ORGANIC

SYNTHESIS

BY QUENCH

REACTIONS

W. K. PARK*, A. R. H O C H S T I M Research Institute for Engineering Sciences, Wayne State University, Detroit, Mich. 48202, U.S.A. and C. P O N N A M P E R U M A Laboratory of Chemical Evolution, Dept. o f Chemistry, University of Maryland, College Park, Md. 20742, U.S.A.

Abstract. The effects of chemical quench reactions on the formation of organic compounds at a water surface under simulated primordial earth conditions were investigated for the study of chemical evolution. A mixture of gaseous methane and ammonia over a water surface was exposed to an arc discharge between an electrode and the water surface. This discharge served as a source of dissociated, ionized and excited atomic and molecular species. Various organic molecules were formed in the gaseous, aqueous, and solid states by a subsequent quenching of these reactive species on the water surface. The effects of these water-surface quench reactions were assessed by comparing the amounts of synthesized molecules to the amounts which formed during the discharge of an arc above the water level. The results showed that: (1) the water-surface quench reaction permitted faster rates of formation of an insoluble solid and (2) the quench discharge yielded twice as much amino acids and 17 times more insoluble solids by weight than the other discharge. The highest yield of amino acids with the quench reaction was 9 • 107 molecules per erg of input energy. These observations indicate that quench reactions on the oceans, rain, and clouds that would have followed excitation by lightning and shock waves may have played an important role in the prebiotic milieu. Furthermore, the possibility exists that quench reactions can be exploited for the synthesis of organic compounds on a larger scale from simple starting materials.

1. Introduction In a n effort to study the chemical events a n d reaction systems of the p r i m o r d i a l Earth, m a n y investigators have performed experiments on the f o r m a t i o n of a m i n o acids u n d e r simulated conditions (Park, 1973; G a b e l and P o n n a m p e r u m a , 1971). Various p r i m a r y a n d secondary sources of n a t u r a l energies were applied to the presumed primeval, atmospheric gas-mixture at several stages of its development resulting in the p r o d u c t i o n of biologically relevant molecules. However, it is only recently that a t t e n t i o n has been given to the roles of chemical a n d physical parameters in the reaction processes. Specifically, q u e n c h i n g has been recognized as a selection m e c h a n i s m in the r e c o m b i n a t i o n process. Faster rates of formation, larger yields, a n d larger molecules were predicted for quench conditions (Hochstim, 1963; Park et al., 1973), which would indicate that q u e n c h i n g m a y have been an i m p o r t a n t process in the abiotic synthesis of organic molecules. * Present address: Laboratory of Chemical Evolution, Department of Chemistry, University of Maryland, College Park, Md. 20742, U.S.A.

Origins o f Life 6 (1975) 99-107. All Rights Reserved Copyright 9 1975 by D. Reidel Publishing Company, Dordreeht-Holland

100

W.K. PARK ET AL.

Gaseous quench processes can be classified by the physical event which causes the temperature to drop: (1) gas expansion, (2) gas mixing, (3) contact with a cool solid surface, and (4) contact with a cool liquid surface. Among these, quenches on surfaces may have been the most significant in the natural processes of chemical evolution because the molecules produced near or at the surfaces were protected from further reaction and decomposition from re-exposure to the energetic sour, ces.

Irt the design of an investigation of quenches on surfaces, the liquid-surface quench (water-surface quench in this work) was thought to be more germane, because of the possibility of rapid mass transfer from the surface to the inner layers of the water, subsequent shielding, and reactions in the water phase. The quench discharge (wet discharge) was carried out (Park, 1973) by forming a vertical arc column between an electrode and the water surface; the non-quench discharge (dry discharge) was done by forming an arc column along the line of the electrodes in the gas phase. In the wet discharge, quenching is achieved due to the contact of the hot arc column with the cold water surface. This causes the temperature of the heated gaseous species to decrease rapidly on the water surface. The experiments were performed at NASA, Ames Research Center in Moffett Field, California and at the University of Maryland at College Park, Maryland.

