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Advances in Space Research 33 (2004) 81­87 www.elsevier.com/locate/asr

Prebiotic, planetary and interstellar chemistry starting from compounds detected in the interstellar medium
Jean-Claude Guillemin *, Miloud Bouyahyi, El Hassan Riague
Laboratoire de Synthses et Activations de Biomolcules, UMR CNRS 6052, ENSCR, Institut de Chimie de Rennes, 35700 Rennes, France e e Received 6 December 2002; received in revised form 30 June 2003; accepted 2 July 2003

Abstract The most important families of reagents used in organic synthesis are represented in the 130 compounds detected up-to-now in the interstellar medium. The ability of these compounds to react together in the gaseous phase or in liquid water is discussed. Some examples of the possible formation of various products in such media are presented. ñ 2003 COSPAR. Published by Elsevier Ltd. All rights reserved.
Keywords: Interstellar chemistry; Organic synthesis; Interstellar medium; Prebiotic chemistry

1. Introduction It is generally accepted that life originated from simple molecules and proceeded via more complex species. About 130 compounds have been detected in the interstellar medium (IM) (Ehrenfreund and Charnley, 2001). Among them, about 90 species are neutral molecules that have been isolated in lab conditions. Several of these compounds have also been detected in comets, in meteorites or in the atmospheres of Planets. Only 12 elements are represented (H, C, N, O, S, Si, P, F, Cl, Al, Na, Mg) among the 90 stable ones of the Periodic Table. Hydrogen, Carbon, Oxygen and Nitrogen are unambiguously the most abundant represented elements. The functional groups are typically those found in organic synthesis: alkynes, alcohols, amines, aldehydes, ketones, imines, nitriles, etc. It is easy to deduce from these observations that organic compounds are (or have been) easily formed in the Universe and that organic chemistry probably occurs (or occurred) in this medium. Radicals and ions are wellknown to easily react on various molecules even at very
* Corresponding author. Tel.: +33-2-2323-8073; fax: +33-2-23238108. E-mail address: jean-claude.guillemin@ensc-rennes.fr (J.-C. Guillemin).

low temperatures but compounds satisfying the octet rule can also react together. Independently of the knowledge of the experimental conditions or the understanding of the chemistry occurring in the gaseous phase or on the grains in the IM, it is of interest to know which products are formed when two or more of these compounds are mixed together in the gaseous phase under a thermal or photochemical activation. On the other hand, in the condensed phase and diluted in water, these components may have played a determining role on the Prebiotic Earth. We report here, from the point of view of organic chemistry, an analysis of the compounds observed in the IM and several examples of reactions between two of them. On four of the examples, the chemistry in gaseous phase and in water of some nitrogen derivatives will be presented.

2. Organic compounds in the interstellar medium 2.1. Alkanes, alkenes, alkynes and aromatic compounds The sole alkane detected in the IM is methane 1. Alkanes do not have a permanent dipolar moment and are quite difficult to detect. Among the unsaturated hydrocarbons, only one alkene, the ethylene 2, and one aromatic compound, the benzene 4, have been observed.

0273-1177/$30 ñ 2003 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2003.07.015


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The presence of propene 3 in the IM, a compound easily detected by microwave spectroscopy, has never been evidenced. On the contrary, numerous alkynes are on the list: acetylene 5, propyne 6, butadiyne 7, pentadiyne 8 and hexatriyne 9 as well as alkynes bearing a nitrile substituant. The HC2n CN compounds, with n ¼ 1­5 (10­ 14), constitute the most abundant series; this family also contains the compound 14 with the largest number of atoms (13) (Scheme 1). 2.2. Alcohols and water Three saturated alcohols, methanol, ethanol and ethyleneglycol 15 have been detected. a; b-Unsaturated alcohols, such as vinylalcohols, present ceto-enol tautomerism. They are stable compounds at low pressure and in the gaseous phase. Only the simplest derivative 16 has been detected in the IM. Several aldehydes, ketones, and ketene have been observed but not the corresponding a; b-unsaturated alcohols. The glyoxal 19, a dimer of formaldehyde 18 (Gabel and Ponnamperuma, 1967; Reid and Orgel, 1967), has been recently detected in the IM by microwave spectroscopy (Scheme 2). Water is another member of the list of compounds with hydroxyl groups. 2.3. Ketones, aldehydes, ketene and carboxylic acids Aldehydes and ketones are oxidized (dehydrogenated) compounds. Formaldehyde 18 and its dimer the glyoxal 19 (already mentioned in the previous paragraph concerning alcohols), acetaldehyde 17, propionaldehyde 20, propiolaldehyde 21, and acetone 22 have been detected in the IM. Ketene 23, formic acid 24, acetic acid 25 and methyl formate 26 are also present and this class of compounds containing a carbonyl group constitutes one of the most important series of the list (Scheme 3). In laboratories, aldehydes can be formed by oxidation of the corresponding alcohol (Heyns and Paulsen, 1957) or by the reduction of the corresponding carboxylic acid. However this last process is often more

