Hydrogen bonding of keto-carboxylic acids in the catemer and dimer motifs

Abstract

We have examined the x-ray structures of more than 60 organic keto-carboxylic acids in our laboratory to determine the nature of the hydrogen bonding between the molecules in the solid state. Keto-carboxylic acids, besides having a ketone function as part of the acid group, contain a second ketone elsewhere in the molecule. As these organic acids contain only one donor (the acidic H atom) and two possible acceptors (the carboxyl ketone, and the second functionality, another ketone), there are five different possibilities for hydrogen bonding: acid-to-acid dimer formation, acid-to-ketone catemer formation, intramolecular H bonding, acid-to-acid catemer formation and acid-to-ketone dimer formation. There is no way of predicting a priori which of these possible hydrogen bonding schemes a previously unstudied keto-carboxylic acid molecule will adopt. We have used single crystal x-ray structure analysis to find out which hydrogen bonding mode prevails in each molecule. Secondly, we also have used the CSD (Cambridge Structural Database) to analyze and compare all the other known keto-carboxylic acid structures in the literature.