Question

Which of the following statement is not correct regarding the solvolysis of 3 -bromo-3-methylhexane? (a) Polar solvents accelerate the reaction. (b) The intermediate involves \(\mathrm{sp}^{2}\) carbon. (c) The rate of the reaction at a particular temperature depends only on the concentration of \(3-\) Bromo- \(3-\) methyl hexane. (d) The reaction involves inversion of configuration.

Short answer

a) Polar solvents accelerate the reaction. b) The intermediate involves \(\mathrm{sp}^{2}\) carbon. c) The rate of the reaction at a particular temperature depends only on the concentration of 3-bromo-3-methylhexane. d) The reaction involves inversion of configuration. Answer: d) The reaction involves inversion of configuration.

Step-by-step solution

Understanding the solvolysis mechanism

The solvolysis of 3-bromo-3-methylhexane involves a nucleophilic substitution reaction, specifically an SN1 reaction. In this mechanism, the reaction occurs in two steps: (1) the leaving group (bromide ion) departs, forming a carbocation intermediate, and (2) the nucleophile (usually the solvent) attacks the carbocation, leading to the final product.

Analyzing option (a)

Option (a) states that polar solvents accelerate the reaction. This statement is correct. Polar solvents stabilize the carbocation intermediate formed in the SN1 reaction mechanism, lowering the overall activation energy and increasing the reaction rate.

Analyzing option (b)

Option (b) states that the intermediate involves \(\mathrm{sp}^{2}\) carbon. This statement is also correct. The carbocation intermediate has a central carbon with three bonds and an empty p orbital, giving it an \(\mathrm{sp}^{2}\) hybridization.

Analyzing option (c)

Option (c) states that the rate of the reaction at a particular temperature depends only on the concentration of 3-bromo-3-methylhexane. This statement is correct. The rate law for an SN1 reaction is given by the equation: Rate \(= k \times [\text{3-bromo-3-methylhexane}]\) The rate law only depends on the concentration of the substrate (3-bromo-3-methylhexane) and the reaction rate constant (k), which is temperature-dependent.

Analyzing option (d)

Option (d) states that the reaction involves inversion of configuration. This statement is not correct. In an SN1 reaction, the nucleophile can attack the carbocation intermediate from either side, leading to a mixture of retention and inversion of configuration. The product will be a racemic mixture if the starting material is optically active. Hence, the statement that is not correct regarding the solvolysis of 3-bromo-3-methylhexane is option (d).

Key concepts

Nucleophilic Substitution

In organic chemistry, nucleophilic substitution reactions are fundamental processes where a nucleophile replaces a leaving group attached to a carbon atom. There are different types of nucleophilic substitution mechanisms, such as SN1 and SN2. The "1" in SN1 refers to the reaction being unimolecular, meaning that the rate-determining step involves only one molecular entity, which is the substrate itself.

A classic example of an SN1 reaction is the solvolysis of 3-bromo-3-methylhexane, where the leaving group is a bromide ion. This type of reaction tends to occur with substrates that can form stable carbocations, such as tertiary alkyl halides. The solvent often plays a dual role as a decisive nucleophile in the reaction.

To remember:

• In SN1, the rate is not affected by the nucleophile's concentration.
• Polar solvents are crucial to stabilize the intermediate.
• The reaction's speed is determined by how quickly the leaving group departs.

Carbocation Intermediate

Carbocations are essential intermediates in many organic reactions, including the SN1 reaction mechanism. Once the leaving group detaches from the substrate, a carbocation is formed—specifically, a positively charged carbon atom known as the carbocation.

In the case of 3-bromo-3-methylhexane, the central carbon is connected to three other carbon atoms and holds a positive charge, making it tertiary. This configuration significantly influences its reactivity, as tertiary carbocations are more stable than primary or secondary ones due to hyperconjugation and the inductive effect.

Key points to understand carbocations better:

• Carbocations have an \(\mathrm{sp}^{2}\\)hybridization with a planar structure.
• They have an empty p-orbital, making them highly reactive.
• Their stabilization is crucial; hence, polar solvents or nearby electron-donating groups help stabilize them.

Reaction Mechanism

A reaction mechanism outlines the step-by-step events occurring during a chemical reaction. For the SN1 reaction, the mechanism involves two main steps.

First Step: Formation of Carbocation
In this initial step, the leaving group departs from the substrate, resulting in the creation of a carbocation. This step is typically the slowest, thereby making it the rate-determining step of the reaction. The ease with which the leaving group departs heavily influences the overall reaction rate.

Second Step: Nucleophilic Attack
After the carbocation forms, a nucleophile—often the solvent—attacks the positively charged carbon. This interaction completes the substitution process and forms the final product. Depending on the reaction conditions, the nucleophile can approach the carbocation from either side, leading to a mixture of stereoisomers.

Key takeaways regarding SN1 reaction mechanisms:

• The two-step nature allows for a possible rearrangement of the intermediate if more stable forms are attainable.
• The use of polar solvents can tremendously accelerate the reaction by stabilizing the carbocation.
• This process often generates racemic mixtures when optically active starting materials are used.