Sequence of chain reactions

Macromolecules ate formed by two types of chain reactions polymerizations and polycondensations. 

Polymerization involves a series of reactions, each adding a simple molecule to an already polymerized molecule. An example is the polymerization of ethylene, where the propagation step is a series of additions of ethylene molecules  

Such a reaction may involve either a single simple molecule, as in the previous example, or alternatively several different simple molecules the latter situation is called copolymerization. 

Polycondensation occnrs like polymerization with a series of reactions where each adds a pattern to the chain. Instead of a simple addition, however, there is an elimination of a small molecnle at each step (water, ammonia, etc.). An example is the prodnction of nylon throngh polycondensation between adipic acid and hexamethylenediamine with the elimination of water  

In both types of reactions (polymerization and poly condensation)  

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Hass and Bender [105] described a reaction of p-nitrobenzyl chloride wit salts of nitroalcanes, for example of 2-nitropropane. The mechanism of the r action was rationalized by Kornbtum and co-workers (106) and Russell ar Danen [107] as a sequence of chain reactions ... 

Reaction (1) is the most important step in the sequence of chain reactions in CVD processes which lead to the conversion of Silrh to Si02 layers. It is also critically important for the determination of the flammability and explosion limits and induction times of SiH4-containing gas mixtures, issues of utmost concern for industrial applications. " The key question is whether the reaction products can be explained by the proposed channels... 

It turns out that CFCs are unreactive in normal conditions, but high up in the atmosphere they become totally different. The CFCs can persist in the atmosphere for about a hundred years. They diffuse up to the stratosphere - and that s where the problem starts. The UV light from the Sun breaks the C—Cl bonds in the CFC molecules. This releases highly reactive chlorine atoms, called chlorine free radicals. These chlorine free radicals react with ozone molecules. In a sequence of chain reactions, it has been estimated that each chlorine free radical can destroy a million ozone molecules (Figure 16.8). 

The step in which the reactive intermediate, in this case A-, is generated is called the initiation step. In the next four equations in the example above, a sequence of two reactions is repeated this is the propagation phase. Chain reactions are characterized by a chain length, which is the number of propagation steps that take place per initiation step. Finally, there are termination steps, which include any reactions that destroy one of the reactive intermediates necessary for the propagation of the chain. Clearly, the greater the frequency of termination steps, the lower the chain length will be. 

Some of the most conclusive studies of the mechanisms of chain reactions come from experiments in which some of the propagating steps have been independently measured directly. This measurement can sometimes be done by the use of flash photolysis and pulse radiolysis (Chapter 11). Such determinations can verify the occurrence of a certain reaction in the sequence and provide its rate constant. 

One proposed mechanism involved an intramolecular rearrangement, while a second involved a free radical chain mechanism composed of the following sequence of elementary reactions ... 

The collision must be sufficiently energetic that enough energy is available to break the chemical bond linking the two bromine atoms. This type of reaction is called an initiation reaction because it generates a species that can serve as a chain carrier or active center in the following sequence of elementary reactions. 

Cyclic chain termination with aromatic amines also occurs in the oxidation of tertiary aliphatic amines (see Table 16.1). To explain this fact, a mechanism of the conversion of the aminyl radical into AmH involving the (3-C—H bonds was suggested [30]. However, its realization is hampered because this reaction due to high triplet repulsion should have high activation energy and low rate constant. Since tertiary amines have low ionization potentials and readily participate in electron transfer reactions, the cyclic mechanism in systems of this type is realized apparently as a sequence of such reactions, similar to that occurring in the systems containing transition metal complexes (see below). 

Multiple covalent bonds are formed in each macromolecule and, in general, statistical, polydispersed structures are obtained. In the case of controlled vinyl polymerizations, the average length of the macromolecule is determined by monomer to initiator ratios. If one views these polymerizations as extraordinarily long sequences of individual reaction steps, the average number of covalent bonds formed/chain may be visualized as shown in Scheme 2 ... 

