Covalent Electrophilic Catalysis of the Breakdown of Hyponitrite to Nitrous Oxide by Aldehydes, Ketones, and Carbon Dioxide

Article abstract of DOI:10.1021/ja00244a034

Catalysis of the breakdown of trans-hyponitrite monoanion (HN2O2^-) into N2O by aldehydes and ketones was studied spectrophotometrically in aqueous solution from pH 6 to pH 12.5 at 25 deg C, ionic strength 1.0 M (KCl), and that of trans-hyponitrite dianion by CO2 was studied similarly from pH 7 to 12.5.The reactions are first order with respect to hyponitroite and catalyst concentrations.Only the monoanion reaction is observed with aldehydes and ketones and only the dianion reaction with CO2.The species active in catalysis are shown to be unhydrated carbonyl and CO2.The correlation between the logarithm of the second-order catalytic rate constant and the Taft ?^* values for substituents at the carbonyl carbon is linear with ρ^* = 1.80 and 1.68 for free aldehydes and ketones, respectively.For carbonyl hydration, ρ^* = 1.68 and for ionization of the hydrates, 1.40.Kinetic arguments support mechanisms (Scheme III) in which trans-hyponitrite forms a nitrogen adduct with a carbonyl or CO2, after which the adduct experiences rate-determining isomerization to its cis isomer.This form of the adduct then reverts to catalyst and cis-hyponitrite, which rapidly breaks down to N2O.The breakdown of cis-hyponitrite involves trans periplanar elimination of hydroxide ion, and this probably accounts for its greater instability relative to trans-hyponitrite.Isomerizaton requires inversion or rotation at N,N-double bonded nitrogen in trans-hyponitrite, and adducts catalyze isomerization by stabilizing resonance forms with N,N-single bond character.These reactions appear to be examples of covalent electrophilic catalysis.Arguments are presented which suggest that the uncatalized breakdown of hyponitrite proceeds by a similar mechanism involving protonation of nitrogen and rate-determining isomerization.