Scheme 2.

Scheme 3.

difficult to perform (March, 2001). Ketene is easily formed by pyrolysis of acetone (Sankaran et al., 1969). Methyl formate is synthesized starting from methanol and formic acid; the by-product is water (Haslam, 1980). Acetic acid can be prepared by oxidation of acetaldehyde (Chinn, 1971) (Scheme 4). It is interesting to note that several alcohols and carboxylic acids corresponding to the observed aldehydes and ketones have not been detected in the IM.

Scheme 4.

Scheme 1.


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2.4. Other oxygenated compounds Dimethyl ether 27 and oxirane 28 have also been observed in the IM. In laboratories, dimethyl ether 27 can be formed starting from two molecules of methanol in the presence of a strong acid (Feuer and Hooz, 1967). The by-product is water. Dehydration of glycol can lead to epoxide but only with specific reagents (Martin et al., 1974). The epoxydation of alkene by peracidic compounds is more usual (Dryuk, 1976) (Scheme 5). 2.5. Amines Ammonia (NH3 ) and methylamine (CH3 NH2 29) have been observed in the IM. The cyanamide (H2 N­ CN 30) is also present. 2.6. Imines Small imines are very unstable compounds in the condensed phase at a temperature higher than )100 °C. The simplest derivative, H2 C@NH 31, which has been detected in the IM, is one of the most kinetically unstable imines (Guillemin and Denis, 1988). It could be formed starting from formaldehyde and ammonia with water as the by-product. However compound 31 has never been synthesized by this approach in labs and, in such mixtures, the formed product is the hexamethylenetetramine (HMT). The latter is formed starting from six molecules of methanimine and with two molecules of ammonia as the by-product (Nielsen et al., 1979) (Scheme 6). 2.7. Nitriles The presence of compounds containing a nitrile group has been discussed above when this functional group was associated to an alkyne. Acrylonitrile 32,

another unsaturated nitrile, has also been detected in the IM. Saturated derivatives are also observed: hydrogen cyanide 33, acetonitrile 34 and propionitrile 35. The presence of hydrogen cyanide is not unexpected as this compound is easily formed in various drastic conditions on the Earth and even sometimes in reactions which are out of control. The corresponding sodium salt 36 has also been detected (Scheme 7). 2.8. Other nitrogen derivatives Isonitrile (HNC 37), methyl isonitrile (CH3 NC 38), isocyanoacetylene (HCBCANC 39), isocyanate (HNCO 40), thioisocyanate (HNCS 41) and formamide ((H2 N)HC@O 42) have also been observed in the IM. Isonitriles can be formed by dehydration of the corresponding amide (Ugi et al., 1965). Isonitriles can be oxidized to form the corresponding isocyanates and isocyanates can be reduced to isonitriles (Baldwin et al., 1983). Similarly isonitriles can be oxidized to thioisocyanates (Tanaka et al., 1977) (Scheme 8). 2.9. Sulfur derivatives Several compounds containing a sulfur atom have been detected in the IM and, among them, two thiols: hydrogen sulfide (H2 S) and methanethiol (CH3 SH). The thioformaldehyde is also present (H2 C@S 43). 2.10. Other compounds Carbon monoxide (CO) and dioxide (CO2 ), its monosulfur derivative (COS), hydrogen fluoride (HF) and chloride (HCl), sodium chloride (NaCl), silane (SiH4 ) and several other inorganic compounds have also been observed. To conclude with this part, it is easy to observe that the simplest derivatives of the most important functional groups in organic chemistry have been detected in the IM. The fact that only the smallest derivatives were observed is easily explained by the increased difficulty to

Scheme 5. Scheme 7.

