Exothermic sequence

It is credibly held that at least three equations are needed for chaotic behavior and it is not surprising that it has been found in the stirred tank when two reactions are taking place. Rbssler, Varna and Kahlert (63) have found it for consecutive reactions, one exo- and the other endothermic Lynch and others (64) have used parallel reactions the exothermic sequence... 

Fig. 12. Typical double-resonance spectra for reactions under conditions of high conversion to product. Curves 1-4 are typical of exothermic sequences. Curves 5-7 are typical of sequences with endothermic steps.

The curing process of PF resins is chemically and thermally complex, characterized by endo- and exothermic sequences. During initial heat input, the free water begins to evaporate at approx. 60 - 80 °C in appreciable quantities. Simultaneously, the non-cured material begins to soften, linked to increasing molecular movement in the chains. Both process steps are endothermic and completed at approx. 110 °C. Next is an initial strong exothermic crosslinking reaction up to approx. 160 °C. 

Shift Conversion. Carbon oxides deactivate the ammonia synthesis catalyst and must be removed prior to the synthesis loop. The exothermic water-gas shift reaction (eq. 23) provides a convenient mechanism to maximize hydrogen production while converting CO to the more easily removable CO2. A two-stage adiabatic reactor sequence is normally employed to maximize this conversion. The bulk of the CO is shifted to CO2 in a high... 

For all of these reactions, the reagents and reaction conditions must be chosen to meet the fundamental requirement for successful chain reactions. Each step in the sequence must be exothermic to permit chain propagation. ... 

Since 1973, several authors have proved that there is a relationship between thermostability of collagen and the extent of hydroxylation of the proline residues31,34). Equilibrium measurements of the peptides al-CB 2 of rat tail and rat skin revealed a higher rm, for al-CB 2 (rat skin)157). The sequence of both peptides is identical except that in the peptide obtained from rat skin, the hydroxylation of the proline residues in position 3 has occurred to a higher extent than in the case of al-CB 2 (rat tail). Thus, a mere difference of 1.8 hydroxy residues per chain causes a ATm of 26 K. Obviously, there are different stabilizing interactions in the triple-helical state, that means al-CB 2 (rat skin) forms more exothermic bonds than al-CB 2 (rat tail) in the coil triple-helix transition. This leads to an additional gain of enthalpy which overcompensates the meanwhile occurring losses of entropy. 

The rate constants for unimolecular dissociation of the intermediate ions suggested earlier indicate that all ions containing seven or more carbon atoms arise from reactions of the dissociation products of Steps 9, 13, and 17 when pressures are of the order of a few torr and of Step 20 and its analogues at pressures in excess of a few hundred torr. The product ions are generally quite complex, and the simple exothermicity rule given earlier will not apply. Thus, we may well expect that there will be inefficient ion-molecule reactions in the sequences originating with these ions as well. 

Due to the exothermic nature of the reaction and the phase separation which occurs, temperature and conversion (and MWD and sequence distribution) can only be assigned local, not global, values in polyurethane reaction molding. 

There are instances where the reflux temperature decreases with conversion of the reagents, and exothermic reactions may even lead to hazardous runaway conditions. In such cases it may become necessary to optimize an addition sequence of reagents to avoid unwanted exotherms. Anderson et al. (1997) have given a practical example for an important intermediate in making the drug monopril. 

In order to develop the safest process the worst runaway scenario must be worked out. This scenario is a sequence of events that can cause the temperature runaway with the worst possible consequences. Typically, the runaway starts with failure that results in an adiabatic course of exothermic reaction, inducing secondary reactions that proceed with a high thermal effect. Such a. sequence of typical events is shown in Fig. 5.4-55 (after Gygax, 1988-1990 Stoessel, 1993). It starts with, for instance, a cooling failure at time tx when the temperature is at the set level, Tv ,- Then the temperature rises up to the Maximum Temperature for Synthetic Reaction (MTSR) within the time Atn. Assuming adiabatic conditions MTSR = + ATad,R... 

The first-order consecutive exothermic reaction sequence, A —> B —> C, is carried out in a thick-walled, jacketed batch reactor, provided with both jacketed heating and cooling, as shown below. 

The fundamental mechanisms of free radical reactions were considered in Chapter 11 of Part A. Several mechanistic issues are crucial in development of free radical reactions for synthetic applications.285 Free radical reactions are usually chain processes, and the lifetimes of the intermediate radicals are very short. To meet the synthetic requirements of high selectivity and efficiency, all steps in a desired sequence must be fast in comparison with competing reactions. Owing to the requirement that all the steps be fast, only steps that are exothermic or very slightly endothermic can participate in chain processes. Comparison between addition of a radical to a carbon-carbon double bond and addition to a carbonyl group can illustrate this point. 

