Core Structure of Carboxylic Acids

Steps:

• Recall that carboxylic acids have the general formula R-COOH, where the functional group is -COOH.
• Evaluate option A: It describes the essential -COOH group present in all carboxylic acids.
• Check option B: Ethanoic acid (CH3COOH) reacts with copper(II) oxide but not metallic copper.
• Assess option C: Vinegar contains ethanoic acid from bacterial oxidation of ethanol, not methanoic acid.
• Verify option D: Saturated carboxylic acids like propanoic do not reduce or decolorize acidified KMnO4.

Why A is correct:

• Carboxylic acids are defined by the -COOH functional group, which includes the -C(=O)OH structure, matching the described −C−OH (shorthand for carboxyl).

Why the others are wrong:

• B: Ethanoic acid does not react with copper metal; it only reacts with copper(II) oxide to form a salt.
• C: Methanoic acid (formic acid) is not in vinegar; vinegar's main acid is ethanoic acid from ethanol oxidation.
• D: Propanoic acid is saturated and does not decolorize acidified potassium manganate(VII), which tests for unsaturation or reducing agents like aldehydes.

Final answer: A