Urey reaction

It appears that conditions in the solar nebula were appropriate for the FTT but not the Miller-Urey reaction. Kinetic calculations (Lewis and Prinn, 1980) as well as observations on comets (Delsemme, 1977) show that CO and COj, not CH, were the principal forms of carbon. And the dust-laden solar nebula was opaque to UV, precluding any photochemical reactions. It seems best, however, to approach the problem empirically, by examining the meteoritic organic compounds themselves for clues to their formation. We shall review these compounds class by class, looking for the signatures of the FTT or Miller-Urey reactions. 

Actually, much of the experimental work on chemical evolution (cf. Lemmon, 19701 utilizes such unstable compoumls, e.g. HCN, HCHO, HC=CCN, H2NCN, etc., on the grounds that they can be made by Miller-Urey reactions. But they can also be made by spontaneous reactions of CO, NH3, and Hj (Anders et al., 1974). Hence this class of reactions provides some common ground between the two main types of abiotic synthesis. 

This resemblance is highly significant if one considers that 10,359 structural isomers exist for saturated hydrocarbons with 16 C atoms (Lederberg, 1972). Apparently the meteoritic hydrocarbons were made by FTT reactions, or some other process of the same extraordinary selectivity. The Miller-Urey reaction, incidentally, shows no such selectivity. Gas chromatograms of hydrocarbons made by electric discharges in methane show no structure whatsoever in the region around Cjg (Ponnamperuma et al., 1969). Apparently all 10 possible isomers are made in comparable yield, as expected for random recombination of free radicals. 

The Miller-Urey reaction fails qualitatively rather than quantitatively. It produces no detectable normal acids above Cg, even in the presence of an alkaline aqueous phase that is known to favor growth of linear chains by formation of a monolayer (Allen and Ponnamperuma, 1967). Given the fundamentally random nature of the Miller-Urey reaction, there is little hope that it will ever achieve the needed selectivity for normal isomers. 

The Miller-Urey reaction has been notably less successful in producing N-hetero-cyclics. Only adenine has thus far been made, by electron irradiation of CH, NHj, HjO, and Hj (Ponnamperuma et al., 1963). The yield was only 0.01 %. Better success was achieved by reactions involving unstable reactants, such as HCN (Lemmon, 1970), but the reactions, being spontaneous, actually are related no less to FTT than to Miller-Urey reactions (Sec. 3.3). 

The Miller-Urey reaction has been quite successful in duplicating these results. All 20 amino acids identified in meteorites, and 12 others, were produced by electric discharges on CH4-NH3-H2O-H2 mixtures, in the presence of an aqueous phase (Ring et al., 1972 Wolman et al., 1972). Even the proportions of the various amino acids resemble those in Murchison to within 1-2 orders of magnitude. 

Polymeric materials also form in Miller-Urey reactions, by both spark discharge (Miller, 1955) and UV irradiation (Sagan and Khare, 1979). These materials have not been studied in detail, but the elemental" analyses show high N contents (36% and 11%, vs. 2.4% for Murchison and 1.23% for FTT polymer). The H/C ratios also are higher (1.28 and 1.23, vs. 0.70 and 0.78), suggesting a predominantly aliphatic and/or alicyclic, rather than aromatic, structure. 

No attempt has yet been reported to produce carbynes by the Miller-Urey reaction. This should not be held against it, since carbynes have only very recently been discovered in meteorites. At least acetylene and some of its simpler derivatives have been made in the Miller-Urey synthesis (Friedman et al., 1971). 

The Miller-Urey reaction gives a fractionation of only —0.4 + 0.2%o (Lancet, 1972). 

Chanical fractionation apparently cannot account for this difference. Kung et al. (1979) have found that N-isotope fractionations in both FTT and MiUer-Urey reactions are too small -l-3%o and + 10-12%o- The high SN of Cl and C2 chondrites could, in principle, be explained by mass fractionation in a Rayleigh process, involving loss of 99% of the N,. But this process would have to be driven to... 