The fragmentation of alkoxyl radicals is especially favorable because the formation of a carbonyl bond makes such reactions exothermic. Rearrangements of radicals frequently occur by a series of addition-fragmentation steps. The following two reactions involve radical rearrangements that proceed through addition-elimination sequences. 

The observed H+(NH3)n and H (NH3)n(PA) clusters are thought to be formed in a two-step reaction sequence taking place after ionization of the PA(NH3) cluster. The first step is a charge transfer (CT) reaction between the resonantly ionized PA+ and the NH3 molecules in the cluster. The second step is an intracluster ion-molecule reaction (ICIMR) of the charged ammonia cluster leading to the formation of an (n - 1) protonated cluster ion this has been previously established for NH3 clusters33 and is sufficiently exothermic for fragmentation of the cluster. 

The dioxirane 8a is much less labile than the carbonyl oxide 7a, and UV irradiation (A > 400 nm) is required to induce the rearrangement to lactone 9a. With 70 kcal/mol this is the most exothermic step in the whole reaction sequence from la to 9a. All of these reaction intermediates were generated in subsequent steps in high yields and characterized by matrix IR spectroscopy. 

Figure 1.1 Schematic representation of a well known catalytic reaction, the oxidation of carbon monoxide on noble metal catalysts CO + Vi 02 —> C02. The catalytic cycle begins with the associative adsorption of CO and the dissociative adsorption of 02 on the surface. As adsorption is always exothermic, the potential energy decreases. Next CO and O combine to form an adsorbed C02 molecule, which represents the rate-determining step in the catalytic sequence. The adsorbed C02 molecule desorbs almost instantaneously, thereby liberating adsorption sites that are available for the following reaction cycle. This regeneration of sites distinguishes catalytic from stoichiometric reactions.

The NCO—CH2(CF2) CH2—OCN monomer series have been characterized by DSC, IR, H-NMR, 19F-NMR, 13C-NMR, and elemental analysis.8 Table 2.1 summarizes the characterization most pertinent to these cyanate ester monomers. The n = 5,1, and 9 members are missing. This is a reflection of the difficulty in obtaining the odd hydrocarbon diol precursors. The trend of a rapid melting point increase with increasing fluoromethylene sequence length is an indication that monomers with n > 10 will probably not be melt-processible since the onset of the cure exotherm in most purified monomers occurs at 200°C. 

The results presented above and their comparison with those for the Al + H2 reaction indicate that addition of the second hydrogen molecule to B1(A7) should be as easy as addition of the first H2 molecule to Al, which is known to occur at laboratory conditions. Indeed, the rate-determining barriers of the reaction sequence, Al + H2 - A3 - A7 and B1(A7) — B3 are calculated to be 21.2 and 19.8 (19.5) kcal/mol, respectively. However, the first process is exothermic by 13 kcal/mol, while the second process is endothermic by 8 kcal/mol. 

The order of addition does not seem to be important. If the sequence that is described is employed, a slight exotherm is observed upon addition of the acetic acid. 

Scheme 9.1 shows a generalized sequence of reactions for the oxidation of an alkane, via alcohol, ketone and carboxylic acid, to the completely oxidized products, water and carbon dioxide. The latter are often referred to as combustion products as they are the same as those formed by burning hydrocarbons. These are not normally desirable chemical products unless it is necessary to destroy a toxic, hazardous or otherwise unwanted waste material. Oxidation itself is not difficult to achieve, and is a highly exothermic or even explosive process. Selective oxidation, however, is a much greater challenge, as it is important to stop the sequence at the desired product without proceeding further down the oxidation pathway. 

From the sequence of reactions (3.32)—(3.35) one finds that although reaction (3.21) terminates the chain under some conditions, under other conditions it is part of a chain propagating path consisting essentially of reactions (3.21) and (3.28) or reactions (3.21), (3.31), and (3.35). It is also interesting to note that, as are most H02 reactions, these two sequences of reactions are very exothermic that is,... 

Cadmium(II) and zinc(II) systems other than cyanides Among the i acceptors of the zinc group, the softness rapidly decreases from the markedly soft Hg2+ to the mildly soft Cd2+ and to the distinctly hard Zn2+. As mentioned above, only very soft ligands such as CN are coordinated to Cd2+ or Zn + by bonds which are essentially covalent. Nevertheless, covalent bonding is still important for the formation of the Cd2+ halide complexes. This is evident from the fact that the values of AHn become more exothermic as the halide becomes larger and consequently more polarizable and susceptible to covalent bonding. This trend results in the (6) or soft sequence for the halide systems of... 

---