CHOLIC-ACID INCLUSION-COMPOUNDS WITH AROMATIC GUESTS - STRUCTURES ANDDECOMPOSITION KINETICS

Cholic acid forms 1:1 (host:guest) inclusion compounds with a variety of aromatic guest molecules. The crystal structures of the inclusion c ompounds with benzonitrile and p-nitro toluene have been elucidated. C A.benzonitrile: space group p2(1), a = 13.642(3), b = 8.133(2), c = 14 .055(3) Angstrom, beta = 114.12(2)degrees, Z = 2, D-c = 1.194 g.cm(-3) , final R factor = 0.0781 for 1853 independent reflections. CA.p-nitro toluene: space group P2(1), a = 13.495(1), b = 8.266(4), c = 14.398(2 ) Angstrom, beta = 114.61(1)degrees, Z = 2, D-c = 1.249 g.cm(-3) final R factor = 0.0608 for 2783 independent reflections. The kinetics of d ecomposition of these and related compounds are investigated and kinet ic parameters Ea and In A so derived compared and contrasted. 3 alpha, 7 alpha, 12 alpha-trihydroxy-5 beta-cholan-24-oic acid or Cholic acid (CA) has been shown to form inclusion compounds with a variety of gue st molecules. The 1:1 inclusion compounds with methanol, ethanol and n -propanol(1,2,3) exhibit extensive host-host and host-guest hydrogen b onding and the guest molecules are accommodated in cavities. The struc tures of the hydrate(4) and hemihydrate(5,6) are composed of hydrogen- bonded bilayers and helical columns respectively while the inclusion c ompound with acetone and three water molecules(7) is composed of bilay ers of host and guest hydrogen bonded together in a system containing ten unique hydrogen bonds. The bulk of CA inclusion compounds crystall ise as tubulate clathrates in which host molecules are hydrogen bonded together in a head to tail fashion to form puckered bilayers with the hydrophobic beta faces exposed on the bilayer surfaces. These bilayer s are in turn packed such that channels remain between them in which g uest molecules are stacked in columns parallel to the b crystallograph ic direction. Guest molecules such as aliphatic ketones(8), lactones(9 ), aliphatic(10) and vinyl esters as well as the aromatic guests nitro benzene, aniline(11), acetophenone(12) and benzene(13) are included in this manner. Given the similarities in the structures of these compou nds we decided to investigate the kinetics of decomposition with the a im of relating reactivity to structure at a molecular level. The kinet ics of decomposition of a variety of inorganic salts and hydrates have been extensively studied(14,15,16) and the effects of changes in reac tion conditions, particle size and sample pretreatment investigated.(1 7,18,19) The decomposition kinetics of solid organic clathrates has no t been considered in spite of the potential insights into stability an d host-guest interactions to be gained therefrom. We present the decom position kinetics of a group of CA inclusion compounds with the aromat ic guest molecules benzonitrile (CABN), aniline (CAAN), nitrobenzene ( CANI), p-nitro-toluene (CAPNOT) and p-toluidine (CAPTOL). Possible mec hanisms for the decomposition reaction are advanced and the crystal st ructures of CA with benzonitrile and p-nitro-toluene guests are presen ted while those of the other inclusion compounds included in this stud y have been published elsewhere.(11,20)