Textbook Question
Given that the indicated pKa values correspond to the acid dissociation reactions shown, calculate the ratio of acid to conjugate base for the reactions shown.
(b)
3. Acids and Bases
Equilibrium Constant
4:10 minutesProblem 17c
Textbook Question
Estimate the Keq for the following reactions based on the stability of the anions involved.
(c) 
Verified step by step guidance1
Analyze the reaction: The left side of the equilibrium involves a cyclopropyl anion, while the right side involves a linear alkyl anion. The reaction is a ring-opening process where the cyclopropyl anion is converted into a linear alkyl anion.
Consider the stability of the anions: Cyclopropyl anions are destabilized due to angle strain in the three-membered ring. The linear alkyl anion on the right side is more stable because it lacks ring strain and allows for better delocalization of charge.
Evaluate the electronic effects: The linear alkyl anion benefits from hyperconjugation and inductive effects, which help stabilize the negative charge. Cyclopropyl anions do not have these stabilizing effects to the same extent.
Relate stability to equilibrium constant (K_eq): Since the linear alkyl anion is more stable, the equilibrium will favor the formation of the products (right side of the reaction). A higher stability of the products corresponds to a larger K_eq value.
Conclude the reasoning: The equilibrium constant (K_eq) for this reaction is expected to be greater than 1, as the reaction favors the formation of the more stable linear alkyl anion over the cyclopropyl anion.
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Video duration:
4mKey Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Equilibrium Constant (K_eq)
The equilibrium constant (K_eq) quantifies the ratio of the concentrations of products to reactants at equilibrium for a given reaction. It is a crucial parameter in understanding the extent of a reaction and is influenced by the stability of the species involved. A higher K_eq indicates a greater tendency for the reaction to favor products, while a lower K_eq suggests a preference for reactants.
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The relationship between equilibrium constant and pKa.
Stability of Anions
The stability of anions is a key factor in determining the favorability of a reaction. Anions that are more stable (due to factors like resonance, electronegativity, and size) will contribute to a higher K_eq for the reaction. Understanding how different structural features affect anion stability helps predict the direction of equilibrium and the relative concentrations of reactants and products.
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Acid-Base Reactions
Acid-base reactions involve the transfer of protons (H+) between species, leading to the formation of anions and cations. The strength of acids and bases can be assessed through their conjugate pairs, where a stronger acid has a weaker conjugate base. This relationship is essential for estimating K_eq, as the stability of the resulting anions directly impacts the equilibrium position of the reaction.
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02:49The Lewis definition of acids and bases.
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