Chemical reactions disproportionation reactions

The basic idea of catalysis can be traced to the writings of J. J. Berzelius/ who in 1836 reviewed a number of curious occurrences in which traces of certain substances seemed to have an effect on chemical reactions disproportionate to then-amounts. In a passage often quoted, but bearing repetition, he wrote/- ... 

Step chronoamperometry (DPSC) conducted on a microsecond timescale reveal that l-methyl-3-carbamidopyridinyl radicals react via a dimerisation mechanism involving direct coupling of the electrogenerated neutral radicals at a rate of approximately 1.6 0.1 X 10 sec in DMF. The l-methyl-4-carbamidopyridinyl and l-methyl-3,4-dicarbamidopyridinyl radicals react via a pH-dependent ECE-DISPI mechanism, where E, C and DISP denote electron transfer, following chemical and disproportionation reactions, respectively. 

The assumption that k values are constant over the entire duration of the reaction breaks down for termination reactions in bulk polymerizations. Here, as in Sec. 5.2, we can consider the termination process—whether by combination or disproportionation to depend on the rates at which polymer molecules can diffuse into (characterized by kj) or out of (characterized by k ) the same solvent cage and the rate at which chemical reaction between them (characterized by kj.) occurs in that cage. In Chap. 5 we saw that two limiting cases of Eq. (5.8) could be readily identified ... 

Mitsubishi Chemical Industries, Ltd. practiced a Henkel II technology starting with toluene to produce benzoic acid. Reaction of benzoic acid with potassium hydroxide resulted in potassium benzoate, which was subjected to a disproportionation reaction to produce dipotassium terephthalate and benzene. Dipotassium terephthalate reacted with sulfuric acid, and the resulting terephthahc acid was recovered by filtration and drying (65,66). Here, dipotassium sulfate was the by-product. 

Even though the chemical reactions are the same (i.e. combination, disproportionation), the effects of compartmentalization are such that, in emulsion polymerization, particle phase termination rates can be substantially different to those observed in corresponding solution or bulk polymerizations. A critical parameter is n, the average number of propagating species per particle. The value of h depends on the particle size and the rates of entry and exit. 

The photolysis of dimethyl sulphoxide (at 253.7 nm) in a wide range of solvents has been studied in detail176. Three primary reactions occur, namely (i) fragmentation into methyl radicals and methanesulphinyl radicals, equation (60), (ii) disproportionation into dimethyl sulphone and dimethyl sulphide, equation (61) and (iii) deactivation of the excited state to ground state dimethyl sulphoxide. All chemical processes occur through the singlet state. Further chemical reactions of the initial photochemical products produce species that have been oxidized relative to dimethyl sulphoxide. 

The relative importance of the disproportionation process (SET between two anion radicals) depends principally on the thermodynamic constant (K). It can be easily determined more or less accurately from the potential difference existing between the first cathodic peak and the second one. (An exact calculation would be possible from the thermodynamic potentials of the two reversible transfers in the absence of proton sources and at reasonable sweep rates so as to inhibit any undesirable chemical reaction.)... 

The number of chemical reactions used in CVD is considerable and include thermal decomposition (pyrolysis), reduction, hydrolysis, disproportionation, oxidation, carburization, and nitrida-tion. They can be used either singly or in combination (see Ch. 3 and 4). These reactions can be activated by several methods which are reviewed in Ch. 5. The most important are as follows ... 

Chain reactions do not continue indefinitely, but in the nature of the reactivity of the free radical or ionic centre they are likely to react readily in ways that will destroy the reactivity. For example, in radical polymerisations two growing molecules may combine to extinguish both radical centres with formation of a chemical bond. Alternatively they may react in a disproportionation reaction to generate end groups in two molecules, one of which is unsaturated. Lastly, active centres may find other molecules to react with, such as solvent or impurity, and in this way the active centre is destroyed and the polymer molecule ceases to grow. 

