Propose mechanisms for the following reactions. In most cases, more products are formed than are shown here. You only need to explain the formation of the products shown, however.
(a)

Verified step by step guidance

1. Step 1: Recognize the reaction conditions. The presence of HCl and ZnCl₂ indicates that this is a Lucas test reaction, which involves the conversion of alcohols to alkyl halides via an SN1 mechanism. ZnCl₂ acts as a Lewis acid, enhancing the reactivity of the alcohol.
2. Step 2: Protonation of the alcohol group. The hydroxyl group (-OH) on the cyclohexylmethanol is protonated by HCl, forming water as a leaving group. This step increases the electrophilicity of the carbon attached to the hydroxyl group.
3. Step 3: Formation of a carbocation intermediate. After the water molecule leaves, a carbocation is formed at the benzylic position (CH₂ group attached to the cyclohexane ring). This carbocation is relatively stable due to hyperconjugation and inductive effects from the cyclohexane ring.
4. Step 4: Rearrangement of the carbocation. A hydride shift occurs from the cyclohexane ring to the carbocation, resulting in a more stable secondary carbocation directly on the cyclohexane ring. This rearrangement explains the formation of the minor product.
5. Step 5: Nucleophilic attack by chloride ion. The chloride ion (Cl⁻) from HCl attacks the secondary carbocation, forming the final product, chlorocyclohexane. This product is labeled as the minor product because the primary product would involve substitution at the original benzylic position without rearrangement.

Key Concepts

Reaction Mechanisms

A reaction mechanism is a step-by-step description of how a chemical reaction occurs at the molecular level. It outlines the sequence of elementary steps, including bond breaking and formation, that lead to the transformation of reactants into products. Understanding mechanisms is crucial for predicting the products of reactions and for explaining why certain pathways are favored over others.

Types of Organic Reactions

Organic reactions can be classified into several types, including substitution, addition, elimination, and rearrangement reactions. Each type has distinct characteristics and mechanisms. Recognizing the type of reaction helps in predicting the behavior of reactants and the nature of the products formed, which is essential for proposing accurate mechanisms.

Stability of Intermediates

The stability of reaction intermediates, such as carbocations, carbanions, and free radicals, plays a significant role in determining the pathway of a reaction. More stable intermediates are generally formed preferentially, influencing the overall reaction mechanism and the products generated. Understanding the factors that affect stability, such as resonance and inductive effects, is vital for accurately proposing mechanisms.