Carbon Condensation

The major uncertainty in the description of the detonation process is the carbon condensation that occurs in explosives with excess carbon. 

Real explosives, which have solid carbon as a detonation product, exhibit behavior that is not described adequately without including some time-dependent phenomenon, such as diffusion-controlled carbon deposition or some other kinetic behavior of the detonation products. A time-dependent carbon deposition is the only process known that could account for the large energy deficits required by the build-up process. 

The calculations support the proposal that the carbon is mostly graphite at 1.55 g/cc and mostly diamond at 1.64 g/cc. This surprising result also supports the proposal that all explosives producing large amounts of solid carbon may exhibit similar slope changes in their detonation velocity-density curves. It also suggests that the carbon present in high density RDX is similar to diamond. 

Since the effect of the form of the carbon is so important to the performance of an explosive, it is important that we examine the evidence for diamond and other forms of carbon in the detonation products. Experiments where the detonation products are recovered at ambient conditions do not represent the composition of the detonation products at the C-J state and do not furnish us the history of the products along the expansion isentrope. The recovered products will have had a complicated history of expansion, being reshocked after interaction with the container walls and associated non-equilibrium composition changes. If diamond is seen, the diamond phase can probably be associated with the higher pressures near the C-J state. Graphite can be formed near the C-J state and during the release process. 

Russian scientists have extensively investigated the carbon detonation products from explosives, as described in references 56 thru 60. They exploded mixtures of TNT and RDX in explosive chambers with inert gas. They collected the solid products, which were 8 to 9 percent of the initial mass of the explosive. They have been able to obtain up to 80% of the solid product as diamond powder with an average particle diameter of 4 nanometers. Larger diamond particles were obtained which are made up of Svelded 4 nanometer particles. From electrical conductivity measurements they found that the carbon production was completed in 0.2 to 0.5 microseconds . They studied the formation of carbon using isotopic carbon methods and found that the condensed carbon isotopic ratio was the same as in the initial explosive. They report producing about a ton of industrial diamonds a year from explosives. 

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As it was established by Geus et a/.[18, 19] the decrease of the rate of carbon deposition is a positive factor for the growth of fibres on metal catalysts. Si02 is an inhibitor of carbon condensation as was shown in Ref [20]. This support also provides possibilities for the stabilization of metal dispersion. Co and Fe, i.e. the metals that give the best results for the tubular condensation of carbon on graphite support, were introduced on the surface of siUca gel... 

The more interesting situation arises in quinones which possess two dissimilar substituents. The site of initial carbon-to-carbon condensation is explicable in terms of the relative electronic effects. Thus condensation of 2-chloro-5-methylbenzoquinone (19) with t-butyl 3-aminocrotonate (20) in hot acetic acid furnished the 4-chloro-7-methylindole (21) in 51% yield. ... 

It is well to add that most of the compounds of carbon condense to molecular liquids and solids. Their melting points are generally low (below about 300°C) and many carbon compounds boil below 100°C. The similar chemistry of the liquid and solid phases shows the retention of the molecular identities. 

Such results were supposed to correspond to an increase in the size and/or number of the sp -carbon condensed ring domains present in the films upon increasing temperature, up to graphitization at 600 °C. After that, the expected behavior for graphitized carbon, the decrease of /d//g upon the increase of crystal size is observed. 

Scheme 2. Trapping of carbon condensates with dicyanogen and chlorine.

Alkylpyrazines are prepared by two main procedures, (1) self-condensation of a-amino carbonyl compounds and (2) alkylation (or acylation) of pyrazines at nuclear or side chain carbons. Condensation of aminoacetone hydrochloride (491) in the presence of the corresponding aldehyde gave the 3- -alkyl-2,5-dimethylpyrazines 20e and 20g (Scheme 61) 36). 2,5-Dimethylpyrazine 20a was prepared from hydroxyiminoacetone (493) by reduction with tin and hydrochloric acid (Scheme 61) 145). 3-Ethyl-2,6-dimethylpyrazine (21b) was prepared along with 20a and 495 by condensation of aminoacetone hydrochloride (491) with 2-aminopentan-3-one hydrochloride (494) in the presence of sodium ethoxide (Scheme 61) 36). 

