Which of the following has highest pKa values?

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This question was previously asked in

DSSSB PGT Chemistry (Female) Official Paper (Held On: 06 Jul, 2018 Shift 1)

Answer 1

CONCEPT:

pKa and Acidity

pKa is the negative logarithm of the acid dissociation constant (Ka) of a compound. It is a measure of the strength of an acid.
A lower pKa value indicates a stronger acid, as the acid dissociates more easily in solution.
A higher pKa value indicates a weaker acid, as the acid dissociates less easily.
The presence of electron-withdrawing groups (such as fluorine atoms) increases the acidity of a molecule by stabilizing the conjugate base, thus lowering the pKa value.

EXPLANATION:

Acetic acid (CH₃COOH) has no electron-withdrawing groups attached to the carbon adjacent to the carboxylic group. Therefore, it has the highest pKa value, as it is the weakest acid.
Fluoroacetic acid (FCH₂COOH) has one fluorine atom attached to the carbon adjacent to the carboxylic group. This fluorine atom is an electron-withdrawing group, which increases the acidity (lowers the pKa value).
Difluoroacetic acid (F₂CHCOOH) has two fluorine atoms, further increasing the acidity compared to fluoroacetic acid.
Trifluoroacetic acid (F₃CCOOH) has three fluorine atoms, making it the most acidic compound among the given options (lowest pKa value).
Since acetic acid lacks electron-withdrawing groups, it has the highest pKa value among the options.

Therefore, the correct answer is Acetic acid.

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