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Difficulty: HardAlkanoic Acids, Esterification, Saponification, Fats, and Oils

A student subjects separate samples of methyl propanoate to two different laboratory reactions:

Reaction 1: Refluxing with dilute HCl(aq)\text{HCl}(aq)
Reaction 2: Refluxing with aqueous NaOH(aq)\text{NaOH}(aq)

Which of the following correctly compares the reversibility of these reactions and the nature of the organic products formed?

  1. Reaction 1 is reversible yielding propanoic acid and methanol, whereas Reaction 2 is irreversible yielding sodium propanoate and methanol.Answer
  2. B
    Both Reaction 1 and Reaction 2 are fully reversible equilibrium processes yielding propanoic acid and methanol.
  3. C
    Reaction 1 is irreversible yielding sodium propanoate, whereas Reaction 2 is reversible yielding propanoic acid.
  4. D
    Both Reaction 1 and Reaction 2 are irreversible single-step neutralizations yielding sodium propanoate and methanol.

Answer

Reaction 1 is reversible yielding propanoic acid and methanol, whereas Reaction 2 is irreversible yielding sodium propanoate and methanol.
The statement specifying that Reaction 1 is reversible yielding propanoic acid and methanol, while Reaction 2 is irreversible yielding sodium propanoate and methanol, is correct. Acid-catalyzed hydrolysis of an ester is an equilibrium process that forms the parent alkanoic acid and alkanol. In contrast, alkaline hydrolysis (saponification) uses hydroxide ions to deprotonate the acid as it forms, producing an unreactive alkanoate salt (sodium propanoate) which prevents the reverse reaction, making the process quantitative and irreversible.

Step-by-Step Solution

1
Analyze Reaction 1 (Acid Hydrolysis)
Methyl propanoate reacts with water in the presence of H+\text{H}^+ catalyst to form propanoic acid (C2H5COOH\text{C}_2\text{H}_5\text{COOH}) and methanol (CH3OH\text{CH}_3\text{OH}).
Acid hydrolysis of an ester is the reverse of esterification and reaches dynamic equilibrium (it is reversible).
2
Analyze Reaction 2 (Alkaline Hydrolysis / Saponification)
Methyl propanoate reacts with OH−\text{OH}^- ions to form the propanoate anion (C2H5COO−\text{C}_2\text{H}_5\text{COO}^-) as sodium propanoate salt and methanol (CH3OH\text{CH}_3\text{OH}).
The base reacts un-reversibly with the carboxylic acid product to form a carboxylate salt, pulling the equilibrium completely to the right and making the overall saponification irreversible.
3
Compare both reactions
Reaction 1 is reversible (producing acid + alcohol), while Reaction 2 is irreversible (producing salt + alcohol).
Differentiates reversible acid-catalyzed ester hydrolysis from irreversible base-promoted ester hydrolysis.

Key Concept

Difference between acid hydrolysis (reversible) and alkaline hydrolysis/saponification (irreversible) of esters.
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Topics

Alkanoic Acids, Esterification, Saponification, Fats, and Oils

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