2. Experimental A 2 liter pyrex flask, cleaned with dichromic acid, was fitted with a water reservoir and electrodes (Figure 1). For each run the whole system was evacuated and maintained at a pressure of 5 # for 12 hr for further out-gassing. Matheson research grade CH 4 was used to flush the flask and its connecting parts 6 times (each time evacuating to a pressure of 30 p). The reaction vessel was then filled with research grade NHa and CH 4 (300 mm Hg each). One hundred ml of distilled water was bubbled with filtered N 2 ir~ the water reservoir for 20 min to remove dissolved 0 2, and was then transferred to the vessel by a pressure differential caused by opening the valve to the water reservoir, which was at atmospheric pressure, just long enough to bring in the water. In the wet discharge a tungsten electrode was located 1 cm above the water which itself Served as a second electrode. A gold wire place within the water was used to complete the circuit, tn the dry discharge two tungsten electrodes with a 1 cm gap were placed 9 cm above the water level. As soon as the water was introduced into the reactor, the ammonia began dissolving with concomitant reduction in the pressure of the system. Twelve minutes after the water had been added, approximate phase equilibrium for ammonia was reached resulting in a total pressure of 357 mm Hg. This process was expedited by agitation of the water with a magnetic stirrer. Two power supplies, transformer and Tesla coil, were used for the discharges. Their characteristi& were determined as follows:

ORGANIC SYNTHESISBY QUENCH REACTIONS

101

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2.1. A R C DISCHARGES AND ENERGY MEASUREMENTS

2.1.1. Low Intensity Arc A high voltage transformer (Peschel Instruments Model No. T20-0.6AR), whose one secondary terminal was grounded, was controlled by an autotransformer on the primary side to supply the necessary low intensity power. In order to limit the current and, at the same time, to measure voltage drops across known resistances, resistors were lined up in series. The gold wire in the wet discharge vessel was grounded to draw the necessary current. Voltage measurements of the secondary terminals were made with a Tektronix (1000• attenuation) high voltage probe connected to a Tektronix 531A oscilloscope. The power (5.1 W) and the total energy (2.2 x 10 s J) for the reaction were calculated from the measured rms voltage (3 kV) and rms current (1.7 mA) and discharge time (12 hr).

t02

W . K . PARK ET AL.

2.1.2. High Intensity Arc A Tesla coil (Sargent-Welch Cat. No. $30978) was used as a source for the high intensity arc. It produced irregular decaying sinusoidal oscillations with random time intervals. The average peak voltage for the load was 8 kV with a frequency of 250 kHz. The rms voltage (2.3 kV) was estimated from the ringing wave by assuming a sinusoidal wave for each half cycle. By monitoring voltage drops across known resistors, rms current (50 mA) was also estimated. For the wet discharge vessel, a piece of aluminum foil was attached to the bottom of the reactor to draw a high frequency transient current. This configuration eliminated any effects due to the gold wire used in the low intensity arc. Average power for each oscillation was 115 watts and the effective total energy that was supplied to a reaction vessel was 2.9 x 105 J. The initial temperature of both systems was 28 ~ which was the ambient room temperature. After an hour of the discharges, temperature distributions in the flask reached steady states showing between 30~ and 35~ near the flask wall. The pressure increased in all the discharge experiments reaching 490 mm Hg in the low intensity discharge and 820 mm Hg in the high intensity discharge. Even though the spark gap was adjusted to be about 1 cm, a carbon deposit on the electrode tips decreased the gap, especially in the dry discharge with the transformer. Frequent tapping on the electrodes prevented the growth of a carbon deposit. In general, the discharge pressure was dependent on arc intensity and spark gap. 2.2.

C H E M I C A L ANALYSIS

The resultant solution (97 ml), after 12 hr discharge, was filtered and reduced to 11 ml by partial evaporation on a rotary evaporator at 50 ~ Of this solution 6 ml was removed for analysis by the computer-controlled Durrum amino acid analyzer and the remainder was used for gas chromatography. One half of each sample was then hydrolyzed in 6 N HC1 at l l 0~ for 12hr. Aliquots of both hydrolyzed and unhydrolyzed samples for the amino acid analyzer were freeze-dried and dissolved in 0.01 N HC1 solution before injection. The hydrolyzed and unhydrolyzed solutions which had been set aside for gas chromatography, were derivatized to form volatile N-triftuoroacetyl amino acid nbutyl esters (Roach and Gehrke, 1969). These were chromatographed on a 3~ OV-17 column in a Perkin-Elmer 990 gas chromatograph (Figure 3). Amino acids were tentatively identified by their retention times in the amino acid analyzer and gas chromatograph. Mass spectra of each resolved peak were later obtained from a Dupont 2t-492 mass spectrometer in tandem with the gas chromatograph. The fragmentation patterns of the spectra were then interpreted and matched with the patterns of standard compounds to confirm the identities of the amino acids. Yields of amino acids were calculated on the basis of carbon. The total carbon present in the 36 millimoles of CH4 with which the system had been charged was 430 rag. The yields of each amino acid were expressed as the per cent of the carboncontent of the amino acid compared to 430 mg of total carbon. Yields of solid material were similarly calculated from the elemental composition of the solid.

