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Ch. 16 - Reactions of Aldehydes and Ketones • More Reactions of Carboxylic Acid Derivatives
Bruice - Organic Chemistry 8th Edition
Bruice8th EditionOrganic ChemistryISBN: 9780135213711Not the one you use?Change textbook
All textbooksBruice 8th EditionCh. 16 - Reactions of Aldehydes and Ketones • More Reactions of Carboxylic Acid DerivativesProblem 95b
Chapter 17, Problem 95b

The pKa values of the carboxylic acid groups of oxaloacetic acid are 2.22 and 3.98.
b. The amount of hydrate present in an aqueous solution of oxaloacetic acid depends on the pH of the solution: 95% at pH 0, 81% at pH 1.3, 35% at pH 3.1, 13% at pH 4.7, 6% at pH 6.7, and 6% at pH 12.7. Explain this pH dependence.

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Step 1: Understand the structure of oxaloacetic acid. It contains two functional groups: carboxylic acids (-COOH) and a ketone group (C=O). The molecule can exist in equilibrium between its keto form and its hydrate form (gem-diol), where the ketone group is converted into two hydroxyl groups (-OH).
Step 2: Analyze the pKa values provided. The pKa values of 2.22 and 3.98 correspond to the dissociation of the two carboxylic acid groups. At lower pH values, the carboxylic acid groups are protonated, while at higher pH values, they are deprotonated. This affects the overall charge and reactivity of the molecule.
Step 3: Examine the pH dependence of the hydrate formation. At very low pH (e.g., pH 0), 95% of oxaloacetic acid exists in the hydrate form. As the pH increases, the percentage of hydrate decreases significantly, reaching only 6% at pH 6.7 and above. This indicates that the keto-hydrate equilibrium is influenced by the protonation state of the molecule.
Step 4: Explain the relationship between pH and hydrate formation. At low pH, the carboxylic acid groups are protonated, making the molecule more electrophilic and favoring the addition of water to the ketone group to form the hydrate. As the pH increases and the carboxylic acid groups become deprotonated, the molecule becomes less electrophilic, reducing the tendency to form the hydrate.
Step 5: Consider the role of steric and electronic effects. The deprotonation of the carboxylic acid groups at higher pH introduces negative charges, which can repel water molecules and destabilize the hydrate form. Additionally, the keto form may be more stable due to resonance stabilization of the carbonyl group, especially at higher pH values.

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Key Concepts

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

pKa and Acid-Base Equilibrium

pKa is a measure of the strength of an acid in solution, representing the pH at which half of the acid is dissociated. Lower pKa values indicate stronger acids, which dissociate more readily in water. Understanding pKa is crucial for predicting the behavior of acids and their conjugate bases in different pH environments, influencing the equilibrium between protonated and deprotonated forms.
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Hydration and pH Relationship

The hydration of oxaloacetic acid is influenced by the pH of the solution, as it affects the ionization of the carboxylic acid groups. At lower pH values, the acid is predominantly in its protonated form, leading to higher hydration levels. As pH increases, the acid loses protons, resulting in decreased hydration due to the formation of less hydrated anionic species.
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Le Chatelier's Principle

Le Chatelier's Principle states that if a system at equilibrium is subjected to a change in concentration, temperature, or pressure, the system will adjust to counteract that change. In the context of oxaloacetic acid, as pH changes, the equilibrium between the hydrated and deprotonated forms shifts, demonstrating how the system responds to changes in proton concentration in the solution.
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Related Practice
Textbook Question

The Baylis-Hillman reaction is a DABCO (1,4-diazabicyclo[2.2.2]octane) catalyzed reaction of an α,β-unsaturated carbonyl compound with an aldehyde to form an allylic alcohol. Propose a mechanism for the reaction. (Hint: DABCO serves as both a nucleophile and as a base in the reaction.)

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Textbook Question

Show how each of the following compounds can be prepared from the given starting material. In each case, you will need to use a protecting group.

a.

b.

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Textbook Question

When a cyclic ketone reacts with diazomethane, the next larger cyclic ketone is formed. This is called a ring-expansion reaction. Draw a mechanism for the following ring-expansion reaction.

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Textbook Question

Identify compounds A and B:

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Textbook Question

To solve this problem, you need to read the description of the Hammett σ\(\sigma\), ρ\(\rho\) treatment given in [Chapter 15, Problem 92]. When the rate constants for the hydrolysis of several morpholine enamines of para-substituted propiophenones are determined at pH 4.7, the ρ\(\rho\) value is positive; however, when the rates of hydrolysis are determined at pH 10.4, the ρ\(\rho\) value is negative.

b. What is the rate-determining step of the hydrolysis reaction when it is carried out in an acidic solution?

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Textbook Question

Propose a mechanism for each of the following reactions:

b.

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