The Reaction Mechanisms

As early as in the first paper on lucigenin chemiluminescence [1] the dioxetan derivative (1) had been postulated as a key intermediate  

Nonetheless, a dioxetan decomposition mechanism for lucigenin chemiluminescence, based on the exergonic processes described in Chap. V, seems well established [3]. A direct demonstration of the intermediacy of this dioxetane was first made [4] in 1969 by treating 10,10 -dimethyl-9,9 -biacrylidene (4) with singlet oxygen from several sources. Emission from N-methyl acridone was unequivocally shown. The lifetime of the intermediate was characteristic of the supposed dioxetane. Intramolecular electron transfer has been suggested as the excitation mechanism in the decomposition of this and other electron-rich dioxetans. 

A more detailed study [5] confirmed these observations 

If the concentrations of (2) in the solutions are higher, singlet-singlet energy transfer from (3) to (2) takes place. This can be seen in Fig. 13 (from [5])  

IV Chemiluminescence from incomplete Reaction of (4) with singlet oxygen. 

It is evident that the interaction of a phosphorus(III) triester and the alkylating species RX can be pictured as an S 2 process (reaction 2) or, for those alkylating reagents capable of forming a carbocation, as an S l process (reaction 3). Several reactions testify to the importance of carbocationic carbon for the Michaelis-Arbuzov reaction in pursuance of its normal course they include the ease of reaction of cyclopropene dihalides, already encountered, and the ready formation of complexes with species having particularly weakly nucleophilic counter ions. Phosphonic acid formation also takes place with cyclic azonium salts and related ions. 9-Chloroacridine reacts with triethyl phosphite to afford a product thought to be the bisphosphonic acid ester 49 The related phosphonic esters 51 are obtainable when the onium salts 50 (X = NH, NR, O or S) are treated with trimethyl 

Following a comparison of the behaviours of trialkyl phosphites, mixed alkyl phenyl phosphites and triphenyl phosphite towards iodomethane and, in the last case, the breakdown of the phosphonium salt when treated with an alcohol, Landauer and Rydon considered that all the reactions involve a stage identical with that of the normal Michaelis-Arbuzov reaction. The absence of any rearrangement during the decomposition of complexes from neopentyl phosphites, and the configurational inversion which occurs when optically active 2-halooctanes are produced from optically active phosphite triesters (themselves obtained from optically active octan-2-ol), suggest that the mode of breakdown of the intermediate complexes is of S 2 character. 

In spite of the large volume of evidence for the participation of ionic intermediates in the Michaelis-Arbuzov reaction, there is also considerable evidence for the formation and breakdown of other species during the course of the same reaction such participation occurs together with, or in place of, that of ionic species. 

When treated with iodomethane, initially at room temperature, a mixture of conform-ers of 5-r r/-butyl-2-methoxy-l,3,2-dioxaphosph(III)orinane (52) of conformational (at phosphorus) composition 77 23 yielded a mixture of 5-r r -butyl-2-methyl-2-oxo-1,3,2-dioxaphosphorinanes (53) of composition 71 29, the principal component being the conformer with P-methyl sited axially (53a) This high degree of stereospecificity was not 

On the basis of such evidence, it now seems to be widely accepted that the intermediates in valence expansion reactions of the Michaelis-Arbuzov type can have either an ionic, or a non-ionic, pentacoordinate structure, or both can be involved, possibly sequentially, or through equilibration, the choice being dependent on the ligands surrounding the central phosphorus atom, i.e. on the nature of the reactants. Thus reaction 1 might well be written as reaction 4. 

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Homogeneous catalysts. With a homogeneous catalyst, the reaction proceeds entirely in the vapor or liquid phase. The catalyst may modify the reaction mechanism by participation in the reaction but is regenerated in a subsequent step. The catalyst is then free to promote further reaction. An example of such a homogeneous catalytic reaction is the production of acetic anhydride. In the first stage of the process, acetic acid is pyrolyzed to ketene in the gas phase at TOO C ... 

The reaction mechanism for these products is not clearly understood, but the introduction of organo-metallic compounds (barium or iron salts in colloidal suspension) has been shown to have a beneficiai action on the combustion of diesel fuel in engines and reduce smoke. However, these products cause deposits to form because they are used in relatively large proportions (on the order 0.6 to 0.8 weight %) to be effective. 

K has been identified as CFl200I-I from its chemistry the reaction mechanism is insertion [115], Collision-induced dissociation (in a SIFT apparatus, a triple-quadnipole apparatus, a guided-ion beam apparatus, an ICR or a beam-gas collision apparatus) may be used to detemiine ligand-bond energies, isomeric fomis of ions and gas-phase acidities. 

