Radical species sequence

The Clemmensen reduction can be formulated to proceed by a sequence of one-electron and proton transfer reactions. It is a heterogenous reaction, taking place at the zinc surface. Initially an electron is transferred from zinc to the carbonyl group of ketone 1, leading to a radical species 3, which is presumed to react further to a zinc-carbenoid species 4 ... 

A radical mechanism sequence requires three distinct types of process initiation, propagation, and termination. Initiation is the formation of two radical species by bond fission, whereas propagation involves reaction of a radical with a neutral molecule, a process that leads to generation of a new radical. Because radicals are so reactive, the propagation process may continue as long as reagent molecules are available. Finally, the reaction is brought to a... 

Free-radical polymerization ideally follows the preceding sequence of reaction steps. However, there are also slow but important steps that complicate this simple model. These involve chain transfer steps. We assumed that the only termination involves two radical species reacting with each other to form a stable dead molecule,... 

Snbsequent detailed kinetic stndies revealed that the reaction mechanism for the hydroxy-lation of arenes is mnch more complicated than that indicated above Furthermore, the active intermediate is likely an anion radical species formed upon interaction of two molecules of the vanadium peroxo complex. The sequence of the various steps is indicated in equations 17-24. The steps indicated in equations 17-21 refer to a radical chain which accounts for decomposition of the peroxo complex to form dioxygen, whereas the subsequent steps are those required for the functionalization of the substrate. 

In a free-radical chain mechanism we want to 1) produce a given product selectively, 2) simultaneously produce radical species which will further propagate the chain. Consider the autoxidation of m-chlorotoluene to m-chlorobenzoic acid in the three ways given on Figure 2. For the sake of argument, we initially start with MCPBA. We will also assume the free radical chain mechanism sequence does not contain a rate determining step. 

Another common feature of free-radical reactions is that they tend to be chain processes. Since any chemical reaction must exhibit conservation of spin, the reaction of a free radical widi a closed-shell (fully electron paired) molecule must result in the production of a new free-radical species which can participate in subsequent free-radical reactions. The series of free-radical reactions leading to product is often a cyclic process in which the initial free radical is produced once again in die last step of the cycle so that the reaction sequence starts over again. The process is termed a chain reaction because each step of the process is linked directly to die preceding step. 

Interaction of the solute with radicals from the water is the first of a sequence of reactions which finally leads to stable products. Kinetic studies of the type cited give valuable information about the primary radical species and their relative reaction rates with molecules of different types. When sufficient data have been accumulated, it should be possible to predict the course of radiolysis in complex molecules. From the nature and yields of the products and by observing the effects on them of various factors such as concentration, pH, 02, and specific radical scavengers, it is often possible to speculate about the mechanisms by which products are formed. More often than not, this is a difficult problem because the products, even from relatively simple compounds, prove to be complex. Furthermore, it is often possible to produce more than one mechanism to fit the experimental data. The proteins are particularly difficult because of their complex structures. They contain approximately 20 different amino acids with an average of more than three carbon atoms in the side chains, which vary considerably in their structure hence, the possible number of products is large. For this reason, model compounds such as peptides and polyamino acids have been studied because they contain the peptide linkage but are free from the complications which arise from the diversity of the amino acid residues in a protein. A further practical difficulty which applies to chem-... 

The sequential treatment of ( , )-l-tosylamido-2, 4-alkadienes with n-butyl-lithium and propynyliodonium triflate 30 results in a cascade addition/bicy-clization sequence leading to bicyclic N-tosyldihydropyrroles 39 (Scheme 60) [165,166]. These transformations are completely stereoselective for the cis-isomers, and appear to proceed by intramolecular addition of alkylidenecarbene intermediates to the C2-C3 double bond of the pentadienyl chain to give azabi-cyclo[3.1.0]hexenes, which rearrange to 39 through diyl radical species. 

The reaction sequence shown above illustrates three important aspects of chemistry that will be shown to be very important in the discussion of atmospheric chemistry in Section 2.8. The first of these is that a reaction may be initiated by a photochemical process in which a photon of light (electromagnetic radiation) energy produces a reactive species, in this case the Cl- atom. The second point illustrated is the high chemical reactivity of free radical species with unpaired electrons and incomplete octets of valence electrons. The third point illustrated is that of chain reactions, which can multiply manyfold the effects of a single reaction-initiating event, such as the photochemical dissociation of Cl2. 

The first type of mechanism involves a redox chain process. As shown in Eqs. (1-3), it begins with the abstraction of a halogen atom from a polyhalo-alkane reagent by the metal complex. This generates a radical species that further adds to an olefin. A chain-transfer reaction ensues and yields back the reduced metal species, hence the acronym ATRA, for the sequence. 

At present, the accepted mechanism of 1,4-addition involves the formation of either a charge-transfer complex or an anion-radical species by partial or complete electron transfer, respectively [Eq. (92)]. Collapse of the charge-transfer complex or transfer of an organic group from the copper(II) species which results from the second process, completes the addition sequence 139). Supporting evidence for this view of the... 

The simultaneous occurrence of the rapid increase of conductivity and spin concentration upon the melting of solid complex (D " ". A ) indicates that the carriers (D" and A ) are generated in the following sequence The molten complex (D +. A ) dissociates into liquid neutral donors and neutral TCNQ molecules. These have an equilibrium with the radical species (D and A ), and the radicals migrate in the liquid. However, the migration of the ionic species may be different from conventional ionic conduction since the ionic conductivity was a minor contribution. 

Obviously, what we would really like to do is not just have a feel for tendencies, useful as this is, but also calculate copolymer composition and sequence distributions, things that can also be measured by spectroscopic methods. We will start by using kinetics to obtain an equation for the instantaneous copolymer composition (it changes as the copolymerization proceeds). Later we will use statistical methods to describe and calculate sequence distributions. In deriving the copolymer equation, we only have to consider the propagation step and apply our old friend, the steady-state assumption, to the radical species present in the polymerization, and... 

The steps (1 )-(5) represent the likely radical reaction sequence, but it is possible that ionic species, such as those observed in mass spectrometer studies are involved in the primary stage of the reaction. In the case of carbon tetrachloride these would be CCls, Cl, CCl" " and CCI2 as well as the negative ion Cl . 

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