Liquid Crystalline Solvents as Mechanistic Probes‐v. An Investigation of the Effect of Cholesteric Order on the Formation Rates of Vitamin D₃ From Pre‐vitamin D₃ and of Pre‐vitamin D₃ From Vitamin D₃*

The rates for conversion of pre‐vitamin D 3 (pre‐D) to vitamin D, (D) and of D to pre‐D in the cholesteric liquid‐crystalline and isotropic phases of a 60/26/14 (wt/wt/wt) mixture of cholesteryl oleate/cholesteryl nonanoate/cholesteryl chloride and in hydrocarbon solvents (hexane and 2,2,4‐tri‐methylpentane) have been measured. The rate constants for the forward and reverse reactions in the cholesteric mixture are greater than those measured in the hydrocarbon solvent. The pre‐D D rate in the liquid‐crystalline phase is independent of whether pre‐D is generated in situ from 7‐dehydrocholes‐terol or from lumisterol 3 . Eyring plots of data from the cholesteric mixture show no perceptible differences between the liquid‐crystalline and isotropic phases: ΔH=83.85 3.0 kJ/mol and ΔS=‐64.3 9.7 J/mol‐degree for pre‐D D; ΔH=104.22.7 kJ/mol and ΔS = ‐13.7 8.4 J/ mol‐degree for D pre‐D. It is concluded that macro‐solvent order does not play an important role in the rates and equilibria for the pre‐D D intcrconversion. Solvent‐solute spatial interactions may be responsible for the high rates obtained in the cholesteric mixture.