Scheme 6.

Scheme 8.


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observe the substituted derivatives. In most cases, the high symmetry of the simplest derivatives is lost in the substituted derivatives, rendering their detection extremely difficult, particularly by microwave spectroscopy. The presence of heavier compounds can be seriously envisaged because several of those observed in the IM correspond to the formal addition of two other ones (sometimes followed by an elimination reaction). Thus, in the second part of this article, we will consider the detected compounds as starting material. Which energy source is necessary to promote reactions between two (or more) of them? Thermal and photochemical reactions can occur in the IM. We will discuss the addition of ammonia or dihydrogen sulfide on cyanoacetylene and the addition of hydrogen cyanide on the methanimine below. The study of the reaction of aldehydes and ketones on amines, which is under progress in our lab, will be reported elsewhere. On the other hand, with the hypothesis that the same compounds were present on the Primitive Earth as endogenous or exogenous compounds, the chemistry of similar mixtures in water is also reported.

a 1:1 ratio is obtained. After distillation in vacuo or heating of the mixture at about 100 °C, the Z:E ratio is higher than 10. The addition of hydrogen sulfide on cyanoacetylene also occurs in the gaseous phase. However only a bisadduct, the bis-acrylonitrile sulfide 45, is observed even when a very large excess of hydrogen sulfide is used. The yield is of 87% based on cyanoacetylene. That can be explained by a second reaction faster than the first one, the addition of the formed cyanovinylthiol on cyanoacetylene. Only the Z,Z-isomer is obtained (Scheme 10). The synthesis of this compound by another approach has been reported (Kishida and Terada, 1968). Water is also a nucleophile. However, until now, no formation of a product has been observed by mixing gaseous water and cyanoacetylene in a cell at room temperature. 3.1.2. Hydrogen cyanide and methanimine The reaction of hydrogen cyanide and methanimine has been studied in our group a long time ago (Guillemin et al., 1986). We observed that the addition of hydrogen cyanide on methanimine to give the aaminonitrile 46 is very fast even at very low temperature. When both gases are condensed on the cooled KBr window (6 K), only compound 46 is observed. On the contrary, the analysis by microwave spectroscopy of the aminonitrile 46 in the gaseous phase at a pressure of few mbar shows the presence of few percent of hydrogen cyanide and methanimine 31 (Scheme 11) (this reaction could be also more important at a very low pressure). The retro-addition reaction is complete at 800 °C in the gaseous phase as observed after flash vacuum pyrolysis of compound 46. The activation barrier of addition in

3. Chemistry in the gaseous phase 3.1. Reaction without UV-irradiation 3.1.1. Addition in the gaseous phase of ammonia, dihydrogen sulfide or water on cyanoacetylene In a cell at room temperature, gaseous ammonia and cyanoacetylene react together to form the 3-aminopropenenitrile 44 (Guillemin et al., 1998) (Scheme 9). At room temperature and using a 50 mbar pressure of each gas, the reaction is complete after one day. However, only few percent of product 44 are obtained after 2 or 3 h. It seems that this reaction generates its own catalyst. It is however difficult to know if this reaction really occurs in the gaseous phase or on the walls of the cell. After the formation of very small amounts of the product, which has a very low partial pressure at room temperature, the reaction could occur in the liquid phase by dissolving the gaseous reagents in the condensed product 44. The formation of compound 44 also occurs in an organic solvent, such as dichloromethane. (Xiang et al., 1994). In both cases, a mixture of Z and E compounds in

Scheme 10.

Scheme 11.

Scheme 9.


