Synthesis and characterisation of selectively fluorinated stearic acids (octadecanoic acids) and their tristearins: the effect of introducing one and two fluorine atoms into a hydrocarbon chain

Abstract

The synthesis and physical properties of 2,2′,2″- and 12, 12′, 12″-trifluorotristearins 2 and 3 and 12, 12, 12′, 12′, 12″, 12″-hexafluorotristearin 4 are described. NMR, X-ray powder diffraction, melting point and differential scanning calorimetry analyses of these compounds are reported and comparisons are drawn with tristearin 1. Analysis of Langmuir film isotherms of the stearic acids gave the average area occupied by each molecule in a condensed monolayer and was used as a method to probe disorder in the hydrocarbon chains. Of the compounds studied, 3 possesses the closest behaviour to tristearin. The incorporation of a CF₂ group into the lipid chain in 4 imparts unique physical properties to the tristearin and a novel thermodynamically stable polymorphic form (β″) is observed. This is attributed to a conformational change in the hydrocarbon chain induced by the CF₂ groups. Langmuir film analysis of the selectively fluorinated stearic acids (octadecanoic acids)7 and 8 reinforce this conclusion.