Internal Reactions

The enormous amount of research at the interface between physical and structural chemistry has been expertly reviewed recently by Schmalzried in a book about chemical kinetics of solids (Schmalzried 1995), dealing with matters such as morphology and reactions at evolving interfaces, oxidation specifically, internal reactions (such as internal oxidation), reactions under irradiation, etc. 

The living microbial, animal, or plant cell can be viewed as a chemical plant of microscopic size. It can extract raw materials from its environment and use them to replicate itself as well as to synthesize myriad valuable products that can be stored in the cell or excreted. This microscopic chemical plant contains its own power station, which operates with admirably high efficiency. It also contains its own sophisticated control system, which maintains appropriate balances of mass and energy finxes through the links of its internal reaction network. 

In the pores of the electrodes, practically no natural convection of the liquid takes place. Reactants dissolved in the liquid can be supplied in two ways from the external surface to the internal reaction zones (and reaction products transported away in the opposite direction) (1) by diffusion in the motionless liquid diffusion electrode),... 

A. Ethyl 3-hydroxy-4-pentenoate. A dry, 2-L, two-necked, round-bottomed flask, capped with septa and equipped with a thermometer (Note 1), magnetic stirring bar, and an argon inlet is flushed with argon and charged with dry tetrahydrofuran (400 mL, Note 2) and diisopropylamine (30.8 mL, 220 mmol, Note 3). The solution Is cooled to -30°C and butyllithium (BuLi) (93.2 mL, 220 mmol, 2.36 M solution in hexanes, Note 4) is added. The reaction is stirred for 15 min and cooled to -76° to -78°C. Dry ethyl acetate (19.5 mL, 200 mmol, Note 5) is added dropwise so that the Internal reaction temperature remains below -66°C (addition time 10-15 min). When addition of the ethyl acetate is complete, the reaction Is stirred for 50 min at -70° to -78°C. A solutior of freshly distilled acrolein (13.4 mL, 200 mmol, Note 6) and 100 mL of dry... 

A Fluke 51 K/J digital thermometer with temperature probe is used to monitor internal reaction temperature. 

Fig. 7 Internal reaction coordinate (IRC) computations for the Bergman cyclization of model enediynes.

A. 1,1-Dibromo-2,2-bis(chloromethyl)cyclopropane (1). Into a 1-L, threenecked, round-bottomed flask, equipped with an efficient mechanical stirrer, a thermometer, and a condenser equipped with a potassium hydroxide drying tube, are placed 54.1 g (0.403 mol) of 3-chloro-2-(chloromethyl)propene (Note 1), 212 g (0.805 mol) of bromoform (Note 2), 1.70-2.00 g (14.4-16.9 mmol) of pinacol (Note 3), and 1.45 g (3.94 mmol) of dibenzo-18-crown-6 (Note 4). With very vigorous stirring (Note 5), 312 g of an aqueous 50% sodium hydroxide solution that has been cooled to 15°C is added in one portion. The reaction mixture turns orange, then brown, then black within 5 min, and the temperature of the reaction mixture begins to rise. Within 20 min, the internal reaction temperature is 49-50°C at which point the reaction flask is cooled with a room-temperature water bath, and the reaction temperature decreases to ca. 

Figure 9.5 Cyclic, hemiacetal structures of D-glucose. The reaction between an alcohol and aldehyde group within an aldohexose results in the formation of a hemiacetal. The only stable ring structures are five- or six-membered. Ketohexoses and pentoses also exist as ring structures due to similar internal reactions.

With the tests of percent hydrogen in air around 25% - 30%, reactions were observed which did not make the lid pop up. For instance, with percent hydrogen in air at 25% - 30% and the pressure at 150 torr, the chamber did not explode with enough force to pop up the lid there was only a quick flash in the chamber. Although an explosion did not occur to over-pressurize the vessel, an internal reaction did, the pressures of which are shown in Figure 5. This reaction, though contained and silent, is evidence that a reaction can occur in deeper vacuum, and in richer and leaner hydrogen concentrations than evidenced by observation of over-pressurization. 

Figure 5. The pressure increase of an internal reaction over time at 25%-30% hydrogen.

It seems that there is a missing link between the scenarios of the prebiotic RNA-world and the compartmentalishc approach and the missing link is how to make RNA by a prebiotic sequence or network of internalized reactions. 

The term chemical autopoiesis indicates the experimental implementation of autopoiesis in the chemistry laboratory. The most well known of these processes is the self-reproduction of micelles and vesicles. This has been discussed in the previous chapter, where the original idea of Francisco Varela and myself was to work with bounded systems that would produce their own components due to an internal reaction, respecting the scheme illustrated in Figure 8.3. We came up with the idea of using reverse micelles (refer back to Figure 7.13) with two reagents. 

The criteria used for the prediction of gas-carbon reactions entering Zone II, for first order reactions, are presented in Table IV, with the results of Thiele (100) for plane and spherical specimens included. Zone II is entered when (f>n, where is the value of (j> for the start of Zone II and is 2, 4, or 6 for a plane, cylinder, or sphere, respectively. In all cases, the specimens approach uniform internal reaction, that is chemical control, when is true activation energy is obtained. 

