Vitamin C, which is also known as ascorbic acid and L-absorbic acid, has a hydroxyl group, an ester functional group and an alkene. Although it behaves like an acid with a carboxyl acid group, Vitamin C lacks this group and has a carbonyl double bond instead, which is characteristic of alkene groups.
The hydroxyl group indicates that an OH, or hydroxide, has bonded to a carbon.The carbonyl double bond, two carbons bonded to each other, has an OH bond attached to each end of its two carbons. Last, its ester functional group indicates where two oxygen groups are bonded to a carbon. However, in this case, one of the oxygen molecules is double bonded to one end of the carbon, and the other is single bonded to the opposite end.
According to the diagram of the structure of Vitamin C on the NCBI website, Vitamin C has several hydroxyl groups, which explains Vitamin C’s solubility. Since hydroxyl groups are water soluble, Vitamin C easily dissolves in water. The hydrogen in the OH groups quickly dissociates and bonds to the oxygen molecules in the water.
According to Michigan State University, Vitamin C’s carbonyl double bond also makes it a strong acid because one side of the carbon double bond represents a beta-hydroxy group and the other end represents an alpha-hydroxyl group. Since the alpha hydroxy side acts like a phenol, Vitamin C has a similar acidity. This explains why Vitamin C is quite acidic.