The energy for the carbon-to-carbon condensations in fatty acid synthesis is supplied by the process of carboxylation and then decarboxylation of acetyl groups in the cytosol. The carboxylation of acetyl CcA to form malonyl CoA is catalyzed by acetyl CoA carboxylase (Figure 16.7), and requires HC03 )and ATP. The coenzyme is the vitamin, biotin, which is covalently bound to a lysyl residue of the carboxylase. 

A possible explanation of these differences is that the reactions of the trivalent phosphorus compounds with carbonyl compounds may contain reversible and irreversible steps. The condensation of one mole of biacetyl with one of the phosphite probably contains several reversible steps leading eventually to the 1 1 oxyphosphorane. If this 1 1 adduct is of high energy as a result of ring strain and/or intramolecular crowding in the TBP, it might not be observable. The carbon-carbon condensation step in the formation of the 2 1 adducts is probably essentially irreversible under most conditions hence, once that step is achieved, there is an opportunity for isolation of the 2 1 phosphoranes. 

A combination of DEPT and IGD spectra allows more precise quantitative analysis of, for example, the number of quaternary aromatic carbons. From such results, the degree of carbon-carbon condensation on the aromatic ring can be defined as the number of quaternary carbon atoms that are neither CL nor C4 nor are methoxyl-substituted C3/C5. This kind of information is particularly useful for following condensation reactions during such processes as steam explosion treatment of aspen wood (Robert et al. 1986, Bardet 1987) and kraft cooking (Robert et al. 1984, Gellerstedt and Robert 1987). 

Of particular interest in the present chapter is the effect of test atmosphere on creep and creep damage mechanisms. While there are undoubtedly several factors that can promote creep cavitation and contribute to the observed changes in stress exponent and activation energy, the fact remains that the strain rates are substantially higher in air than in inert atmospheres, as shown in Fig. 8.12. This phenomenon is a direct consequence of the topotactic oxidation reaction of SiC whiskers exposed at the surface. As described by Porter and Chokshi,38 and subsequently by others,21,22 at high temperatures in air, a carbon-condensed oxidation displacement reaction occurs in which graphitic carbon and silica are formed at the whisker interface via... 

Although silver carbonate although is not a very powerful oxidizing agent, it is extremely useful for the oxidation of alcohols to carbonyl compounds. The silver carbonate condensed on Celite is known as Fetizon s reagent which oxidizes primary alcohols to aldehydes (e.g. 7.5 is converted into 7.6) and secondary alcohols to ketones. 

This interpretation has been questioned by Chang and Mack (1978), who found that water-soluble organics, especially amino acid extracts from the Murchison meteorite, were almost as heavy as carbonate C [8C (PDB) = -1-23 to -l-44%o vs. +44.4%o], in contrast to the much lighter insoluble polymer (—15.3 to —16.8%o) or benzene-methanol extracts ( + 5.0%o)- They suggested that these various forms of carbon represent several stages of carbon condensation in the solar nebula, in different environments separated in space and possibly in time . 

C. a carbon-carbon condensation between a ketone and an ester... 

Common synthetic methods for the preparation of cyclic 8-enamino esters are the condensation between a lactim ether and benzyl cyanoacetate followed by hydrogenolytic decarboxylation, or the imino ester carbon-carbon condensation with tert-butyl cyanoacetate followed by a trifluoroacetic acid treatment. The use of a thiolactam condensed with ethyl bromoacetate gives, after sulfur extrusion by triphenylphosphine, cyclic 8-enamino esters. Compared with these methods, the Meldrum s acid condensation followed by the monodecarboxylating transesterification described here is more convenient and practical. An extension of this procedure permits preparation of smaller... 

We D see later in this chapter and again in Chapter 29 that carbon condensation reactions occur frequently in metabolic pathways. Almost all classes of biomolecules—carbohydrates, lipids, proteins, nucleic acids, and many others—are biosynthesized through routes that involve carbonyl condensation reactions. 

Industrially, a phosphoric acid solution is used which is neutralized with the appropriate quantity of sodium hydroxide or sodium carbonate. Condensation of the monophosphate solution is carried out in one or more stages in spray towers or rotary kilns. 

R—(OCH2CH2) OH with n > 4 as a matter of fact the best results are obtained for n = 2. Moreover the activity seems to decrease when R, being linear, the carbon condensation increases. 

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