Reaction Mechanisms Initiation step

Just as the E2 mechanism shares features of the Sn2 mechanism, the El mechanism shares features of the Sn 1 reaction. The initial step is formation of a carbocation intermediate through loss of the leaving group. This slow step becomes the rate-determining step for the whole reaction, i.e. the El mechanism is unimolecular. In general terms, the reaction can be represented as follows. 

The mechanism of the N03 radical reaction with DMDS is complex. The rate constants for the N03 reaction with CH3SCH3, CH3SH, and CH3SSCH3 (Table 8.17) are similar and much larger than that for the reaction with H2S. This suggests that by analogy with the dimethyl sulfide reaction, the initial step is addition to a sulfur atom to form an adduct that then reacts further ... 

The reaction mechanism for this process is fairly well established and may serve as a more complex example of a chain reaction. The initiation step in ethane pyrolysis is thermal dissociation of the ethane molecule,... 

The paramagnetic 1 1 adduct is probably the active catalyst in these reactions. The initial step may involve hydrogen transfer or electron transfer to give aryloxy radicals that react further immediately or diffuse out of the solvent cage and react with another molecule of catalyst or with themselves (Scheme 3). More work is necessary to resolve the mechanism of this interesting and synthetically useful reaction. 

Oxidation, which can occur by autoxidation or photo-oxidation (initiation by light), is a complex chain reaction. The initial step of oxidation is the formation of hydroperoxides. This initial step is followed by secondary reactions in which species, such as aldehydes, acids, alcohols and hydrocarbons, are formed. Since it is a mechanism based on the formation of radicals, dimerization of some intermediates can occur, leading to formation of higher molecular weight products. Oxidative polymerization can also occur. In addition to these mechanisms, fuel deterioration can also occur hydrolytically through the presence of water. A detailed book on oxidation has been published by Frankel (2005). 

The mechanism proposed for the peroxide effect involves a radical chain reaction. The initiation step (equation 9.31) produces a bromine atom, which then attaches to the less alkyl-substituted carbon atom of a carbon-carbon double bond (equation 9.32). The resulting alkyl radical then abstracts a hydrogen atom from HBr to produce the anti-Markovnikov product and regenerate a bromine atom in the second propagation step (equation 9.33). Termination steps, not shown, interrupt the chain reaction. 

The mechanism of skeletal isomerization of saturated hydrocarbons can be considered as a chain reaction. The initiation step is to form the first car-bocation (eq. 42) followed by its isomerization to another, usually more stable intermediate carbocation (eq. 43). The final propagation step is chain transfer through hydride ion transfer between the isomeric carbocation and the starting... 

Assuming stationary states in this reaction mechanism from step 1 to step 3, the rate equation (185), is derived for the initial reaction, where 1 2> -2>3ndlr3... 

By far, photostimulation is the most used method to initiate reactions. The initiation step by photoinduced ET can conceivably be accomplished in one of the followings ways (i) homolytic cleavage of the C—X bond in excited ArX, (ii) ET from the Nu" to the excited ArX (or from the excited Nu" to the ArX), and (iii) ET within an excited charge-transfer complex (CTC), among others (Scheme 10.3). Depending on the nature of the ArX, the Nu", and the experimental eonditions, any of these mechanisms could probably be considered as an initiation step. It is noteworthy that the t-BuOK can form [ArX] by ET under irradiation [14,15]. 

Decomposition of the diethyl acetal of formylmethylcobalamin occurs via a multistage reaction the initial step of which has a second-order rate constant of 2.6 X 10 1 mol s. These kinetic results are used to explain conflicting reports concerning the synthesis and nature of formylmethylcobalamin. A second communication has appeared with the same objective, but the studies are not as extensive as those in ref. 25. Electrophilic cleavage of cobalt-alkyl reactions between Hg + and complexes [Co(Me)(L)(H20)] (L = macrocyclic chelate) in which the rate-determining step is considered to be reaction of Hg + with the neutral cobalt complex according to the equation... 

The reaction between phenol and formaldehyde under acidic conditions proceeds through a mechanism different from that described previously for the base-catalyzed reaction. The initial step involves the protonation of formaldehyde to give a carbonium ion ... 

Thus there are three main steps to this reaction mechanism initiation, propagation and termination (Figure 10.68). 

This series of kinetic steps typifies a chain reaction. A chain reaction is a reaction whose mechanism consists of steps whose products are intermediates that react to form other intermediates, usually in an apparently cyclical fashion. Reaction a, which started the chain reaction in this example, is called an initiation reaction (or initiation step). Reactions b and c of this example use one intermediate and produce another intermediate. They are called propagation reactions. The reactions d represent a loss of the intermediates that propagate the chain reaction. They are called termination steps. All steps can be generally characterized by the change in the reactive intermediates over the course of the reaction. Initiation steps form reactive intermediates from reactants, propagation steps use an intermediate but form another (so there is no net change in the amount of reactive intermediate), and... 

