Using pK_a values, calculate an approximate K_eq value for the following substitution reaction.

검증된 단계별 안내

Step 1: Identify the acids and conjugate bases involved in the reaction. In this substitution reaction, the acetylide ion (CH≡C⁻) acts as a base, and the alkyl iodide (R-I) undergoes substitution to form a new alkyne (R-CH≡C) and iodide ion (I⁻).

Step 2: Determine the pKₐ values of the relevant acids. The pKₐ of the terminal alkyne (R-CH≡C-H) is approximately 25, and the pKₐ of the conjugate acid of iodide (HI) is approximately -10.

Step 3: Use the relationship between pKₐ values and equilibrium constants (Keq). The equilibrium constant can be calculated using the formula: Keq = 10^(pKₐ of acid on the reactant side - pKₐ of acid on the product side).

Step 4: Substitute the pKₐ values into the formula. For this reaction, the acid on the reactant side is HI (pKₐ = -10), and the acid on the product side is the terminal alkyne (pKₐ = 25).

Step 5: Calculate the approximate Keq value using the formula. The difference in pKₐ values (-10 - 25) will determine the exponent for the equilibrium constant calculation. This step provides the theoretical basis for determining the equilibrium constant.

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주요 개념

질문에 올바르게 답하기 위해 반드시 이해해야 하는 핵심 개념들은 다음과 같습니다.

pKₐ and Acid-Base Equilibrium

pKₐ is a measure of the strength of an acid in solution, defined as the negative logarithm of the acid dissociation constant (Kₐ). It indicates how readily an acid donates protons (H⁺) to a base. In equilibrium reactions, the difference in pKₐ values between the acids and bases involved can be used to estimate the equilibrium constant (K_eq) for the reaction, as K_eq is related to the relative strengths of the acids and bases.

Equilibrium Constant (K_eq)

The equilibrium constant (K_eq) quantifies the ratio of the concentrations of products to reactants at equilibrium for a reversible reaction. It is calculated using the formula K_eq = [products]/[reactants], where the concentrations are raised to the power of their stoichiometric coefficients. A larger K_eq indicates a reaction that favors product formation, while a smaller K_eq suggests that reactants are favored.

Substitution Reactions

Substitution reactions involve the replacement of one functional group in a molecule with another. In organic chemistry, these reactions can be classified as nucleophilic or electrophilic, depending on whether the attacking species is a nucleophile (electron-rich) or an electrophile (electron-poor). Understanding the mechanism and the stability of intermediates is crucial for predicting the outcome and the equilibrium of these reactions.

Using pKa values, calculate an approximate Keq value for the following substitution reaction.