Most radicals are highly reactive, and there are few examples where one would produce a stable radical product in a reaction. Reference to a radical reaction in synthesis or in Nature, almost always concerns a sequence of elementary reactions that give a composite reaction. Multistep radical sequences are discussed in general terms in this section so that the elementary radical reactions presented later can be viewed in the context of real conversions. The sequences can be either radical chain reactions or radical nonchain reactions. Most synthetic apphcations involve radical chain reactions, and these comprise the bulk of organic synthetic sequences and commercial applications. Nonchain reaction sequences are largely involved in radical reactions in biology. Some synthetic radical conversions are nonchain processes, and some recent advances in commercial polymerization reactions involve nonchain sequences. 

The fuel and oxygen are consumed primarily by a sequence of chain-branching reactions that yield a net production of active free radicals ... 

The aim of the present Section is to discuss existing information concerning sequences of glycosylation reactions during the assembly of polysaccharide chains. Data concerning identification of intermediates in the process, and on solubilization and purification of the enzymes involved, are also included. 

Concerns about nonproductive reactions of radicals apply not only to carbon-carbon bond forming steps, such as additions and cyclizations, but to every step in a sequence of radical reactions. For example, to obtain a good yield of product from the reaction in equation (2), not only must the addition of R to A=B occur but the conversion of R—A—B- to a nonradical product must also be efficient. In a chain reaction, the slowest propagation step must still be rapid relative to loss of the radicals by radical-radical or radical-solvent reactions. In practice, reactions with rates of product formation of 102—103 s 1 are experimentally difficult to conduct. Reactions with rates of 104—10s s 1 are manageable and those with rates >106 s l are usually conducted with ease. 

The PTOC carbamate method for efficient and controlled generation of aminyl radicals allows kinetic studies that previously were not possible with tetrazene precursors. As is the case with carbon radicals, optimum synthetic utility of chain reaction sequences is found when absolute rate constants or ratios of rate constants for competing reactions are known, i.e., Scheme 8, step D vs step E. If an absolute rate constant is known for one reaction, then other absolute rate constants can be determined for other reactions from the product distributions in competitions of the reactions of interest with the reaction with a known rate constant. 

This section ends with two problems, which between them cover a lot of the aspects of chain reactions and use of one of the Rice-Herzfeld rules. The second problem explains how it is that many chain reactions have observed overall activation energies which are considerably less than the bond dissociation energy involved in the first step of the sequence. 

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. 

Two types of primer DNA (Sections 4.2.1. and 4.8.1.) may be employed for the chain-extension reaction. When the desired sequence, of chain length up to —400 nucleotides, is cloned into M13mp2, a flanking primer which hybridizes to a region of the... 

Examples. There are many reasons why a real or an apparent induction stage will occur in a sequence of chemical reactions. Tobolsky, Metz, and Mesrobian developed equations to show that the induction time in the autoxidation of hydrocarbons is related to the ratio of the steady state concentration of hydroperoxide to the initial concentration, [ROOH]00/[ROOH]0 or expressed another way, to the chain length of the propagation steps Equations (3) and (4) below (2). 

A very similar picture will also be developed for the establishment of a cpiasi-stationary state with respect to the concentrations of metastable intermediates in a complex sequence of consecutive reactions. In a system such as II2 + CI2 —> 2HC1, which proceeds via an atomic chain... 

Long-chain approximation. In most chain reactions, a short sequence of steps, once initiated, repeats itself many times until it is terminated. The initiation and termination reactions then contribute very much less to product formation than do the self-repeating steps. The long-chain approximation neglects these minor contributions. It will be taken up in the context of chain reactions (see Section 9.3) and also used in chain-growth polymerization (Sections 10.3 and 10.4). 

The chain-growth polymerization of Eq. (5-5) actually represents a sequence of monomer reactions which is initiated by a small concentration of a strong acid ... 

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