Séminaire d'OSCAR N.Ventura

And what if there is no oxygen? 

Abstract : 

Although the systems of reactions in the atmosphere are infinitely complex, the prevalence of certain species in the gas mixture determines much of the type of individual reactions. Volatile organic compounds {VOCs) are oxidized mainly by OH, followed by halogens (principally Cl, but also Br at the marine boundary), N03 (especially in the absence of solar energy) CH3, H02 and 03 in different reg ions and altitudes. 
Focusing mainly on the reactions with OH and Cl, additions at and H-abstractions from different chemical groups are possible, especially when the molecules are unsaturated or especial atom centers are present. ln this last case, sulfur and selenium are the focus of a large portion of the research we have done in the last years, and I will focus primarily on them. 
Once the initial radicals have been formed, the general reaction path is the addition of 02, since this is the foremost reactive component of the atmosphere. The initial radicals add oxygen, forming peroxyl radicals, later evolving to alkoxy radicals, through the interaction with NO or N02, until the final products. However, not all atmospheres are the same. One could conceive oxidative environments in which oxygen is not present in any significant amount and the initial radicals have to follow alternative paths. Obvious reactions could be fragmentation or dimerization, but more interesting reaction systems may appear in case the initial radicals react with a further oxidant molecule or atom. 
For instance, one could think of an initial reaction with Cl, followed by a secondary reaction with another Cl atom. Or the same, first with an OH molecule and then with a second one. Or a mixture, first HO and then Cl, or first a and then OH. What all these things have in common is that 

one cou Id think them as reactions of the initial species with Cl2, H202 or HOCI respectively, dissociated photochemically to produce the radicals. ln this speculative talk I will describe some of the reaction mechanisms we have studied for the oxidation of S-containing compounds in the absence of oxygen and I will make a proposai of further studies involving species with the chalcogen triad [0, S, Se] in different combinations.