Protonation converts the hydroxy group of an alcohol to a good leaving group. Suggest a mechanism for each reaction.
(a)

Verified step by step guidance

1. Step 1: Protonation of the alcohol group: The hydroxyl group (-OH) in the alcohol is a poor leaving group. In the presence of HBr, the oxygen atom of the hydroxyl group is protonated, forming a positively charged oxonium ion (R-OH2+), which is a much better leaving group.
2. Step 2: Determine the reaction mechanism (SN1 or SN2): The reaction mechanism depends on the structure of the alcohol. In this case, the alcohol is a secondary alcohol (cyclohexanol). Secondary alcohols can undergo either SN1 or SN2 mechanisms, but the reaction conditions (heat and the presence of a strong acid like HBr) favor the SN1 mechanism.
3. Step 3: Formation of the carbocation intermediate: In the SN1 mechanism, the protonated hydroxyl group (R-OH2+) leaves, forming a carbocation intermediate (cyclohexyl carbocation). This step is the rate-determining step of the reaction.
4. Step 4: Nucleophilic attack by bromide ion: The bromide ion (Br⁻), which is generated from the dissociation of HBr, acts as a nucleophile and attacks the carbocation intermediate. This results in the formation of the final product, cyclohexyl bromide.
5. Step 5: Formation of water as a byproduct: During the reaction, the leaving group (H2O) is formed as a byproduct when the protonated hydroxyl group departs. The overall reaction converts cyclohexanol to cyclohexyl bromide with water as a byproduct.

Key Concepts

Protonation of Alcohols

Protonation is the addition of a proton (H+) to a molecule, which in the case of alcohols, converts the hydroxy (-OH) group into a better leaving group, water (H2O). This process enhances the electrophilicity of the carbon atom bonded to the hydroxy group, facilitating nucleophilic substitution reactions.

Nucleophilic Substitution Mechanisms (SN1 and SN2)

Nucleophilic substitution reactions can occur via two primary mechanisms: SN1 and SN2. SN1 is a two-step process involving the formation of a carbocation intermediate, while SN2 is a one-step process where the nucleophile attacks the substrate simultaneously as the leaving group departs. The choice between these mechanisms depends on factors like substrate structure and the nature of the nucleophile.

Role of HBr in Reaction

Hydrobromic acid (HBr) serves as both a proton source and a nucleophile in the reaction. It protonates the alcohol, converting it into a better leaving group, and the bromide ion (Br-) acts as a strong nucleophile that can attack the electrophilic carbon, leading to the substitution product. The heat applied can also facilitate the reaction by increasing the kinetic energy of the molecules involved.