Cyclodextrin catalysis in the basic hydrolysis of alkyl nitrites

Abstract

The influence of β-cyclodextrin (β-CD) on the hydrolysis reaction of a variety of structural types of alkyl nitrites (RONO) is studied in acid and in basic aqueous solutions. Acid-catalysed hydrolysis of alkyl nitrites is inhibited by the presence of β-cyclodextrin. The results are accounted for by assuming the formation of host–guest complexes between β-cyclodextrin and alkyl nitrite, which are unreactive or much less reactive than the uncomplexed RONO. We propose that the result is a consequence of the orientation of the alkyl nitrite in the cavity of CD. The degree of inhibition increases with the greater inclusion of the alkyl nitrite in the CD cavity. The kinetic data are quantitatively analyzed to afford the stability constants of the host–guest complexes. On the contrary, the presence of β-cyclodextrin strongly increases the rate of hydrolysis of alkyl nitrites in a basic medium, that is at a pH value higher than the pK_a of β-cyclodextrin. This feature suggests the formation of a reactive complex between the alkyl nitrite and β-cyclodextrin, whose ionized CD hydroxy group promotes a nucleophilic attack in the rate-limiting step. This behaviour is consistent with a higher reactivity towards alkyl nitrites of an ionized CD hydroxy group as compared with the OH^–; the contrary occurs in the case of esters, whose cleavage by cyclodextrins in basic aqueous media has been studied extensively over the last several years.