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gaseous phase or in water has been calculated quite recently (Arnaud et al., 2000) but the theoretical data (13­ 43 kcal/mol) is not consistent with the one expected from our experimental results by condensation of the reagents on a cold window (Guillemin et al., 1986). Contrary to the examples reported above, numerous addition reactions between two compounds do not occur at a reasonable temperature (<350 K) without photolysis. As an example, although ammonia or dihydrogen sulfide reacts on cyanoacetylene, the addition of phosphine or silane on HC3 N is performed under irradiation (Guillemin et al., 1998). Similarly with simple alkynes like acetylene, propyne or butadiyne, the addition of NH3 , H2 S or PH3 needs light. 3.2. Photochemical additions reactions 3.2.1. Addition of NH3 , H2 S or H2 O on alkenes and alkynes The photochemical addition of NH3 or H2 S on the C­C multiple bond of a hydrocarbon occur by abstraction of a hydrogen from NH3 or H2 S followed by the addition of a radical on the carbon­carbon multiple bond. Depending on the nature of the heteroatom and consequently of the strength of the X­H bond, two reaction pathways can occur, the first one involving a radical recombination and the second one, a radical chain process. With ammonia, the first step is the formation of NH2 and H radicals (1). H radicals are not able to abstract easily a hydrogen radical from another molecule of NH3 . The addition of H is about 700 times faster than the addition of the NH2 radical on a C­C multiple bonds; the second step is the addition of H on the unsaturated hydrocarbon (2). The formed radical then react with a NH2 radical and this radical recombination leads to the amine (3). This reaction pathway is consistent with the nature of the products observed starting from ammonia and an alkene. Starting from a dissymmetric alkene, the proposed reaction pathway is consistent with the nature of the more abundant product, the hydrogen radical mainly adding on the less substituted carbon (Guillemin et al., 1995, 1997, 1998). On the contrary, starting from NH3 and an alkyne, the formation of vinylamine has never been unambiguously evidenced (Ferris et al., 1992) (Scheme 12). NH3 * NHå2 × H
hv å

Scheme 12.

ability of hydrogen radicals to easily abstract a hydrogen from another molecule of H2 S (5). So, after a short time of irradiation, the most abundant radical in the reaction mixture is the thiyl radical. This latter adds on the C­C multiple bond to form a radical (6), which is also able to abstract a hydrogen from H2 S (7). A radical chain process thus occurs (Guillemin et al., 1997, 1998). This reaction can be performed with a 254 nm lamp. At this wavelength, the unsaturated hydrocarbon does not absorb the light. H2 S ! HSå × Hå Hå × H2 S ! HSå × H
2 hv

Ï 4÷ Ï 5÷ Ï 6÷

Ï 7÷ In the case of dissymmetric unsaturated hydrocarbons, the regioselectivity of the reaction is consistent with the proposed reaction pathway, the thyil radical adding on the less substituted carbon. It is also interesting to note that vinylthiols, which are quite kinetically

Ï 1÷
2

Hå × CH2 @CH2 * H3 CACå H

Ï 2÷ Ï 3÷

With H2 S, the first step is quite similar. Hydrogen and HS radicals are formed (4). The main difference of this reaction, compared to the one with ammonia, is the

Scheme 13.


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stable compounds, are easily observed in the reactions starting from an alkyne and H2 S (Scheme 13). The addition of H2 O on unsaturated hydrocarbon is much more difficult to study. Photolysis of water requires more energy (short wavelength <185 nm) than the one of the unsaturated hydrocarbon and the latter absorbs most of the light. Moreover the strength of the O­H bond is very high and few radicals are able to abstract a hydrogen from water. In most cases, the products are mainly those formed by the photolysis of the hydrocarbon by itself.

