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Ch. 5 - Chemical Reaction Analysis: Thermodynamics and Kinetics
Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition
Mullins1st EditionOrganic Chemistry: A Learner Centered ApproachISBN: 9780137566471Not the one you use?Change textbook
All textbooksMullins 1st EditionCh. 5 - Chemical Reaction Analysis: Thermodynamics and KineticsProblem 63b
Chapter 4, Problem 63b

Calculate Keq for the following acid–base reaction.

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Step 1: Identify the acid and base on both sides of the reaction. In this reaction, the methyl anion (CH₃⁻) acts as the base, and isopropanol (CH₃CHOHCH₃) acts as the acid on the left side. On the right side, methane (CH₄) is the conjugate acid, and the isopropoxide ion (CH₃CH⁻OCH₃) is the conjugate base.
Step 2: Recall the relationship between pKa values and equilibrium constants (Keq). The equilibrium constant for an acid-base reaction can be calculated using the formula: Keq = 10^(pKa(acid on left) - pKa(acid on right)).
Step 3: Look up the pKa values for the acids involved. The pKa of isopropanol (CH₃CHOHCH₃) is approximately 16. The pKa of methane (CH₄) is approximately 50.
Step 4: Substitute the pKa values into the formula for Keq. Using the formula Keq = 10^(pKa(acid on left) - pKa(acid on right)), substitute pKa(acid on left) = 16 and pKa(acid on right) = 50.
Step 5: Simplify the expression to determine the magnitude of Keq. The calculation will show that Keq is a very small number, indicating that the equilibrium strongly favors the reactants (left side of the reaction).

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Key Concepts

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

Acid-Base Reactions

Acid-base reactions involve the transfer of protons (H+) between reactants. In organic chemistry, these reactions are fundamental for understanding the behavior of various compounds, particularly in determining their reactivity and stability. The strength of acids and bases is often quantified using the pKa scale, which helps predict the direction of the reaction.
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The Lewis definition of acids and bases.

Equilibrium Constant (K_eq)

The equilibrium constant (K_eq) is a numerical value that expresses the ratio of the concentrations of products to reactants at equilibrium for a given reaction. It provides insight into the favorability of a reaction; a K_eq greater than 1 indicates that products are favored, while a K_eq less than 1 suggests that reactants are favored. Calculating K_eq is essential for understanding the extent of acid-base reactions.
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The relationship between equilibrium constant and pKa.

Relationship Between pKa and K_eq

The relationship between pKa and K_eq is crucial in acid-base chemistry. The pKa of an acid is the negative logarithm of its acid dissociation constant (Ka), and it can be used to derive K_eq for acid-base reactions. Specifically, K_eq can be calculated using the difference in pKa values of the acids and bases involved, allowing for a quantitative assessment of the reaction's equilibrium position.
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Related Practice
Textbook Question

Keto–enol tautomerism is a reaction we discuss in detail in Chapter 19. Estimate the equilibrium constant of this reaction (BDE for C―C π bond = 65 kcal/mol ; for C―O π bond = 85 kcal/mol).

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Textbook Question

Reaction (c), on the other hand, is favored (∆G° < 0). Identify the bonds formed and broken and explain this result in light of (a) and (b).

(c)

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Textbook Question

In light of your answers to parts (b) and (c), where both were shown to be quite favorable, imagine a scenario where either reaction is possible. Of the two, which would you expect to be faster? Which would you expect to be more favored? Explain each in the context of the important thermodynamic and/or kinetic parameters.

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Textbook Question

The hydrogenation of alkenes is a reaction we study in Chapter 9.

(b) Is this reaction favored or disfavored in terms of entropy?

Textbook Question

Parts (a)–(d) of this assessment assist in the development of what will become a common theme in organic reactions and should be worked in order. [Think carefully about how each question relates to the others.]

(c) Without worrying about the mechanism of the reaction, estimate an equilibrium constant for the following carbonyl addition reaction based on the relative stability of the Lewis bases.

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Textbook Question

In Chapter 13, we discuss the ring-opening reactions of epoxides, such as the one shown here.

(a) Based on the bonds formed and the bonds broken, calculate ∆H°.

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