The acid-catalyzed hydration we learned here in Chapter 8 is reversible.

(a) Propose a mechanism for the formation of an alkene from an alcohol.

Verified step by step guidance

1. Step 1: Recognize that the acid-catalyzed dehydration of an alcohol to form an alkene is the reverse of the acid-catalyzed hydration of an alkene. This reaction typically occurs under acidic conditions and involves the removal of a water molecule.
2. Step 2: Protonation of the alcohol: In the presence of an acid (e.g., H₂SO₄ or H₃PO₄), the hydroxyl group (-OH) of the alcohol is protonated to form a better leaving group. This step increases the electrophilicity of the carbon attached to the hydroxyl group. The reaction can be represented as: ${R-OH}_{+}H+\to {R-OH₂}_{+}$
3. Step 3: Formation of the carbocation: The protonated alcohol undergoes elimination of water (H₂O), leaving behind a carbocation intermediate. This step is crucial and depends on the stability of the carbocation formed. The reaction can be represented as: ${R-OH₂}_{+}\to {R-+}_{}+H₂O$
4. Step 4: Rearrangement (if necessary): If the initial carbocation is not the most stable form, a rearrangement (e.g., hydride shift or alkyl shift) may occur to form a more stable carbocation. This step is important for ensuring the formation of the most stable alkene product.
5. Step 5: Formation of the alkene: The carbocation undergoes deprotonation at an adjacent carbon atom, leading to the formation of a double bond (alkene). A base (often the conjugate base of the acid used, such as HSO₄⁻) abstracts a proton from the β-carbon, resulting in the formation of the alkene. The reaction can be represented as: ${R-+}_{}\to {R=R}_{}+H+$

Key Concepts

Acid-Catalyzed Dehydration

Acid-catalyzed dehydration is a reaction where an alcohol is converted into an alkene through the removal of a water molecule. This process typically involves protonation of the alcohol's hydroxyl group, making it a better leaving group, followed by the elimination of water and the formation of a double bond. Understanding this mechanism is crucial for proposing a detailed pathway for the transformation.

Carbocation Stability

Carbocations are positively charged carbon species that play a key role in many organic reactions, including dehydration. The stability of a carbocation is influenced by its degree (primary, secondary, tertiary) and the presence of electron-donating groups. A more stable carbocation is more likely to form during the reaction, which affects the regioselectivity and outcome of the alkene formation.

Elimination Reactions (E1 and E2)

Elimination reactions, such as E1 and E2, describe the mechanisms by which alkenes are formed from alcohols. E1 involves a two-step mechanism where a carbocation intermediate is formed, while E2 is a concerted mechanism that occurs in a single step. Understanding these pathways helps in predicting the conditions and products of the dehydration process.