called the ``Strecker reaction''; the a-aminonitrile 46 is thus formed (Guillemin and Denis, 1988) (Scheme 15). 4.1.2. Addition of NH3 , H2 S or H2 O on alkenes and alkynes The solubility in water of alkenes and alkynes is very low and such experiments to study the addition of ammonia or hydrogen sulfide on unsaturated hydrocarbons have probably never been performed. A first alternative could be the bubbling of both reagents in water. Another one could be to work under a very high pressure to thus simulate the deep oceans. However ammonia, dihydrogen sulfur and water do not react easily with alkenes or alkynes without photolysis as reported above. Such experiments under irradiation and in liquid water have been recently investigated in our lab and are currently under progress. 5. Conclusion Starting from compounds detected in the IM, numerous reactions can be performed in the gaseous phase with or without photolysis. Similarly numerous products can be prepared using water as solvent. On the light of the examples reported above, it is too early to conclude on similarities between the chemistry of the interstellar medium and the one that possibly occurred in the lakes and oceans on the Primitive Earth. The detection of new compounds in the IM as well as many and various laboratory simulations of the chemistry of the Primitive Earth, comets or atmospheres of Planets are necessary to improve our understanding of their chemical evolution. 6. Experimental part Preparation of 3-propenenitrile 44 in gaseous phase. The compound was prepared in a one necked flask connected by a stopcock to a vacuum line equipped with a gauge. Ammonia (200 mbar) was added to cyanoacetylene (200 mbar) in the flask and the mixture was abandoned. A yellow­brown liquid was observed on the walls of the flask after one day. The gaseous part was then removed in vacuo and the high-boiling compounds were diluted in dichloromethane (5 mL). Purification was performed by distillation in vacuo (bp0:1 : 63 °C, Yield: 53%). (Z). 1 H NMR (CDCl3 ) d 3.96 (d, 1H, 3 JHH ¼ 8.4 Hz, CH­CN); 4.85 (s brd, 2H, NH2 ); 6.77 (dt, 1H, 3 JH H ¼ 10.6 Hz, 3 JHH ¼ 8.4 Hz, CH­N). I3 C NMR (CDCl3 ) d 63.6 (d, 1 JCH ¼ 179.9 Hz, CH­CN); 118.3 (s, CN); 149.4 (d, 1 JCH ¼ 169.4 Hz, CH­N). (E) 1 H NMR (CDCl3 ) d 4.27 (d, 1H, 3 JHH ¼ 14.0 Hz, CH­CN); 4.60 (s brd, 2H, NH2 ); 6.99 (dt, 1H, 3 JHH ¼ 14.0 Hz 3 JHH ¼ 10.7 Hz, CH­N). 13 C NMR (CDCl3 ) d 66.1 (d, 1 JCH ¼ 176.6 Hz, CH­CN); 121.2 (s, CN); 151.0 (d, 1 JCH ¼ 167.8 Hz, CH­N).

4. Chemistry in water It is quite easy to imagine that numerous comets containing such chemicals failed in the lakes and oceans of the Primitive Earth. Thus a chemistry was able to occur in the condensed phase and with water as solvent. 4.1. Addition of ammonia, dihydrogen sulfide or water on cyanoacetylene Cyanoacetylene reacts with ammonium hydroxide in an aqueous solution to give the compound 44. We never observed compound 45 by bubbling for one hour dihydrogen sulfur in an aqueous solution of cyanoacetylene. Similarly, no product is observed starting from cyanoacetylene in neutral liquid water. By addition of a base (as for example NaOH), the reaction occurs and give the corresponding alkoxide. Cautious acidification of the reaction mixture leads to the cyanoacetaldehyde 47, a compound which has been largely investigated for its role in prebiotic chemistry (Ferris et al., 1974; Orgel, 2002) (Scheme 14). 4.1.1. Hydrogen cyanide and methanimine Methanimine is a very unstable compound which has never been diluted in water. Moreover, in water, small imines are very quickly hydrolyzed to give the corresponding carbonyl compound and amine. However, such imines can be generated in water as intermediates and quickly trapped by a nucleophile. Using hydrogen cyanide as nucleophile, the reaction is well-known and

Scheme 14.

Scheme 15.


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General procedures for the photolysis. Photolyses were performed in a 17 cm long á 3.0 cm diameter cylindrical quartz cells. The gas mixtures were prepared on a mercury-free vacuum line equipped with a vacuum gauge. A low pressure mercury lamp with principal emissions at 184.9 and 253.7 nm was used as light source for experiment with ammonia. A mercury lamp with principal emission at 253.7 nm (and without emission at 184.9 nm) was used for experiments with dihydrogen sulfide. The products of photochemical reactions and controls were identified by 1 H NMR spectroscopy and comparison of their spectra with those obtained from authentic samples. Controls were always performed to establish that there were indeed photochemical reactions in which the gas mixtures were allowed to stand at room temperature in the absence of light for the same period of time that corresponding samples were irradiated. In a typical experiment, ammonia (50 mbar) was added to ethylene (50 mbar) in the quartz photolysis cell and the mixture was immediately irradiated. For several samples, oligomeric products formed a film on the Suprasil window at the top of the cell. After 1 h irradiation the cell was washed with an NMR solvent (CDCl3 ) and the solution was collected in a NMR tube. Acknowledgements We thank the CNES, the GDR CNRS ``Exobiologie'' and the ``Programme National de Plan etologie'' (INSUCNRS) for financial support and, Dr. Pandarinathan Lakshmipathi for helpful suggestions in writing the publication. References
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