Closed-Sequence Mechanisms Chain Reactions

In some reactions involving gases, the rate of reaction estimated by the simple collision theory in terms of the usually infened species is much lower than observed. Examples of these reactions are the oxidation of H2 and of hydrocarbons, and the formation of HC1 and of HBr. These are examples of chain reactions in which very reactive species (chain carriers) are initially produced, either thermally (i.e., by collision) or photochemically (by absorption of incident radiation), and regenerated by subsequent steps, so that reaction can occur in chain-fashion relatively rapidly. In extreme cases these become explosions, but not all chain reactions are so rapid as to be termed explosions. The chain 

Chapter 7 Homogeneous Reaction Mechanisms and Rate Laws 

The experimental detection of a chain reaction can be done in a number of ways  

Chain carriers are usually very reactive molecular fragments. Atomic species such as H and Cl, which are electrically neutral, are in fact the simplest examples of free radicals, which are characterized by having an impaired electron, in addition to being electrically neutral. More complex examples are the methyl and ethyl radicals, CH and QH, respectively. 

We may use the reaction mechanism for the formation of ethylene from ethane (CjHg - C2H4 + H2), Section 6.1.2, to illustrate various types of steps in a typical chain reaction  

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This mechanism involves a closed sequence, or chain, consisting of reactions (52) and (53). The two free radical chain carriers are the neopentyl radical and the... 

Atoms and free radicals are highly reactive intermediates in the reaction mechanism and therefore play active roles. They are highly reactive because of their incomplete electron shells and are often able to react with stable molecules at ordinary temperatures. They produce new atoms and radicals that result in other reactions. As a consequence of their high reactivity, atoms and free radicals are present in reaction systems only at very low concentrations. They are often involved in reactions known as chain reactions. The reaction mechanisms involving the conversion of reactants to products can be a sequence of elementary steps. The intermediate steps disappear and only stable product molecules remain once these sequences are completed. These types of reactions are refeiTcd to as open sequence reactions because an active center is not reproduced in any other step of the sequence. There are no closed reaction cycles where a product of one elementary reaction is fed back to react with another species. Reversible reactions of the type A -i- B C -i- D are known as open sequence mechanisms. The chain reactions are classified as a closed sequence in which an active center is reproduced so that a cyclic reaction pattern is set up. In chain reaction mechanisms, one of the reaction intermediates is regenerated during one step of the reaction. This is then fed back to an earlier stage to react with other species so that a closed loop or... 

The essential characteristic of a chain reaction mechanism is the existence of a closed cycle of reactions in which unstable or highly reactive intermediates react in propagation steps with stable reactant molecules or other intermediates and are regenerated by the sequence of reactions... 

A reaction mechanism may involve one of two types of sequence, open or closed (Wilkinson, 1980, pp. 40,176). In an open sequence, each reactive intermediate is produced in only one step and disappears in another. In a closed sequence, in addition to steps in which a reactive intermediate is initially produced and ultimately consumed, there are steps in which it is consumed and reproduced in a cyclic sequence which gives rise to a chain reaction. We give examples to illustrate these in the next sections. Catalytic reactions are a special type of closed mechanism in which the catalyst species forms reaction intermediates. The catalyst is regenerated after product formation to participate in repeated (catalytic) cycles. Catalysts can be involved in both homogeneous and heterogeneous systems (Chapter 8). 

The mechanism of aminyl radical generation from PTOC carbamates follows closely the radical chain mechanism of alkyl radical generation from PTOC esters. The chain reaction sequence involves the series of steps shown in Scheme 8. Several methods for inducing N —O bond cleavage are possible. Photochemical decomposition of 29 via visible light irradiation is used to initiate the chain reaction sequence at ambient or subambient temperature reactions have been run as low as -78°C (91TL6493). 

In our previous discussion of the elementary steps involved in chemical reactions we used the decomposition of diethyl ether as an example of a chain reaction in which a cycle of elementary steps produces the final products. Many reactions are known to occur by chain mechanisms, and in the following discussion we refer primarily to those that generally correspond to the closed sequence in the classification of Boudart. Here active centers (also called active intermediates or chain carriers) are reacted in one step and regenerated in another in the sequence however, if we look back to reaction (IV) a closer examination discloses that some of the steps have particular functions. In (IVa) active centers are formed by the initial decomposition of the ether molecule, and in (IVd) they recombine to produce the ether. The overall products of the decomposition, C2H6 and CH3CHO, however, are formed in the intermediate steps (IVb) and (IVc). In analysis of most chain reactions we can think of the sequence of steps as involving three principal processes ... 

The identification of the amino acids at the active center is very important if the mechanism of the enzymic reaction is to be understood, but unambiguous evidence concerning the nature of such acids is extremely diflScult to obtain. (It should be emphasized that the amino acids of the active site need not be close to one another in the amino acid sequence of the protein, as folding of the polypeptide chains can bring them together.) For catalytic activity, many enzymes require a non-protein ion or molecule, bound to (or in close association with) the enzyme—protein. The nature of this prosthetic group, or coenzyme, must be investigated in order that the enzymic action may be fully understood. 

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