Q34P Given the structure of ascorbic ... [FREE SOLUTION]

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Chapter 20: Q34P (page 1038)

Given the structure of ascorbic acid (vitamin C):
(a) Is ascorbic acid a carboxylic acid?
(b) Compare the acid strength of ascorbic acid (pK_a=4.71) with acetic acid.
(c) Predict which proton in ascorbic acid is the most acidic.
(d) Draw the form of ascorbic acid that is present in the body (aqueous solution, pH = 7.4).

Short Answer

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(a) Ascorbic acid is not a carboxylic acid, but it is an example of ene-diol structure.

(b) The acid strengths of both acids are almost identical.

(d) The form of ascorbic acid present in the body is,

Step by step solution

Ascorbic acid

Ascorbic acid is the purest form of vitamin C, having a chemical formula C₆ H₈ O₆, and its deficiency in the body leads to scurvy disease.Many citrus fruits and vegetables contain vitamin C like orange, lemon, and kiwi.

Ascorbic acid is not a carboxylic acid

(a) Ascorbic acid is not a carboxylic acid; instead, it is an example of ene-diol structure having one alkene bond and two OH groups (diol). Here, one of the OH groups is unusually acidic because of the presence of the carbonyl group at the adjacent carbon. The structure of ascorbic acid is shown as follows,

Comparison of acidic strength of ascorbic acid with acetic acid

(b) ThepK_a value of ascorbic acid is 4.71, and that of acetic acid is 4.74. Thus, the acid strengths of both acids are almost identical.

The most acidic proton in ascorbic acid

(c) The most acidic proton in ascorbic acid is the one that gives us the most stable conjugate base when removed. The resonance forms after the removal of one proton in ascorbic acid are shown as,

The form of ascorbic acid which is present in the body

(d) The form of ascorbic acid, which is present in the body, has a slightly basic pH of physiological fluid and is present as the conjugate base 鈥榓scorbate鈥�. Its structure is represented in part (c) at the left with three resonance forms.