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Difficulty: Very hardAlkanoic Acids, Esterification, Saponification, Fats, and Oils

An organic compound XX with the molecular formula C4H8O2\text{C}_4\text{H}_8\text{O}_2 does not react with sodium hydrogentrioxocarbonate(IV) solution to liberate gas. Upon refluxing XX with dilute sodium hydroxide solution, two organic products, YY and ZZ, are formed. Acidification of product ZZ yields ethanoic acid, while mild oxidation of product YY yields an alkanal that gives a positive Tollen's test. Which of the following correctly identifies the IUPAC name of compound XX and the chemical nature of its reaction with dilute sodium hydroxide?

  1. Ethyl ethanoate; an irreversible alkaline hydrolysis reactionAnswer
  2. B
    Ethyl ethanoate; a reversible esterification equilibrium reaction
  3. C
    Methyl propanoate; an irreversible alkaline hydrolysis reaction
  4. D
    Butanoic acid; a reversible acid-base neutralization reaction

Answer

The IUPAC name of compound XX is ethyl ethanoate, and its reaction with dilute sodium hydroxide is an irreversible alkaline hydrolysis reaction.
Compound XX does not evolve carbon(IV) oxide gas with sodium hydrogentrioxocarbonate(IV), ruling out alkanoic acids and establishing that XX is an ester. Alkaline hydrolysis of XX yields sodium ethanoate (ZZ) and ethanol (YY), because acidification of ZZ produces ethanoic acid (22 carbons) and oxidation of ethanol (YY) yields ethanal, an alkanal that gives a silver mirror with Tollen's reagent. Therefore, XX is ethyl ethanoate (CH3COOCH2CH3\text{CH}_3\text{COOCH}_2\text{CH}_3). Base hydrolysis of esters converts the carboxyl moiety into a resonance-stabilized carboxylate ion, preventing the reverse reaction and rendering the hydrolysis irreversible.

Step-by-Step Solution

1
Determine the functional group class of compound XX
Compound XX is an ester.
Isomers with formula C4H8O2\text{C}_4\text{H}_8\text{O}_2 can be alkanoic acids or esters. Since XX does not react with NaHCO3\text{NaHCO}_3 to evolve CO2\text{CO}_2 gas, it lacks the free carboxyl acid group (−COOH-\text{COOH}) and must be an ester.
2
Deduce the structure of the alkanoate (acid) portion of the ester
The alkanoate part is ethanoate (CH3COO−\text{CH}_3\text{COO}^-).
Alkaline hydrolysis of ester XX produces carboxylate salt ZZ. Acidification of ZZ yields ethanoic acid (CH3COOH\text{CH}_3\text{COOH}), which contains 2 carbon atoms.
3
Deduce the structure of the alkyl (alcohol) portion of the ester
The alkyl group is ethyl (−C2H5-\text{C}_2\text{H}_5), making YY ethanol (CH3CH2OH\text{CH}_3\text{CH}_2\text{OH}).
The total number of carbon atoms in XX is 4. Subtracting 2 carbons from the acid part leaves 2 carbons for the alcohol YY (ethanol). Mild oxidation of ethanol yields ethanal (an alkanal), which reduces Tollen's reagent.
4
Identify the reaction type with dilute NaOH\text{NaOH}
Irreversible alkaline hydrolysis (saponification).
Hydrolysis of an ester with alkali consumes hydroxyl ions (OH−\text{OH}^-) to form an unreactive carboxylate anion (CH3COO−\text{CH}_3\text{COO}^-), driving the reaction to completion irreversibly.

Key Concept

Chemical differentiation between carboxylic acids and esters, ester hydrolysis kinetics, and structural deduction.
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Topics

Alkanoic Acids, Esterification, Saponification, Fats, and Oils

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