Now consider the other extreme condition where diffusion is rapid relative to chemical reaction [i.e., hT( 1 — a) is small]. In this situation the effectiveness factor will approach unity for both the poisoned and unpoisoned reactions, and we must retain the hyperbolic tangent terms in equation 12.3.124 to properly evaluate Curve C in Figure 12.11 is calculated for a value of hT = 5. It is apparent that in this instance the activity decline is not nearly as sharp at low values of a as it was at the other extreme, but it is obviously more than a linear effect. The reason for this result is that the regions of the catalyst pore exposed to the highest reactant concentrations do not contribute proportionately to the overall reaction rate because they have suffered a disproportionate loss of activity when pore-mouth poisoning takes place. 

A solution of the isolated platinum blue compound usually contains several chemical species described in the previous section. Such complicated behaviors had long been unexplored, but were gradually unveiled as a result of the detailed equilibrium and kinetic studies in recent years. The basic reactions can be classified into four categories (l)HH-HT isomerization (2) redox disproportionation reactions (3) ligand substitution reactions, especially at the axial coordination sites of both Pt(3.0+)2 and Pt(2.5+)4 and (4) redox reactions with coexisting solvents and atmosphere, such as water and 02. In this chapter, reactions 1-4 are summarized. 

Chemically, nonmetals are usually the opposite of metals. The nonmetallic nature will increase towards the top of any column and toward the right in any row on the periodic table. Most nonmetal oxides are acid anhydrides. When added to water, they will form acids. A few nonmetals oxides, most notably CO and NO, do not react. Nonmetal oxides that do not react are neutral oxides. The reaction of a nonmetal oxide with water is not an oxidation-reduction reaction. The acid that forms will have the nonmetal in the same oxidation state as in the reacting oxide. The main exception to this is N02, which undergoes an oxidation-reduction (disproportionation) reaction to produce HN03 and NO. When a nonmetal can form more than one oxide, the higher the oxidation number of the nonmetal, the stronger the acid it forms. 

Second-order irreversible chemical reaction following a reversible electron transfer disproportionation. The disproportionation reaction can be represented as ... 

Chemical reactions are designated as C, so if the product of electron transfer undergoes a homogeneous chemical reaction we say that it is an EC reaction. The C terms are often given a superscript or subscript to show why type of chemical reaction occurs, e.g. disproportionation, dimerization or catalytic. Table 6.4 lists many of the commonly encountered Reinmuth terms. 

Disproportionation A chemical reaction in which a single compound serves as both an oxidizing and reducing agent. It converts to a more oxidized and a more reduced derivative. 

Transalliyiation. A chemical reaction involving the movement of a group from one molecule to another molecule, dealkylating the first, alkylating the second. An example would be the reaction in which two molecules of toluene form benzene and j -lene, which involves a quick two-step of a methyl group and a hydrogen between toluene molecules (also called disproportionation). 

The study of Li28 + DMF solutions [60] also allowed to characterize the electrochemical properties of polysulfides only redox couples of the type 8 /8 are involved. The chemical reactions coupled to charge transfers are classical dissociation and disproportionation equilibria no complex rearrangement reaction or transient species has been necessary. Redox potentials and charge-transfer coefficients of the redox couples involved in sulfur and polysulfide solutions are summarized in Table 2. 

E=electrochemical reaction, C=chemical reaction, and disp=disproportionation). 

This cascade however may be propagated throughout the cell unless terminated by a protective mechanism (see below) or a chemical reaction such as disproportionation, which gives rise to a non-radical product. Polyunsaturated fatty acids, found particularly in membranes, are especially susceptible to free radical attack. The effects of lipid peroxidation are many and various. Clearly, the structural integrity of membrane lipids will be adversely affected. In the lipid radical produced, the sites of unsaturation may change, thereby altering the fluidity of the membrane (see chap. 3). Lipid radicals may interact with other lipids and... 

A summary of the major chemical reactions of free radicals is given in Table 4.3. Broadly speaking these can be classified as unimolecular reactions of dissociations and isomerizations, and bimolecular reactions of additions, disproportionations, substitutions, etc. The complexity of many photochemical reactions stems in fact from these free radical reactions, for a single species formed in a simple primary process can lead to a variety of final products. 

---