Retardation effect of jojoba chain length on the chemical reactivity of the liquid wax

Abstract

Jojoba wax and its derivatives are slow-reacting compounds. To elucidate the reasons for this phenomenon, we reacted jojoba mono-and bis-epoxide and trans-jojoba bis-epoxide (C 38 -C 44 long-chain esters), as well as side chain esters of three steroid skeleton mono-epoxide derivatives with NaI under acidic conditions to yield the corresponding iodohydrins, which then formed the respective bis-keto (or mono-ketone) derivatives. The kinetics, activation energies, and thermodynamic parameters of activation of nucleophilic epoxide opening and pinacol rearrangement were determined for all these compounds. The reaction rates of the jojoba derivatives were similar to those of two of the epoxides derived from the steroid skeleton compounds, and in the third case the steroid derivative reacted somewhat faster than all the rest. This pattern of rate retardation could stem either from folding of the long jojoba chain, resulting in steric hindrance around the reaction centers, or from repeated unproductive collisions along the long hydrocarbon chain of the jojoba wax (statistical effect). Our results appear to suggest that the multiple unsuccessful collisions were the dominant factor, although steric hindrance cannot be ruled out.Paper no. J10980 in JAOCS 82, 373-379 (May 2005). KEY WORDS:Chemical reactivity, jojoba iodohydrin, jojoba mono-and bis-epoxide, mono-and bis-ketojojoba wax, pinacol rearrangement, retardation, rigid steroid skeleton vs. flexible long chain.Jojoba liquid wax (I) is composed of long-chain esters (1), as follows (Scheme 1):These esters are relatively slow-reacting molecules (2). It seems reasonable to suppose that their slowness is related to the considerable length of the ester chain, which constitutes 38-44 carbon atoms. Folding of the chain, which causes steric hindrance around the active sites along the chain (double bonds, allylic positions, and ester group), could retard reactions along the chain. Retardation might also be due to numerous unproductive collisions with the large inert portions of the hydrocarbon chain (statistical effect). Whatever the cause, the net result is that more drastic conditions, such as higher temperatures and longer reaction periods, are required to achieve the types of chemical change (2) that take place under much milder conditions within shorter times in short-chain molecules.In a recent study, the jojoba bis-epoxides (II) (derived from the natural wax, which contains cis double bonds) and IIa [derived from the isomerized trans-jojoba (Ia) (3)], were reacted with NaI under acid catalysis to form an iodohydrin (III), which formed the bis-ketojojoba (IV) (Scheme 3) (4). Higher temperatures and longer periods were required for these reactions than for analogous reactions with short-chain epoxides (5). In the present paper, we describe the kinetics of these reactions for the two bis-epoxides-as well as for the mono-epoxide (IIb). We then discuss the kinetics of reactions with epoxides cis-DED (IX; cis-dihydrocholesteryl 6,7-epoxydecanoate), trans-DEH (X; trans-dihydroch...