Question

The correct order of increasing pKa values of the compounds:
(A) CH$_3$COOH
(B) CH$_2$OCH$_2$COOH
(C) Cl$_3$COOH
(D) (CH$_3$)$_3$CCOOH
Choose the correct answer from the options given below:

• A. A $<$ B $<$ C $<$ D
• B. A $<$ C $<$ B $<$ D
• C. B $<$ A $<$ C $<$ D
• D. C $<$ B $<$ A $<$ D

Answer 1

The Correct Option is A

The pKa value is inversely related to the strength of an acid. Trichloroacetic acid (Cl$_3$COOH) is the strongest acid with the lowest pKa due to the electron-withdrawing effect of the chlorine atoms. Glycolic acid (CH$_2$OCH$_2$COOH) is more acidic than acetic acid (CH$_3$COOH) due to its ability to stabilize the conjugate base. The bulky tert-butyl group in (CH$_3$)$_3$CCOOH decreases acidity, giving it the highest pKa.

Answer 2

The pKa value is inversely related to the strength of an acid: the lower the pKa, the stronger the acid. This is because a lower pKa indicates that the acid dissociates more readily, releasing more hydrogen ions in solution.

Trichloroacetic acid (Cl₃COOH) is one of the strongest acids, with the lowest pKa, due to the strong electron-withdrawing effect of the three chlorine atoms attached to the carbonyl group. This effect stabilizes the negative charge on the conjugate base (the acetate ion), making it easier for the acid to dissociate and release a proton.

Glycolic acid (CH₂OHCOOH) is more acidic than acetic acid (CH₃COOH) because of the -OH group attached to the alpha-carbon (next to the carboxyl group). This -OH group can form hydrogen bonds and stabilize the conjugate base through inductive and hydrogen-bonding interactions, enhancing the acidity of glycolic acid relative to acetic acid.

Acetic acid (CH₃COOH) is less acidic than glycolic acid, as it lacks the stabilizing effect of the -OH group and therefore has a higher pKa value compared to glycolic acid.

The bulky tert-butyl group (CH₃)₃CCOOH significantly decreases the acidity of the molecule, leading to a higher pKa. The large size of the tert-butyl group creates steric hindrance, making it difficult for the molecule to stabilize the negative charge on the conjugate base. This reduced ability to stabilize the conjugate base results in a lower tendency to dissociate, hence increasing the pKa value.

In conclusion, the strength of an acid is determined by its pKa value, with factors such as electron-withdrawing effects, conjugate base stabilization, and steric hindrance influencing its acidity. Trichloroacetic acid is the strongest, while the bulky tert-butyl group in (CH₃)₃CCOOH leads to the weakest acidity.