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Ch. 7 - Structure and Synthesis of Alkenes; Elimination
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook
All textbooksWade 9th EditionCh. 7 - Structure and Synthesis of Alkenes; EliminationProblem 52c,d
Chapter 7, Problem 52c,d

Predict the major products of acid-catalyzed dehydration of the following alcohols.
(c)
(d)

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1
Step 1: Understand the reaction mechanism. Acid-catalyzed dehydration of alcohols typically proceeds via an E1 or E2 mechanism, depending on the structure of the alcohol. The reaction involves the removal of a water molecule to form an alkene. The stability of the carbocation intermediate (if E1) or the transition state (if E2) determines the major product.
Step 2: Analyze the structure of 2-methylcyclohexanol (part c). Protonation of the hydroxyl group (-OH) by the acid converts it into a better leaving group (water). This is followed by the loss of water to form a carbocation. In this case, the carbocation forms at the carbon bearing the hydroxyl group. Consider possible rearrangements to form a more stable carbocation, such as a hydride or alkyl shift.
Step 3: Apply Zaitsev's rule to predict the major product for 2-methylcyclohexanol. Zaitsev's rule states that the more substituted alkene is generally the major product. After the carbocation forms, elimination of a proton (H⁺) from a β-carbon leads to the formation of the alkene. Identify all possible β-carbons and determine which elimination gives the more substituted alkene.
Step 4: Analyze the structure of 2,2-dimethylpropan-1-ol (part d). Protonation of the hydroxyl group by the acid converts it into a better leaving group (water). However, the primary carbocation that would form is highly unstable. Therefore, a rearrangement occurs to form a more stable tertiary carbocation. This involves a methyl shift from an adjacent carbon.
Step 5: Predict the major product for 2,2-dimethylpropan-1-ol. After the rearranged tertiary carbocation forms, elimination of a proton (H⁺) from a β-carbon leads to the formation of the alkene. Since there is only one β-carbon available, the product will be a single alkene. Ensure the structure of the product reflects the rearrangement and elimination steps.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Acid-Catalyzed Dehydration

Acid-catalyzed dehydration is a reaction where an alcohol is converted into an alkene through the removal of a water molecule, facilitated by an acid. The acid protonates the hydroxyl group, making it a better leaving group, which then leads to the formation of a carbocation intermediate. This process is crucial for predicting the major products, as the stability of the carbocation influences the outcome.
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General features of acid-catalyzed dehydration.

Carbocation Stability

Carbocation stability is a key factor in determining the products of dehydration reactions. Carbocations can be classified as primary, secondary, or tertiary, with tertiary carbocations being the most stable due to hyperconjugation and inductive effects. The more stable the carbocation, the more likely it is to form during the reaction, guiding the formation of the major product.
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Zaitsev's Rule

Zaitsev's Rule states that in elimination reactions, the more substituted alkene is typically the major product. This principle applies to dehydration reactions, where the elimination of water leads to the formation of alkenes. Understanding this rule helps predict which alkene will be favored based on the structure of the starting alcohol and the stability of the resulting alkenes.
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Related Practice
Textbook Question

Write a balanced equation for each reaction, showing the major product you expect.

(b)

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Textbook Question

Write a balanced equation for each reaction, showing the major product you expect.

(a)

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Textbook Question

Predict the products of the following reactions. When more than one product is expected, predict which will be the major product.

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Textbook Question

What halides would undergo E2 dehydrohalogenation to give the following pure alkenes?

d. methylenecyclohexane

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Textbook Question

Predict the major products of acid-catalyzed dehydration of the following alcohols.

(a)

(b)

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Textbook Question

What halides would undergo E2 dehydrohalogenation to give the following pure alkenes?

e. 4-methylcyclohexene

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