Accommodation of hydroxyl groups and their hydrogen bond system in a hydrocarbon matrix

From data of a single crystal analysis of 12-D-hydroxyoctadecanoic acid methyl ester principles for the incorporation of hydroxyl groups into a hydrocarbon chain matrix can be deduced. In the crystalline compound infinite hydrogen bond systems are accommodated in an orthorhombic perpendicular chain arrangement. The orthorhombic perpendicular hydrocarbon subcell is expanded towards a hexagonal packing pattern, allowing more space and optimal geometry for the hydrogen bond system. The arrangement of the bond system in the orthorhombic perpendicular subcell requires that hydrogen bonded carbon chains carry alternatingly hydroxyl groups with opposite configuration. For the enantiomeric compound this requirement is met by a head to tail packing of molecules in a single layer arrangement. The corresponding racemates on the other hand pack head to head in double layers as confirmed by X-ray powder and IR studies. In monolayers both enantiomers and racemates behave identically. The hydrogen bonding of the hydroxyl groups apparently leads to the formation of lipid clusters, in which the geometric conditions for both a close packing of hydrocarbon chains and the formation of an extensive hydrogen bond system do not exist.