Exothermic reaction sequence

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. 

Both the routes mentioned earlier pertain to cell metabolism, which broadly speaking cormotes any biologically inspired reaction sequence. Most usually, however, the term applies to oxidative or exothermic reaction sequences involving energy sources such as glucose or glycogen, and the expression of primary metabolism is more definitive, as distinguished from secondary metabolism. 

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 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. 

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. 

Rearrangement of radicals frequently occurs by a series of addition-fragmentation steps. The fragmentation of alkoxyl radicals is especially common because the formation of a carbonyl bond makes such reactions exothermic. The following two reaction sequences are examples of radical rearrangements proceeding through addition-elimination. 

The results show that as the R substituent increases in size it becomes possible to see a reaction pattern in which there is an endothermic reaction followed by an exothermic reaction followed by an endothermic reaction which in turn is followed by an exothermic reaction. This reaction sequence is particularly noticeable for the cyclohexyl derivative and is tentatively interpreted by the following scheme ... 

We rray also note that very similar sequences arise for a CSTR in which two non-competitive exothermic reactions proceed, coupled only through the... 

The most fundamental and obvious observation to be made concerning the thermochemistry in Table I is that no individual step in this reaction sequence is exothermic enough to break carbon—carbon bonds except the termination step 3a of —97.5 kcal mole 1. Consequently procedures or conditions that minimize the atomic fluorine population or decrease the mobility of hydrocarbon radical intermediates, such as keeping them in the solid state during reaction, are desirable. It is necessary to decrease the reaction rate to the extent that these hydrocarbon radical intermediates have finite lifetimes so that the advantages of fluorination in individual steps may be realized experimentally. It has been demonstrated by EPR(8) methods that under reaction conditions with high fluorine dilution, the various radicals do indeed have appreciable lifetimes. 

The use of aryllithium instead of the Grignard reagent resulted in a higher ratio of cis-isomer formation [34], In reaction calorimetric studies, it was found that both steps, the formation of 3-methoxyphenyllithium and its addition to ketone, are pretty exothermic with worst-case temperature rises of up to 62 and 133 °C, respectively. The lithium intermediate has to be kept at very low reaction temperatures to prevent decomposition. We concluded that a continuous reaction may be a good alternative to batch synthesis to improve the reaction yield and to minimize the safety concerns because of the exothermicity of the reaction sequence. 

It may be worth remarking in passing that Eq. (5.2.5) shows that a falling temperature sequence is always required with an exothermic reaction, for on differentiating this equation we have... 

The exothermic peak evidently does not account for the full amount of exothermic reaction taking place, since the exotherm is partly masked by the endotherm. Fig. 5 c shows, for comparison, a statistic copolymer containing AN and VCl in an approximately 1 1 ratio (commercial product). Although sequences of AN units can be assumed to be present in this copolymer, the picture is very similar to Fig. 5b. 

Above the troposphere is the stratosphere, which consists of nitrogen, oxygen, and ozone. In the stratosphere, the air temperature rises with altitude. This warming effect is the result of exothermic reactions triggered by UV radiation from the sun (to be discussed in Section 17.3). One of the products of this reaction sequence is ozone (O3), which, as we will see shortly, serves to prevent harmful UV rays from reaching Earth s surface. 

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