The reaction mechanisms of plasma polymerization processes are not understood in detail. Poll et al [34] (figure C2.13.6) proposed a possible generic reaction sequence. Plasma-initiated polymerization can lead to the polymerization of a suitable monomer directly at the surface. The reaction is probably triggered by collisions of energetic ions or electrons, energetic photons or interactions of metastables or free radicals produced in the plasma with the surface. Activation processes in the plasma and the film fonnation at the surface may also result in the fonnation of non-reactive products. 

A transition structure is the molecular species that corresponds to the top of the potential energy curve in a simple, one-dimensional, reaction coordinate diagram. The energy of this species is needed in order to determine the energy barrier to reaction and thus the reaction rate. A general rule of thumb is that reactions with a barrier of 21 kcal/mol or less will proceed readily at room temperature. The geometry of a transition structure is also an important piece of information for describing the reaction mechanism. 

Consider the fact that some reactions have no barrier. You might also be making incorrect assumptions about the reaction mechanism. Consider these possibilities and start over. 

An ensemble of trajectory calculations is rigorously the most correct description of how a reaction proceeds. However, the MEP is a much more understandable and useful description of the reaction mechanism. These calculations are expected to continue to be an important description of reaction mechanism in spite of the technical difficulties involved. 

An interesting rearrangement of the (4-methyl-2-thiazolyl)thioureas (263) has recently been reported (Scheme 160) (303). The reaction mechanism is currently under investigation. This reaction does not occur if the 4-methyl substituent in the thiazole ring of 263 is replaced by an hydrogen, which suggests an electrophilic attack on C-5 as the mechanism of this reaction. 

Methoxythiazoles are converted to the corresponding N-methyl-A-4-thiazoline-2-ones by heating with excess methyl iodide (29, 243). The reaction mechanism can be considered initially as the formation of a... 

In the first chapter, devoted to thiazole itself, specific emphasis has been given to the structure and mechanistic aspects of the reactivity of the molecule most of the theoretical methods and physical techniques available to date have been applied in the study of thiazole and its derivatives, and the results are discussed in detail The chapter devoted to methods of synthesis is especially detailed and traces the way for the preparation of any monocyclic thiazole derivative. Three chapters concern the non-tautomeric functional derivatives, and two are devoted to amino-, hydroxy- and mercaptothiazoles these chapters constitute the core of the book. All discussion of chemical properties is complemented by tables in which all the known derivatives are inventoried and characterized by their usual physical properties. This information should be of particular value to organic chemists in identifying natural or Synthetic thiazoles. Two brief chapters concern mesoionic thiazoles and selenazoles. Finally, an important chapter is devoted to cyanine dyes derived from thiazolium salts, completing some classical reviews on the subject and discussing recent developments in the studies of the reaction mechanisms involved in their synthesis. 

Since then, the fundamental physicochemical aspects of the synthesis and properties of ev anines have been exhaustively reviewed by Heseltine and Stunner in the fourth edition of Mee s treatise (3) and by Sturmer in Weissberger s edition of the Chemistry of Heterocyclic Compounds (4). So the purpose of this section dealing especially with thiazolomethine dyes is to give, apart from a complete and recent list of dyes and references, a description of the particularities of their chemistry and chiefly of the reaction mechanisms involved in their synthesis that have remained unknown or have not been discussed until now. 

Several ESR spectra of thiazolyl and benzothiazolyl radicals have been recorded (667, 859), and this type of studies may be used in elucidation of the reaction mechanism. 

Our first three chapters established some fundamental principles concerning the structure of organic molecules and introduced the connection between structure and reactivity with a review of acid-base reactions In this chapter we explore structure and reactivity m more detail by developing two concepts functional groups and reaction mechanisms A functional group is the atom or group m a molecule most respon sible for the reaction the compound undergoes under a prescribed set of conditions How the structure of the reactant is transformed to that of the product is what we mean by the reaction mechanism... 

The regioselectivity and syn stereochemistry of hydroboration-oxidation coupled with a knowledge of the chemical properties of alkenes and boranes contribute to our under standing of the reaction mechanism... 

Compound A (C7Hi3Br) is a tertiary bromide On treatment with sodium ethoxide in ethanol A IS converted into B (C7H12) Ozonolysis of B gives C as the only product Deduce the struc tures of A and B What is the symbol for the reaction mechanism by which A is converted to B under the reaction conditions ... 

A key step in the reaction mechanism appears to be nucleophilic attack on the alkyl halide by the negatively charged copper atom but the details of the mechanism are not well understood Indeed there is probably more than one mechanism by which cuprates react with organic halogen compounds Vinyl halides and aryl halides are known to be very unreactive toward nucleophilic attack yet react with lithium dialkylcuprates... 

