Consecutive unimolecular first-order reactions

This corresponds to the formation of a product via a single intermediate in irreversible steps, usually written as Equation 4.1  

The concentration versus time curves are plotted in Fig. 4.1 (the same concentration-time dependences would be observed for any two consecutive first-order processes A —B —C even if they are not unimolecular, e.g. if pseudo-first-order steps are involved) [10]. 

On the other hand, if k2 k, B is a reactive intermediate (see Chapter 9) and the expression for the formation of C maybe approximated to Equation 4.2  

We see, therefore, that observation of a first-order rate law could be due to a single-step first-order reaction, or a first-order initial step followed by very rapid subsequent reactions of all intermediates. This is true when the fast subsequent steps are other than first order. 

Strategies for detecting reactive intermediates are required to distinguish between these possibilities (Chapter 9). We also see in the inset to Fig. 4.1 that the concentration of the reactive intermediate B in the mechanism of Equation 4.1 builds up to a value determined by the ratio kpky, this is relevant to our later consideration of the steady-state approximation (Section 4.2.3.1). 

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The first product of a chemical degradation reaction successively undergoes another degradation reaction to give a second intermediate or a final product. Consecutive unimolecular first-order reactions are represented as ... 

Irreversible Reactions in Series. We first consider consecutive unimolecular-type first-order reactions such as... 

The NH acidities of some sterically hindered ureas, namely the ureido esters (93), have been reported.81 The kinetics and mechanism of the alkaline hydrolysis of urea and sodium cyanate, NaCNO, have been studied at a number of temperatures.82 Urea hydrolysis follows an irreversible first-order consecutive reaction path. Tetrahedral intermediates are not involved and an elimination-addition mechanism operates. Sodium cyanate follows irreversible pseudo-first-order kinetics. The decomposition of the carcinogen /V-mcthyl-/V-nitrosourca (19) was dealt with earlier.19 The pyrolysis of /V-acctylurca goes by a unimolecular first-order elimination reaction.83... 

Products are olefins and the corresponding acids. These reactions are among the most widely studied and best understood of all gas phase unimolecular reactions. With few exceptions they are experimentally and kinetically well behaved cleanly first-order, no surface sensitivity, and no free radical chain complications. Reactions involve 1,5-hydrogen transfer from the f -carbon to the carbonyl oxygen, migration of the carbonyl Jt-bond, rupture of the ester (C-O) bond, and formation of a (Cg-Cf) 7t-bond. All present evidence favors a mechanism in which the above occur in a concerted manner. However, a two-step consecutive mechanism (see later) cannot be entirely ruled out at this time. 

Important evidence in favor of reaction (iv) as the key reaction in the catalytic peroxide decomposition is the fact that the rate of formation of the primary hydrogen peroxide complex is high enough to account for the entire catalytic activity, provided a mechanism for its rapid decomposition is available. The bimolecular formation velocity constant has a value of about 3 X 107 M.-1 see.-1 as compared with the value of the overall bimolecular constant for the catalytic reaction of 3 — 3.5 X 107 M.-1 sec.-1. Any simple mechanism with consecutive reactions including the unimolecular decomposition must be excluded, for the decomposition rate of the primary complex is extremely low, the first order constant being about 0.02 sec.-1 (Chance, 69). 

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