Ch. 13 - Alcohols, Ethers and Related Compounds: Substitution and Elimination

Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition

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Mullins 1st Edition

Ch. 13 - Alcohols, Ethers and Related Compounds: Substitution and Elimination

Problem 46c

Chapter 12, Problem 46c

Predict the product of the following pinacol rearrangements.
(c)

Verified step by step guidance

Step 1: Recognize that the reaction involves a pinacol rearrangement, which typically occurs under acidic conditions (H2SO4) and heat (Δ). This rearrangement converts a vicinal diol into a ketone or aldehyde through a carbocation intermediate.

Step 2: Protonate one of the hydroxyl groups using H2SO4 to form a good leaving group (water). This step increases the electrophilicity of the carbon attached to the protonated hydroxyl group.

Step 3: Eliminate the water molecule to generate a carbocation intermediate. The carbocation will form at the carbon that was initially bonded to the protonated hydroxyl group.

Step 4: Perform a 1,2-methyl shift (or hydride shift, depending on the structure) to stabilize the carbocation. This rearrangement moves a neighboring group to the carbocation center, creating a more stable carbocation.

Step 5: Deprotonate the remaining hydroxyl group to form the final product, which is typically a ketone or aldehyde. In this case, the product will likely be a cyclic ketone due to the structure of the starting material.

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Key Concepts

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

Pinacol Rearrangement

The pinacol rearrangement is a chemical reaction involving the conversion of a pinacol (a 1,2-diol) into a ketone or aldehyde through a series of steps that include dehydration and rearrangement. This reaction typically occurs under acidic conditions and involves the migration of an alkyl group, leading to the formation of a more stable carbonyl compound.

Carbocation Stability

Carbocation stability is a crucial concept in organic chemistry, as the stability of the carbocation intermediate formed during the pinacol rearrangement significantly influences the reaction pathway and product distribution. Tertiary carbocations are more stable than secondary, which are more stable than primary, due to hyperconjugation and inductive effects from surrounding alkyl groups.

Mechanism of Rearrangement

Understanding the mechanism of the pinacol rearrangement is essential for predicting the products. The mechanism involves protonation of the hydroxyl group, loss of water to form a carbocation, and subsequent rearrangement of the carbocation to a more stable form, followed by deprotonation to yield the final carbonyl product. This stepwise process highlights the importance of both the initial substrate structure and the reaction conditions.

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Related Practice

Textbook Question

Show a reaction coordinate diagram for the two processes in Figure 13.41 that rationalizes pathway B as the one that gives the major product.

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Textbook Question

Suggest an alkene that, in two steps, could be converted into each of the following ketones. Each sequence should involve a pinacol rearrangement.

(c)

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Textbook Question

The intermediates for the Swern oxidation, a reaction introduced in Section 13.9.4, are shown. Provide the arrow-pushing mechanism that rationalizes the formation of each intermediate and the final product(s).

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Textbook Question

In Assessment 13.48, identify the individual step that represents the general mechanism for all alcohol oxidation reactions.

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Textbook Question

Predict the product of the following pinacol rearrangements.

(b)

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Textbook Question

Identify two different alcohols that can be dehydrated (one without rearrangement) to form the alkene shown.