ORGANIC SYNTHESISBY QUENCHREACTIONS

103

Using the same spark gap in identical reactors and feeding the same amount of reactants for both dry and wet discharges, the eIectrical parameters were characterized by current, voltage, and frequency. In this investigation, peak voltage, rms voltage and rms current were controlled for a given input power supply of finite frequency so as to apply qualitatively and quantitatively the same energy to both discharge systems. Accordingly, a difference in product-distribution would be attributable to chemical reactions in the two different discharges that took different forms in relating chemical kinetics of fragmentation and recombination, transportation properties, and arc variables. 3. Results 3.1. INSOLUBLE SOLIDS

A characteristic difference between the dry and wet discharges of this study was made vividly apparent by the solid particles which separated from the liquid phase. The wet discharge produced the particles within one minute after the onset of arcing. The quantity increased continuously during the discharge, producing an orangecolored solution with orange colored solids in suspension. On the other hand, in the dry discharge, black carbon particles, which dropped from the electrode, were apparent after 30 rain, and particles due to reaction products were noticed only after one hour. The particles in both discharges are known to have been formed on the water surface (rather than in solution) from the observation that they were generated as a surface slick when the stirring bar was stopped during the discharge process. The immediate formation of the solid particles in the wet discharge means that the solid could be formed before the system variables (e.g., temperature, pressure, and possibly concentrations) changed very much and that these changes of the system variables did not affect appreciably their continuous formation. In fact, solid particles were formed as long as the gas phase contained an excess of carbon in relation to nitrogen. Since this insoluble solid is generally believed to be a cross-Iinked high molecular weight polymer (Wilson, 1960; Oro, 1963; Henrey, 1969), a rationalization of the aforementioned observations permits the assertion that the solid was produced by a rapid process which could combine a large number of fragmented species together into a finite structure within a short period of time. Such fast reactions are considered to have occurred in the formation of many other organic compounds in the wet discharge. Nonetheless, this process was insignificant in the dry discharge in which most of the solid formed slowly as a film on the flask wall, possibly through a wall recombination of reactive species - partial solid surface quench. 3.2. AMINOACIDS As a result of the faster reactions, yields were larger in the wet discharge than in the dry discharge. Amino acids (see Figure 2) were more abundant in the wet discharge products as flee amino acids (before hydrolysis) and especially in the form

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ORGANICSYNTHESISBYQUENCHR~ACTIONS

105

of their precursors which yielded the amino acids after hydrolysis. Except for aspartic acid, yields of all other amino acids were greatest in the wet discharge. The ratio of yields o f the wet to the dry discharge for major products are shown in Figure 3. The figure indicates that especially high ratios were associated with the formation of N-alkyl amino acids and the insoluble solid. This fact might be related to the similarly higher yield of N-alkyl amino acids in the low intensity arc discharge than in the high intensity arc discharge. TABLE I Comparison of yields of major products for initial energy input Amino acids

Solid

/~M j-1

~oo

mg j-1

Total %

Low intensity Arc discharge

Dry Wet

8.2 x 10-4 14.1 • 10.4

1.55 2.77

1.4 • 10-5 25 • 10-5

0.55 10.16

2.10 12.93

High intensity Arc discharge

Dry Wet

1.4 • 10.4 3.8 • 10.4

0.39 0.94

1.7 • 10-5 29 • 10-5

0.83 14.20

1.22 15.44

TABLE II Absolute yields of major amino acids after hydrolysis (high intensity arc) Dry discharge

Wet discharge

/zM(/zM M -1 CH4)

Ha

/tM(/zM M -1 CH4)

