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Chapter 3: Problem 6

Phenol is an acid but does not react with saturated \(\mathrm{NaHCO}_{3}\) solution. Why is it so?

Short Answer

Expert verified
Answer: Phenol doesn't react with saturated \(\mathrm{NaHCO}_{3}\) solution because it is a weaker acid compared to carbonic acid. As a result, the equilibrium of the reaction lies towards the reactant side, making the reaction unfavorable.

Step by step solution

01

Write down the chemical reaction involved

Write the chemical reaction between phenol (C\(_{6}\)H\(_{5}\)OH) and sodium bicarbonate (NaHCO\(_{3}\)) if it were to react: C\(_{6}\)H\(_{5}\)OH + NaHCO\(_{3}\) <=> C\(_{6}\)H\(_{5}\)O\(^{-}\) + Na\(^{+}\) + H\(_{2}\)O + CO\(_{2}\)
02

Determine if the reaction should occur

We need to compare the acid strength of phenol with that of carbonic acid, which forms when \(\mathrm{NaHCO}_{3}\) dissolves in water: H\(_{2}\)O + CO\(_{2}\) <=> H\(_{2}\)CO\(_{3}\) Generally, an acid-base reaction occurs when the conjugate base of the acid is weaker than the base in the reaction (the one that donates the proton). In this case, the base in the reaction is bicarbonate, HCO\(_{3}^{-}\). The conjugate base of phenol is the phenoxide ion, C\(_{6}\)H\(_{5}\)O\(^{-}\).
03

Compare the acid dissociation constants (K\(_{a}\)) for phenol and carbonic acid

We must find the K\(_{a}\) values for both the phenol and carbonic acid to compare their relative acidic strengths. K\(_{a}\) for phenol: \(\approx 10^{-10}\) K\(_{a}\) for carbonic acid: \(\approx 10^{-6.4}\) (for the first proton) Since the K\(_{a}\) value of phenol is smaller than that of carbonic acid, it means that phenol is a weaker acid than carbonic acid.
04

Analyze the equilibrium between reactants and products

As phenol is a weaker acid compared to carbonic acid and the conjugate base of phenol (C\(_{6}\)H\(_{5}\)O\(^{-}\)) is stronger than that of carbonic acid (HCO\(_{3}^{-}\)), the equilibrium of the reaction lies far to the left favoring the reactants. Consequently, phenol does not react significantly with a sodium bicarbonate solution. So, phenol doesn't react with saturated \(\mathrm{NaHCO}_{3}\) solution because it is a weaker acid compared to carbonic acid. As a result, the reaction isn't favorable, and the equilibrium lies towards the reactant side.

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