Crystal Structure of the Hydrate Form of Racemic Acid

Abstract

As was shown by Pasteur, racemic acid is composed of the two stereoisomers of tartaric acid. Although the chemical properties of racemic and tartaric acids are the same, the existence of a small heat of formation of racemic acid from d-tartaric acid, and differences in the physical properties of the two acids both in crystal form and in concentrated solution, suggest the formation of a definite compound. In early X-ray work on the crystallography of the anhydrous form of racemic acid, Astbury¹ concluded that racemic acid did not exist in the crystal as a double molecule of the two forms of tartaric acid. This view is supported by the present work, although it is possible to identify, in the structure, a column of d- and l-molecules parallel to the c-axis, held together by square systems of hydrogen bonds. This arrangement, which is not possible in tartaric acid, is likely to be responsible for the difference in physical properties of the two acids.