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The typical organic acid group is called the carboxylic acid group which consists of an OH group bonded to a carbon that also has an oxygen double bonded to it. This -CO2H, or alternatively -C(=O)-O-H, group enhances the acidity of the hydrogen on the OH group causing the molecule to be acidic. The majority of simple organic acids have Ka values in the range of 10-5. In contrast, ascorbic acid has two adjacent OH groups that can interact by hydrogen bonding. A combination of the carbon - carbon double bond the adjacent C=O group, and the stable hydrogen bonding increases the acidity of the first of the alcohol groups so that it's Ka is 8.0 x 10-5 similar to a typical acid, while the second is 1.2 x 10-2, still an acid, but much weaker than the first OH group. It is easier to see these relationships by examining the structure of ascorbic acid illustrated below. |