A 500-ml flask equipped with a reflux condenser, stirrer, and a dropping funnel was treated with the step 4 product (0.027 mol) and 200 ml CH2C12. The solution was then treated with the dropwise addition of boron tribromide (0.135 mol) over 30 minutes at such a rate that the internal reaction temperature did not exceed 10°C. The mixture was then stirred at ambient temperature for 3 hours and then treated with water to stop the reaction. Posttreatment was carried out in the same manner as described in step 1 and 10.01 g of product isolated. 

Where the accidental fuel release is the result of an internal combustion or other reaction, the released fuel is at an elevated temp and contains a concn of free radicals derived from the internal reaction such that the ignition delay is short. (Also, the ignition can be characterized as multipoint where the blast effects approach that of a bursting pressurized sphere)... 

Dimethoxyacetophenone (15 g, 83 mmol) was added over lh to coned HN03 (90 mL) with stirring, keeping the internal reaction temperature in the range 18-22 °C. After the addition was complete, the soln was stirred for an additional hour and then poured into H20 (1200 mL). After chilling, the product was collected and recrystallized from EtOH yield 8.4 g (45%) mp 130-132 °C. 

This is an interesting result. We cannot always neglect the space-charge width compared to the recombination length crystals with varying disorder types will be further discussed in Chapter 9. 

The foregoing results have been derived with the tacit assumption that there are neither internal nor external redox reactions that may influence the majority defect concentrations. Internal reactions could be, for example,... 

Systematically speaking, so-called internal oxidation reactions of alloys (A,B) are extreme cases of morphological instabilities in oxidation. Internal oxidation occurs if oxygen dissolves in the alloy crystal and the (diffusional) transport of atomic oxygen from the gas/crystal surface into the interior of the alloy is faster than the countertransport of the base metal component (B) from the interior towards the surface. In this case, the oxidation product BO does not form a stable oxide layer on the alloy surface. Rather, BO is internally precipitated in the form of small oxide particles. The internal reaction front moves parabolically ( Vo into the alloy. Examples of internal reactions are discussed quantitatively in Chapter 9. 

Chapters 6 and 7 dealt with solid state reactions in which the product separates the reactants spatially. For binary (or quasi-binary) systems, reactive growth is the only mode possible for an isothermal heterogeneous solid state reaction if local equilibrium prevails and phase transitions are disregarded. In ternary (and higher) systems, another reactive growth mode can occur. This is the internal reaction mode. The reaction product does not form at the contacting surfaces of the two reactants as discussed in Chapters 6 and 7, but instead forms within the interior of one of the reactants or within a solvent crystal. 

By a change of temperature or pressure, it is often possible to cross the phase limits of a homogeneous crystal. It supersaturates with respect to one or several of its components, and the supersaturated components eventually precipitate. This is an additive reaction. It occurs either externally at the surfaces, or in the crystal bulk by nucleation and growth. Reactions of this kind from initially homogeneous and supersaturated solid solutions will be discussed in Chapter 12 on phase transformations. Internal reactions in the sense of the present chapter occur after crystal A has been brought into contact with reactant B, and the product AB forms isothermally in the interior of A or B. Point defect fluxes are responsible for the matter transport during internal reactions, and local equilibrium is often established throughout. 

A third type of internal solid state reaction (see later in Fig. 9-12) is characterized by two (solid) reactants A and B which diffuse into a crystal C from opposite sides. C acts as a solvent for A and B. If the reactants form a stable compound AB with each other (but not with the solvent crystal C), an internal solid state reaction eventually takes place. It occurs in the solvent crystal at the location of maximum supersaturation of AB by internal precipitation and subsequent growth of the AB particles. Similar reactions can be observed on a crystal surface which, in this case, plays the role of the solvent matrix C. Surface transport of the reactants leads to a product band precipitated on the surface at some distance from each of the two reactants and completely analogous to the internal reactions described before. In addition, internal reactions have also been observed if (viscous) liquids are chosen as the reaction media (C). 

There is still another type of internal solid state reaction which we will discuss and it is electrochemical in nature. It occurs when an electrical current flows through a mixed conductor in which the point defect disorder changes in such a way that the transference of electronic charge carriers predominates in one part of the crystal, while the transference of ionic charge carriers predominates in another part of it. Obviously, in the transition zone (junction) a (electrochemical) solid state reaction must occur. It leads to an internal decomposition of the matrix crystal if the driving force (electric field) is sufficiently high. The immobile ionic component is internally precipitated, whereas the mobile ionic component is carried away in the form of electrically charged point defects from the internal reaction zone to one of the electrodes. 

Internal nucleation and growth can occur coherently or incoherently while the reaction volume can be negative or positive. The severe constraints which the matrix crystal exerts on the internal reaction can lead to the formation of metastable (or even unstable) phases, which do not exist outside the matrix. Often, heavy plastic flow and anisotropic growth has been found. 

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