Question

In the solvolysis of 3 -methyl-3-bromohexane, which of the following statements is not correct? (a) It involves carbocation intermediate (b) The intermediate involves \(\mathrm{sp}^{2}\) carbon (c) Polar solvents accelerates the reaction (d) It involves inversion of configuration

Short answer

(d) It involves inversion of configuration is not correct.

Step-by-step solution

Understand the Reaction Type

The solvolysis of 3-methyl-3-bromohexane is a nucleophilic substitution reaction. Solvolysis typically refers to the SN1 (unimolecular nucleophilic substitution) mechanism, especially when the alkyl halide is highly substituted as in 3-methyl-3-bromohexane.

SN1 Mechanism Characteristics

SN1 reactions proceed via a two-step mechanism. The first step involves the formation of a carbocation intermediate when the leaving group (bromide ion in this case) departs. This results in a planar, sp2 hybridized carbocation.

Evaluate Each Statement

Evaluate statement (a), which says the reaction involves a carbocation intermediate: This is true because SN1 reactions form a carbocation. Evaluate statement (b), which says the intermediate involves an sp2 carbon: This is also true since the carbocation intermediate is sp2 hybridized. Statement (c) claims polar solvents accelerate the reaction: This is true, as polar solvents stabilize carbocations. Statement (d) involves inversion of configuration: This is generally false for SN1 reactions, as they can lead to racemization rather than inversion due to the planar nature of the carbocation.

Identify the Incorrect Statement

Based on the analysis, statement (d) is incorrect because SN1 does not lead to inversion of configuration but rather can result in racemization.

Key concepts

Carbocation Intermediate

During an SN1 reaction, such as the solvolysis of 3-methyl-3-bromohexane, one of the critical steps is the formation of a carbocation intermediate. Unlike other reaction mechanisms that involve direct attacks by nucleophiles, the SN1 mechanism first involves the leaving group departing. This departure creates a positively charged carbon atom called a carbocation. Carbocations are electron-deficient species, making them highly reactive. They are pivotal intermediates because their formation is key to the progress of the SN1 reaction. Understanding the stability of carbocations is essential because it influences the reaction rate and its feasibility. Generally, more substituted carbocations (like tertiary carbocations) are more stable due to the additional electron-donating alkyl groups that help stabilize the positive charge.

sp2 Hybridization

Once the carbocation is formed during an SN1 reaction, the carbon atom involved becomes planar due to sp2 hybridization. This change in hybridization provides a unique structural trait that plays a significant role in the reaction's outcome. In sp2 hybridization, the carbon atom's orbitals mix to form three equivalent sp2 hybrid orbitals, with the remaining p-orbital left unhybridized. This gives the carbon an angle of 120 degrees between bonds, creating a trigonal planar configuration. The planar structure is crucial because it allows the nucleophile to attack the carbocation from either side, which can lead to different stereochemical results. This geometric feature is part of what distinguishes SN1 reactions from other substitution mechanisms.

Nucleophilic Substitution

Nucleophilic substitution reactions involve the replacement or substitution of a leaving group by a nucleophile. In the case of SN1 reactions, this is a two-step process, with the departure of the leaving group forming a carbocation intermediate being the first step. Following the formation of the carbocation, the nucleophile comes into play. Because the carbocation has an empty orbital and a positive charge, it is highly susceptible to attack by a nucleophile. The nucleophile then bonds with the carbon that was previously part of the leaving group's bond, completing the substitution. The nature of nucleophilic substitution in SN1 is facilitated by the intermediate's structure and the potential for mixing or retaining of stereochemistry, depending on various factors at play.

Polar Solvents

Polar solvents are particularly important in SN1 reactions. These solvents have a significant impact on the reaction rate and stability of the intermediates involved. The main advantage of polar solvents in an SN1 reaction is their ability to stabilize the charged species, such as the carbocation. They achieve this by surrounding and solvating the ions, effectively lowering the energy barrier of the transition state and promoting the reaction's forward progress. Additionally, polar solvents can help in the dissociation of the leaving group by stabilizing both the carbocation and the leaving group anion. This dual stabilization increases the likelihood of carbocation formation and, consequently, the overall reaction rate.

Racemization

One unique aspect of SN1 reactions is the potential for racemization. This phenomenon occurs due to the planar geometry of the sp2 hybridized carbocation formed during the reaction. Since the carbocation's trigonal planar configuration allows nucleophile attack from both sides of the p-orbital, the product can exist as a mixture of two stereochemical forms: enantiomers. This is why SN1 reactions can lead to a racemic mixture, where there is no preference for the nucleophile attacking from either side of the planar carbocation, resulting in an equimolar mixture of both possible enantiomers. Racemization contrasts with inversion, where only one stereoisomer would dominate, highlighting another key characteristic that differentiates the mechanisms of SN1 and SN2 reactions.