Show the product(s) you expect from dehydration of the following alcohols when they are heated in sulfuric or phosphoric acid. In each case, use a mechanism to show how the products are formed.
(b)

Verified step by step guidance

1. Step 1: Recognize that the reaction involves the dehydration of an alcohol. Dehydration typically occurs under acidic conditions (e.g., sulfuric or phosphoric acid) and heat, leading to the formation of an alkene.
2. Step 2: Protonation of the alcohol group occurs first. The hydroxyl group (-OH) is protonated by the acid, converting it into a better leaving group (water, H₂O). This step increases the likelihood of the alcohol undergoing elimination.
3. Step 3: Formation of a carbocation intermediate. After the protonation, the water molecule leaves, generating a carbocation at the carbon where the hydroxyl group was originally attached. In this case, the carbocation forms at the secondary carbon.
4. Step 4: Consider carbocation rearrangement. Secondary carbocations can rearrange to form more stable tertiary carbocations if possible. In this molecule, the carbocation is already at a secondary position, and no rearrangement to a tertiary carbocation is possible.
5. Step 5: Elimination of a proton from a β-carbon occurs, forming the double bond. A base (often the conjugate base of the acid used) removes a proton from a β-carbon adjacent to the carbocation, resulting in the formation of the alkene. The major product will follow Zaitsev's rule, favoring the more substituted alkene.

Key Concepts

Dehydration of Alcohols

Dehydration of alcohols involves the elimination of a water molecule from the alcohol, typically in the presence of an acid catalyst like sulfuric or phosphoric acid. This process leads to the formation of alkenes. The reaction can proceed via either an E1 or E2 mechanism, depending on the structure of the alcohol and the reaction conditions.

E1 and E2 Mechanisms

The E1 mechanism is a two-step process where the alcohol first loses a proton to form a carbocation intermediate, followed by the elimination of a leaving group to form the alkene. In contrast, the E2 mechanism is a one-step process where the base abstracts a proton while the leaving group departs simultaneously, leading to the formation of the alkene. The choice between these mechanisms depends on the substrate and conditions.

Regioselectivity and Zaitsev's Rule

Regioselectivity refers to the preference for the formation of one constitutional isomer over others in a chemical reaction. Zaitsev's Rule states that in elimination reactions, the more substituted alkene is typically favored as the major product. This principle is crucial when predicting the products of dehydration reactions, especially when multiple alkene products are possible.