positive charge.
• Inductive effect: The electron-donating nature of surrounding alkyl groups stabilizes the positive charge.

This step is crucial because:

• The formation of a stable carbocation intermediate allows the reaction to proceed even when the nucleophile is weak or present in low concentration.
• The reaction continues as the nucleophile can attack the positively charged carbon, resulting in the product.

Understanding carbocation stability is essential for predicting the outcome of SN1 reactions and determining whether a reaction will follow the SN1 or SN2 pathway.

Transition State in Reactions

The concept of a transition state is fundamental in understanding reaction mechanisms, particularly in SN2 reactions. In chemistry, a transition state refers to a high-energy state that occurs during the formation of products from reactants. It's pivotal in the one-step concerted movement typical of SN2 responses.

During an SN2 reaction, a nucleophile approaches the substrate, and a leaving group departs simultaneously, leading to the formation of a transition state. This state is characterized by:

• A pentavalent carbon center that is partially bonded to both the incoming nucleophile and the leaving group.
• Partial charges and elongation of bonds, reflecting a state of maximum energy along the reaction pathway.

The transition state is crucial because:

• It represents the point of highest energy in the pathway, determining the reaction rate.
• The tight alignment of reactants is necessary for the reaction to occur—a concept known as steric and electronic compatibility.

Understanding transition states provides insight into why SN2 reactions are sensitive to steric hindrance and reinforces the fundamentals of reaction kinetics.