All these facts—the observation of second order kinetics nucleophilic attack at the carbonyl group and the involvement of a tetrahedral intermediate—are accommodated by the reaction mechanism shown m Figure 20 5 Like the acid catalyzed mechanism it has two distinct stages namely formation of the tetrahedral intermediate and its subsequent dissociation All the steps are reversible except the last one The equilibrium constant for proton abstraction from the carboxylic acid by hydroxide is so large that step 4 is for all intents and purposes irreversible and this makes the overall reaction irreversible... 

CH3SCH3 + CH3(CH2)ioCH2l will yield the same sulfonium salt This combination is not as effective as CH3I + CH3(CH2)ioCH2SCH3 because the reaction mechanism is 8 2 and CH3I... 

Studies of the reaction mechanism of the catalytic oxidation suggest that a tit-hydroxyethylene—palladium 7t-complex is formed initially, followed by an intramolecular exchange of hydrogen and palladium to give a i yW-hydtoxyethylpalladium species that leads to acetaldehyde and metallic palladium (88-90). 

The reaction mechanism and rates of methyl acetate carbonylation are not fully understood. In the nickel-cataly2ed reaction, rate constants for formation of methyl acetate from methanol, formation of dimethyl ether, and carbonylation of dimethyl ether have been reported, as well as their sensitivity to partial pressure of the reactants (32). For the rhodium chloride [10049-07-7] cataly2ed reaction, methyl acetate carbonylation is considered to go through formation of ethyUdene diacetate (33) ... 

The reaction mechanisms by which the VOCs are oxidized are analogous to, but much more complex than, the CH oxidation mechanism. The fastest reacting species are the natural VOCs emitted from vegetation. However, natural VOCs also react rapidly with O, and whether they are a net source or sink is determined by the natural VOC to NO ratio and the sunlight intensity. At high VOC/NO ratios, there is insufficient NO2 formed to offset the O loss. However, when O reacts with the internally bonded olefinic compounds, carbonyls are formed and, the greater the sunshine, the better the chance the carbonyls will photolyze and produce OH which initiates the O.-forming chain reactions. 

Hydrogen peroxide may react directiy or after it has first ionized or dissociated into free radicals. Often, the reaction mechanism is extremely complex and may involve catalysis or be dependent on the environment. Enhancement of the relatively mild oxidizing action of hydrogen peroxide is accompHshed in the presence of certain metal catalysts (4). The redox system Fe(II)—Fe(III) is the most widely used catalyst, which, in combination with hydrogen peroxide, is known as Fenton s reagent (5). 

Direct oxidation yields biacetyl (2,3-butanedione), a flavorant, or methyl ethyl ketone peroxide, an initiator used in polyester production. Ma.nufa.cture. MEK is predominandy produced by the dehydrogenation of 2-butanol. The reaction mechanism (11—13) and reaction equihbtium (14) have been reported, and the process is in many ways analogous to the production of acetone (qv) from isopropyl alcohol. 

White Phosphorus Oxidation. Emission of green light from the oxidation of elemental white phosphoms in moist air is one of the oldest recorded examples of chemiluminescence. Although the chemiluminescence is normally observed from sotid phosphoms, the reaction actually occurs primarily just above the surface with gas-phase phosphoms vapor. The reaction mechanism is not known, but careful spectral analyses of the reaction with water and deuterium oxide vapors indicate that the primary emitting species in the visible spectmm are excited states of (PO)2 and HPO or DPO. Ultraviolet emission from excited PO is also detected (196). 

Absorption of Nitrogen Oxides. There have been numerous studies and reports on the reaction mechanisms and rate-controlling steps for the absorption of nitrogen oxides into water (43—46). The overall reaction to form nitric acid may be represented by equation 14, where Ai/298 K kJ/mol ofNO consumed. 

Because of the delay in decomposition of the peroxide, oxygen evolution follows carbon dioxide sorption. A catalyst is required to obtain total decomposition of the peroxides 2 wt % nickel sulfate often is used. The temperature of the bed is the controlling variable 204°C is required to produce the best decomposition rates (18). The reaction mechanism for sodium peroxide is the same as for lithium peroxide, ie, both carbon dioxide and moisture are required to generate oxygen. Sodium peroxide has been used extensively in breathing apparatus. 

Novolaks. Novolak resins are typically cured with 5—15% hexa as the cross-linking agent. The reaction mechanism and reactive intermediates have been studied by classical chemical techniques (3,4) and the results showed that as much as 75% of nitrogen is chemically bound. More recent studies of resin cure (42—45) have made use of tga, dta, gc, k, and nmr (15). They confirm that the cure begins with the formation of benzoxazine (12), progresses through a benzyl amine intermediate, and finally forms (hydroxy)diphenyknethanes (DPM). 

