Chemical Reactions on Surfaces

Abstract

Recent investigations of the chemistry of meteorites have shown that certain meteorites (carbonaceous chondrites) contain appreciable amounts of organic compounds containing linear hydrocarbon chains. Because all naturally occurring compounds of this type are believed to be of biological origin and ordinary free radical reactions lead to branched chain compounds, some authors have suggested that this is evidence for extra-terrestrial life. The purpose of this work was to investigate the possibility that reactions taking place on surfaces may lead to the production of a predominance of straight chain compounds and to investigate the suggestion that these reactions are of importance in cosmochemistry. This work led to the experimental demonstration in the laboratory that free radical reactions on a surface do, in fact, lead to straight chain hydrocarbon synthesis. The details of this investigation are summarized below. 1. It was shown that it is possible to build up straight chain hydrocarbon material on a surface by free radical addition (methyl or ethyl) if the reacting chains are crowded together so that only the ends of the chains are available for reaction. Monolayers of straight chain fatty acid ions on an aqueous surface were shown to react with free methyl and ethyl radicals resulting in lengthening of the chains with negligible quantities of branched chain acids being formed. 2. Solvent-insoluble materials (macromolecules) were shown to be formed on the aqueous surface by the action of spark and corona discharges on mixtures of methane, ammonia and hydrogen sulphide over liquid water. Both nitrogen and sulphur were shown to be present in these products. Similar products were produced when the ammonia and hydrogen sulphide were absent, but not in the absence of liquid water. The rate of production of the product in the corona discharges was shown to be temperature dependent, indicating that hydroxyl radicals (from the decomposition of water vapour in the discharge) were important intermediates in the reactions. The infra-red spectra of the products indicated that they contained hydrocarbon chains of about three methylene groups, although pyrolytic studies showed that hydrocarbon chains of seven, and up to eighteen methylene groups were present. Some of the discharge products were shown to have; (a) similar appearances to microstructure found in some carbonaceous chondrites, and (b) similar pyrolytic products as those from micrometeorites. 3. The possible use of this synthetic procedure was investigated. It was shown that; (a) it was possible to produce large flat polymer molecules by extensively cross-linking together monomolecular films of surface active compounds, and (b) it was possible to link short chain surface active molecules in pairs by ω -cross-linking, thus producing hydrocarbon chains with the same functional group at each end.