On the other hand, one cannot rule out the possibility that D-rich, ionic species formed at some stage in the solar nebula, and reacted with previously-produced, polymeric material. This is essentially a Miller-Urey reaction with a built-in, isotopic tracer. Perhaps these two alternatives can be distinguished by isotopic analysis of carefully separated fractions of the organic material. 

More recently, Lewis and Prinn (1980) have systematically examined the reduction kinetics of CO and Nj in the solar nebula. Taking into account gas-phase reactions as well as surface-catalyzed reactions (for the rather inefficient catalysts pr nt above 400 K), they conclude that reaction rates were so slow relative to the rates of radial mixing or nebular evolution that no more than 1 % of the and CO would have been reduced to NHj and CH over the lifetime of the nebula. Methane, the starting material of the Miller-Urey reaction, apparently was only a minor constituent of the solar nebula. 

The CO2 pressure on Venus is plausibly regulated by the Urey reaction, ... 

Although weathering of siliceous minerals by CO2 (the so-called Urey reactions but see (Berner and Maasch, 1996)) contributed to the long-term flux of silica to the oceans (Berner, 1990), and potentially fostered the radiation of diatoms in the Cenozoic, by itself, orogeny cannot explain the relatively sharp increase in diatoms at the Eocene/Oligocene boundary. Indeed, the seawater strontium isotope record does not correspond with these radiations in diatoms (Raymo and Ruddiman, 1992). We must look for other contributing processes. 

On geological time scales, CO2 cycles between rocks, often by way of the ocean and atmosphere. The rock reservoirs include the mantle, continental carbonates, carbon in reduced form mostly in continental shales, and carbon (mostly carbonate) in or on the sea floor. The small volatile reservoir (ocean plus atmosphere) cycles through carbonate rock in a hundred thousand to a million years. Over longer periods free CO2 is dynamically controlled by processes that form carbonates at low temperatures and processes that decompose carbonates at high temperatures by (Urey) reactions of the form... 

Weathering of silicates occurs through many different hydrolysis reactions whereby CO2 is converted to HCOj ions and Si is either bound in alteration minerals or liberated as H4Si04. Alkali and alkaline earth elements are progressively removed in solution. Of these, Ca and Mg are largely ultimately bound in carbonate, so that the whole process can be summarized in a generic way by the Urey reaction ... 

There are, however, two main reasons why this must be a minimum estimate of the amount of sediment recycled into the mantle over geological time. These are, first, reintroduction of CO2 into the atmosphere via arc volcanism, and second, the fact that purely clastic sediments are ignored in this mass balance. CO2 is reintroduced into the atmosphere as a result of decarbona-tion reactions in calc-silicate rocks (the reverse Urey reaction ), requiring further weathering and photosynthesis to remove it. Fluxes associated with these processes over Phanerozoic time have been reviewed by Berner et al. (1983) and Berner (1991), who concluded that such addition of CO2 by (mainly arc) volcanism and its drawdown by silicate weathering have been the major long-term fluxes over this period, and that drawdown has only slightly outstripped... 

Scientists are particularly interested in the interactions that may be taking place on Venus between materials in the atmosphere and those contained in surface minerals. The very high temperature and pressure at the planet s surface may increase the rates of such interactions, which on Earth are relatively modest. Scientists have studied a number of reactions that would control the rate at which atmospheric or surface components—or both—are generated and removed. The so-called Urey reaction is an example. In the Urey reac-... 

The Urey reaction is significant on Earth because it is thought to be one way in which carbon dioxide is removed from the atmosphere thus, it may influence climate change. It may be that a similar reaction takes place on Venus, likewise controlling the concentration of carbon dioxide in the Venusian atmosphere. As yet, however, there are no data to suggest that carbonates exist in abundance on the planet s surface. 

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