Chemical, or abiotic, transformations are an important fate of many pesticides. Such transformations are ubiquitous, occurring in either aqueous solution or sorbed to surfaces. Rates can vary dramatically depending on the reaction mechanism, chemical stmcture, and relative concentrations of such catalysts as protons, hydroxyl ions, transition metals, and clay particles. Chemical transformations can be genetically classified as hydrolytic, photolytic, or redox reactions (transfer of electrons). 

The kinetics of hydrolysis reactions maybe first-order or second-order, depending on the reaction mechanism. However, second-order reactions may appear to be first-order, ie, pseudo-first-order, if one of the reactants is not consumed in the reaction, eg, OH , or if the concentration of active catalyst, eg, reduced transition metal, is a small fraction of the total catalyst concentration. 

For heavy drain discharges of alkaline cells, there is no useful capacity after this point because the rate of discharge of the ZnMn204 is quite slow. But for lighter drain discharges, further reduction of the cathode is possible. The reaction mechanisms are not entirely clear, but there is some evidence for the formation of a final reaction product resembling hausmannite [1309-55-3] Mn O. ... 

The total concentration or amount of chlorine-based oxidants is often expressed as available chorine or less frequendy as active chlorine. Available chlorine is the equivalent concentration or amount of Cl needed to make the oxidant according to equations 1—4. Active chlorine is the equivalent concentration or amount of Cl atoms that can accept two electrons. This is a convention, not a description of the reaction mechanism of the oxidant. Because Cl only accepts two electrons as does HOCl and monochloramines, it only has one active Cl atom according to the definition. Thus the active chlorine is always one-half of the available chlorine. The available chlorine is usually measured by iodomettic titration (7,8). The weight of available chlorine can also be calculated by equation 5. 

Polymerization Reactions. The polymerization of butadiene with itself and with other monomers represents its largest commercial use. The commercially most important polymers are styrene—butadiene mbber (SBR), polybutadiene (BR), styrene—butadiene latex (SBL), acrylonittile—butadiene—styrene polymer (ABS), and nittile mbber (NR). The reaction mechanisms are free-radical, anionic, cationic, or coordinate, depending on the nature of the initiators or catalysts (194—196). 

The main by-products of the Ullmaim condensation are l-aniinoanthraquinone-2-sulfonic acid and l-amino-4-hydroxyanthraquinone-2-sulfonic acid. The choice of copper catalyst affects the selectivity of these by-products. Generally, metal copper powder or copper(I) salt catalyst has a greater reactivity than copper(Il) salts. However, they are likely to yield the reduced product (l-aniinoanthraquinone-2-sulfonic acid). The reaction mechanism has not been estabUshed. It is very difficult to clarify which oxidation state of copper functions as catalyst, since this reaction involves fast redox equiUbria where anthraquinone derivatives and copper compounds are concerned. Some evidence indicates that the catalyst is probably a copper(I) compound (28,29). 

The pH dependency of enzyme-catalyzed reactions also exhibits an optimum. The pH optima for enzyme-catalyzed reactions cover a wide range of pH values. Eor instance, the subtihsins have a broad pH optima in the alkaline range. Other enzymes have a narrow pH optimum. The nature of the pH profile often gives clues to the elucidation of the reaction mechanism of the enzyme-catalyzed reaction. The temperature at which an experiment is performed may affect the pH profile and vice versa. 

Primary and secondary aliphatic and aromatic amines react readily with thiiranes to give 2-mercaptoethylamine derivatives (Scheme 76) (76RCR25, 66CRV297). The reaction fails or gives poor yields with amines which are sterically hindered e.g. N,iV-dicyclohexylamine) or whose nitrogen atom is weakly basic e.g. N,A/ -diphenylamine). Aromatic amines are less reactive and higher reaction temperatures are usually required for them. The reaction mechanism is Sn2 and substituted thiiranes are attacked preferentially at the least hindered... 

Treatment of cyclic carbonates of 1,2-diols with thiocyanate ion at temperatures of 100 °C or higher yields thiiranes (Scheme 145) (66CRV297, 75RCR138). Thiourea cannot replace thiocyanate satisfactorily, and yields decrease as the carbonate becomes more sterically hindered. The reaction mechanism is similar to the reaction of oxiranes with thiocyanate (Scheme 139). As Scheme 145 shows, chiral thiiranes can be derived from chiral 1,2-diols (77T999, 75MI50600). 

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