The mechanism of photooxidation of aromatic species in the atmosphere is perhaps the area of greatest incertainty in atmospheric hydrocarbon chemistry. The principal reaction of aromatics is with the hydroxyl radical. Absolute rate constants have recently been determined at room temperature for the reaction of OH radicals with benzene and toluene (l8,l ) and with a series of aromatic hydrocarbons (19). Recently, absolute rate constants for the reaction of OH radicals with a series of aromatic hydrocarbons have been determined over the temperature range 296-U73K (20). For aromatic-OH reactions, the initial step can be either abstraction or addition to the aromatic ring. For toluene, for example. 

Vibrational motion is thus an important primary step in a general reaction mechanism and detailed investigation of this motion is of utmost relevance for our understanding of the dynamics of chemical reactions. In classical mechanics, vibrational motion is described by the time evolution and l t) of general internal position and momentum coordinates. These time dependent fiinctions are solutions of the classical equations of motion, e.g. Newton s equations for given initial conditions and I Iq) = Pq. 

In the above examples the size of the chain can be measured by considering the number of automobile collisions that result from the first accident, or the number of fission reactions which follow from the first neutron capture. When we think about the number of monomers that react as a result of a single initiation step, we are led directly to the degree of polymerization of the resulting molecule. In this way the chain mechanism and the properties of the polymer chains are directly related. 

The initiators which are used in addition polymerizations are sometimes called catalysts, although strictly speaking this is a misnomer. A true catalyst is recoverable at the end of the reaction, chemically unchanged. Tliis is not true of the initiator molecules in addition polymerizations. Monomer and polymer are the initial and final states of the polymerization process, and these govern the thermodynamics of the reaction the nature and concentration of the intermediates in the process, on the other hand, determine the rate. This makes initiator and catalyst synonyms for the same material The former term stresses the effect of the reagent on the intermediate, and the latter its effect on the rate. The term catalyst is particularly common in the language of ionic polymerizations, but this terminology should not obscure the importance of the initiation step in the overall polymerization mechanism. 

A special type of substituent effect which has proved veiy valuable in the study of reaction mechanisms is the replacement of an atom by one of its isotopes. Isotopic substitution most often involves replacing protium by deuterium (or tritium) but is applicable to nuclei other than hydrogen. The quantitative differences are largest, however, for hydrogen, because its isotopes have the largest relative mass differences. Isotopic substitution usually has no effect on the qualitative chemical reactivity of the substrate, but often has an easily measured effect on the rate at which reaction occurs. Let us consider how this modification of the rate arises. Initially, the discussion will concern primary kinetic isotope effects, those in which a bond to the isotopically substituted atom is broken in the rate-determining step. We will use C—H bonds as the specific topic of discussion, but the same concepts apply for other elements. 

Certain kinetic aspects of free-radical reactions are unique in comparison with the kinetic characteristics of other reaction types that have been considered to this point. The underlying difference is that many free-radical reactions are chain reactions that is, the reaction mechanism consists of a cycle of repetitive steps which form many product molecules for each initiation event. The hypothetical mechanism below illustrates a chain reaction. 

The result of the steady-state condition is that the overall rate of initiation must equal the total rate of termination. The application of the steady-state approximation and the resulting equality of the initiation and termination rates permits formulation of a rate law for the reaction mechanism above. The overall stoichiometry of a free-radical chain reaction is independent of the initiating and termination steps because the reactants are consumed and products formed almost entirely in the propagation steps. 

Henbest and Jackson have rationalized these remote directive effects on the basis of the well-supported mechanism of alkaline epoxidations. The initial step in the reaction is the reversible addition of the hydroperoxide ion... 

The reaction has been applied to more complex enamines 13) and to dienamines 19). The reduction may be rationalized by initial protonation at the enamine carbon and subsequent decarboxylation of formate ion and addition of the hydride ion to the iminium cation. This mechanism has been given support by the reaction of the enamine (205) with deuterated formic acid 143) to give the corresponding amines. The formation of 206 on reaction with DCOOH clearly indicates that protonation at the enamine carbon is the initial step. 

The term Knoevenagel reaction however is used also for analogous reactions of aldehydes and ketones with various types of CH-acidic methylene compounds. The reaction belongs to a class of carbonyl reactions, that are related to the aldol reaction. The mechanism is formulated by analogy to the latter. The initial step is the deprotonation of the CH-acidic methylene compound 2. Organic bases like amines can be used for this purpose a catalytic amount of amine usually suffices. A common procedure, that uses pyridine as base as well as solvent, together with a catalytic amount of piperidine, is called the Doebner modification of the Knoevenagel reaction. 

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