The Crystal Structures of the 4-Bromocamphorenic Acids: a Unique Isomorphous Series Among Isomeric Carboxylic Acids

Abstract

The crystal structure determinations of the isomeric 4-bromocamphorenic acids [(1R)-4-bromo-1,2,2-trimethylcyclohex-3-ene-1-carboxylic acid and (1R)-4-bromo-1,6,6-trimethylcyclohex-3-ene-1-carboxylic acid], and that of crystals of the normal isomeric mixture of the two from the synthetic preparation, have been completed. This set provides an example of an unusual isomorphous organic crystal series that shows little variation in melting points between pure isomeric end-members and the ‘isomeric mixture’, which is typically 70 : 30%. This ratio is consistent with the observed occurrence of two conformational modifications of the acid found among the three independent molecules in the crystallographic asymmetric unit; this gives two different but typically cyclic hydrogen-bonded carboxylic acid associations. In the ‘isomeric mixture’, random associations of both isomers, consistent with the reaction ratio, result in the isomorphous crystalline structure in which the double bond is statistically distributed over the two adjacent ring sites. This phenomenon also provides an explanation of the physical anomalies that have been associated with these compounds since their first reported synthesis in 1896.