Question: The pK_a of ascorbic acid (vitamin C, page 9) is 4.17 , showing that it is lightly more acidic than acetic acid (CH₃COOH, pK_a = 4.74).

(a)Show four different conjugate bases that would be formed by deprotonation of the four different OH groups in ascorbic acid.

(b) Compare the stabilities of these four conjugate bases, and predict which OH group of ascorbic acid is the most acidic.

(c) Compare the most stable conjugate base of ascorbic acid with the conjugate base of acetic acid, and suggest why these two compounds have similar acidities, even though ascorbic acid lacks the carboxylic acid (COOH) group

The residual part of acid after losing a proton (H⁺) will have a tendency to accept a proton (H⁺). Therefore, it will behave as a base. These pairs of substances which differ from one another by a proton (H⁺) are known as conjugate acid-base pairs. Consider a general example of an acid:

A single structural formula sometimes cannot explain all the properties of a compound that is given. In such cases, the compound may be represented by two or more structural formulae which differ from each other only in the arrangement of electrons. None of this structural formula alone can explain all the observed properties of the compound. The compound is then said to show resonance. The various structures are known as resonating structures. The true structure of the molecule is not represented by any of the resonating structures but is considered to be a resonance hybrid of the various resonating structures. Resonance delocalization is considered as a dominating effect in stabilizing the conjugate base.

K_a is known as the acid dissociation constant and its value indicates the relative strength of the acid. pK_a is defined as the negative logarithm (base 10 ) of K_a. Mathematically, pK_a = -logK_a. pK_avalue of strong acids is usually zero or even negative while the pK_a value of weaker acids are greater than 4.

pK_avalues of different compounds can be compared to determine which acid is better. Lower value of pK_a implies higher acidity.

(a)

Conjugate bases formed by deprotonation of four different groups in ascorbic acid.

(b) Resonance delocalization is considered as a dominating effect in stabilizing the conjugate base.

Conjugate bases in order from most stable to least stable

The most stable conjugate base is stabilized by resonance where the negative charge is delocalized within the molecule.

most acidic OH group of ascorbic acid is rounded

(c)

conjugate base of ascorbic acid conjugate base of acetic acid

The conjugate base of ascorbic acid is more stable than the conjugate base of acetic acid because the conjugate base of ascorbic acid has three resonance forms (as can be seen in part ‘b’) while the conjugate base of acetic acid has only two resonance forms. The negative charge present in both the acid is delocalized between two oxygen atoms. This implies for the similarity in pK_a values of the two acids.