Atoms, as intermediates

The authors favour a two-equivalent oxidation in order to avoid the necessity to postulate free hydrogen atoms as intermediates in the reaction, viz. 

However, if reaction does involve bromine atoms as intermediates, one would expect to find products (including acetolysis) derived from addition of these radicals to the double bond, especially with olefms, such as norbornene, that possess unreactive allylic positions. Obviously the detailed mechanism of this reaction remains to be resolved. 

When Sten Andersen had completed his experiments (32) at 25° he was able to propose a mechanism containing oxygen atoms as intermediates whose results conformed with the experiments so far as the dependence on the different concentrations was concerned. However, when he extended his measurements (33) to other temperatures ranging from 10°C. to 35°C., the choice of oxygen atoms as intermediates could not very well be maintained. The reason was that with this choice the ratio between two velocity constants belonging to reactions of the bimolec-ular type became practically independent of temperature and of the... 

Reactions in acid often involve the formation of carbocations—trivalent, positively charged carbon atoms—as intermediates. The order of stability of carbocations containing only alkyl substituents is 3° > 2° > 1° > CHj. Cation stability is influenced by several factors ... 

This rate law indicates that a reaction between H2(ad) and sulfur ions on Ag2S is the rate determining step. Alternatively, one may assume a sequence of steps involving hydrogen atoms as intermediates as is discussed for removal of oxygen from wiistite in Section V.A. 

Hydrogen atoms as intermediates occur in many reactions. In view of evidence obtained in numerous investigations, the participation of adsorbed hydrogen atoms in reactions discussed in Sections IV, V.A., VI.B, VI.C, and VII has, therefore, been assumed without explicit proof. The following typical hydrogen transfer reactions are quoted. 

Adsorbed hydrogen atoms as intermediates occur especially during the catalytic hydrogenation of hydrocarbons involving double bonds at elevated temperatures (109-111), e.g.,... 

In an atomic level simulation, the bond stretch vibrations are usually the fastest motions in the molecular dynamics of biomolecules, so the evolution of the stretch vibration is taken as the reference propagator with the smallest time step. The nonbonded interactions, including van der Waals and electrostatic forces, are the slowest varying interactions, and a much larger time-step may be used. The bending, torsion and hydrogen-bonding forces are treated as intermediate time-scale interactions. 

The O atom uses one of its sp or sp hybrids to form the CO a bond and antibond. When sp hybrids are used in conceptualizing the bonding, the other sp hybrid forms a lone pair orbital directed away from the CO bond axis one of the atomic p orbitals is involved in the CO n and 71 orbitals, while the other forms an in-plane non-bonding orbital. Alternatively, when sp hybrids are used, the two sp hybrids that do not interact with the C-atom sp2 orbital form the two non-bonding orbitals. Hence, the final picture of bonding, non-bonding, and antibonding orbitals does not depend on which hybrids one uses as intermediates. 

CX-Aminonitriles are compounds containing both cyano and amine substituents attached to the same carbon atom. They are versatile synthetic intermediates that are used to make aminoacids, agrichemicals, chelants, radical initiators, and water-treatment chemicals. In some cases, aminonitriles produced as intermediates are not isolated, but immediately further reacted, for example by hydrolysis, as is the case in producing... 

Chloroformates are versatile, synthetic intermediates, based on the affinity of the chlorine atoms for active hydrogen atoms. Chloroformates should be considered as intermediates for syntheses of pesticides, perfumes, dmgs, polymers, dyes, and other chemicals. Some of these products, eg, carbonates, are used as solvents, plastici2ers, or as intermediates for further synthesis. A significant use of chloroformates is for conversion to peroxydicarbonates, which serve as free-radical initiators for the polymeri2ation of vinyl chloride, ethylene, and other unsaturated monomers. The most widely used percarbonate initiators are diisopropyl peroxydicarbonate (IPP), di-2-ethyIhexylperoxydicarbonate (2-EHP), and di-j -butylperoxydicarbonate (SBP). The following Hst includes most of the commercially used percarbonates. 

A chlorohydrin has been defined (1) as a compound containing both chloio and hydroxyl radicals, and chlorohydrins have been described as compounds having the chloro and the hydroxyl groups on adjacent carbon atoms (2). Common usage of the term appHes to aUphatic compounds and does not include aromatic compounds. Chlorohydrins are most easily prepared by the reaction of an alkene with chlorine and water, though other methods of preparation ate possible. The principal use of chlorohydrins has been as intermediates in the production of various oxitane compounds through dehydrochlorination. 

This makes possible numerous further decomposition reactions involving the hydrogen atom as an intermediate in chain reactions. 

Another widely used approach to the elucidation of metabolic sequences is to feed cells a substrate or metabolic intermediate labeled with a particular isotopic form of an element that can be traced. Two sorts of isotopes are useful in this regard radioactive isotopes, such as and stable heavy isotopes, such as or (Table 18.3). Because the chemical behavior of isotopically labeled compounds is rarely distinguishable from that of their unlabeled counterparts, isotopes provide reliable tags for observing metabolic changes. The metabolic fate of a radioactively labeled substance can be traced by determining the presence and position of the radioactive atoms in intermediates derived from the labeled compound (Figure 18.13). 

The versatility of pyrimidine substituted chloroquinazolines as intermediates is due to the ready replacement of the halogen atoms by hydrogen, alkyl, alkoxyl, amino, and mercapto groups (see Section VI, A). 

We call the carbocation, which exists only transiently during the course of the multistep reaction, a reaction intermediate. As soon as the intermediate is formed in the first step by reaction of ethylene with H+, it reacts further with Br in a second step to give the final product, bromoethane. This second step has its own activation energy (AG ), its own transition state, and its own energy change (AG°). We can picture the second transition state as an activated complex between the electrophilic carbocation intermediate and the nucleophilic bromide anion, in which Br- donates a pair of electrons to the positively charged carbon atom as the new C-Br bond starts to form. 

Amide reduction occurs by nucleophilic addition of hydride ion to the amicle carbonyl group, followed by expulsion of the oxygen atom as an alumi-nate anion leaving group to give an iminium ion intermediate. The intermediate iminium ion is then further reduced by JL1AIH4 to yield the amine. 

Numerous reactions have been described in which the oxygen of the oxepin system is removed to give benzene derivatives. The formation of the aromatic products can be rationalized by an arene oxide as intermediate. A suitable reagent for the elimination of an oxygen atom from this heterocycle is triphenylphosphane, e.g. formation of l,24 2a,12 and 2b.1,9... 

Adsorbed Atomic Species as Intermediates in Heterogeneous Catalysis Carl Wagner... 

C. Wagner, Adsorbed Atomic Species as Intermediates in Heterogeneous Catalysis, in Adv. Catal., (1970), pp. 323-381. 

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