Aspartic Acid Sarcosine Glutamic Acid Glycine Alanine c~-Amino Butyric Acid Norvaline a-Hydroxy y-ABA fl-Alanine cqT-Diamino Butyric Acid N-Ethyl Glycine N-Methyl Alanine N-Methyl fl-Alanine

9.4 (260) 0.9 (25) Trace 7.1 (196) 11.4 (3.7) 0.7 (18) 0.4 (10) 2.0 (56) 7.2 (199) 2.6 (72) Trace Trace Trace

0.10 0.01 0.04 0.I0 0.01 0.01 0.03 0.06 0.03

4.6 (128) 8.0 (222) 0.8 (22) 29.6 (820) 26.4 (732) 2.2 (60) 1.0 (27) 4.3 (118) 12.8 (354) 15.7 (436) 2.1 (58) 1.3 (37) 0.8 (22)

0.05 0.07 0.01 0.16 0.22 0.02 0.01 0.07 0.11 0.18 0.02 0.01 0.01

Total

41.7 (1153)

0.39

109.6 (3036)

0.94

a Percentage based on mass of carbon atom. The low intensity wet discharge yielded 1.8 times more amino acids than the low intensity dry discharge and the high intensity wet discharge yielded 2.7 times more than the high intensity dry discharge as shown in Tables I and II. 3.3. GASES The m o s t a b u n d a n t gas in the discharges was hydrogen. The generation o f hydrogen accounted for a pressure build-up in the reaction vessel during the discharges. In

106

W . K . P A R K ET AL.

a typical experiment 51% of the total gases after 12 hr discharge was hydrogen. Other major gases included 34% unreacted methane and C2-C 4 hydrocarbons. A quantitative comparison of the gaseous products from the wet and dry discharges was not made.

4. Conclusion For both amino acids and insoluble solid, the ratio of yields of the low intensity wet discharge to the low intensity dry discharge was 6 and the ratio of yields of the high intensity wet discharge to the high intensity dry discharge was 12. Summarizing the quench effect, it is concluded that the quench process led to (1) faster rates of reactions, (2) higher molecular weight organic compounds, and (3) one order of magnitude larger yields. As described in the introduction, any cold solid surface can also affect the reaction sequences, i.e., solid surface quench. In many experiments conducted so far, with all sorts of reaction vessel configurations designed for synthesis of amino acids, the wall surfaces could have played a significant role in the formation of micromolecular species through adsorption and recombination on the wall. Similarly, processes for the formation of micromolecular species near or at the earth's surface might have been more important than those processes which took place in the atmosphere where the yield would be smaller and where there would be more likely subsequent decomposition of the products. This proposition is further emphasized by the results of our experiments on the water-surface quench, which could have occurred at the atmospheric-hydrospheric interface of the earth and on water droplets of rain and clouds. Among the major products formed by using the direct discharge on the water surface was an insoluble solid. Even though its detailed structure is not known, it can be postulated that it could be a good reactive surface for macromolecular condensations because of its inert unsaturated structure. We may conclude that lightning and shock waves, interacting with water via quench reactions, might have played an important role in the primordial milieu leading to the chemical evolution of organic molecules.

Acknowledgements This work was partially supported by NASA Grant NGR 05-007-215 to the University of California at Los Angeles, through the courtesy of Prof. W. F. Libby to Prof. A. R. Hochstim, and partially by the Research Institute for Engineering Sciences at Wayne State University.

References Gabel, N. W. and Ponnamperuma,C. : 1971,in C. Ponnamperuma (ed.),Exobiology, North-Holland, Amsterdam. Henley, R. R. : 1969, Ph.D. Thesis, Stanford University,Stanford. Hochstim, A. R.: 1963,Proe. Nat. Aead. Sci. 50, 200.

ORGANIC SYNTHESIS BY QUENCH REACTIONS

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Or6, J. : 1963, Nature 197, 862. Park, W. K.: 1973, Ph.D. Thesis, Wayne State University, Detroit. Park, W. K., Hochstim, A. R., and Ponnamperuma, C. : 1973, Am. Chem. Soe. Regional Meeting, Wash., D.C., U.S.A. Roach, D. and Gehrke, C. W.: 1969, J. Chromatog. 44, 269; Wilson, A. T.: 1960, Nature 188, 1007.

Organic synthesis by quench reactions.

The effects of chemical quench reactions on the formation of organic compounds at a water surface under simulated primordial earth conditions were inv...
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