Parallel or Competitive Reactions

If both of these reactions occur simultaneously in aqueous solution, it would be expected that the absorption rate could be written as 

If one assumes that in more concentrated solutions the rate of the forward reaction continues to follow this rate expression, what forms of the reverse rate are thermodynamically consistent in concentrated acid solution Equilibrium is to be established with respect to equation (A) when written in the N2O4 form. It may be assumed that the dependence on the concentrations of NO2 and N2O4 may be lumped together by using equation (C). 

At equilibrium we require that the net rate of reaction be zero. If we postulate a net rate expression of the general power function form 

Thermodynamically consistent forms may be obtained by choosing different positive values of n. 

The following table indicates the reaction orders corresponding to selected arbitrary values of n. 

---

Under the influence of solar irradiation photochemical reactions are caused producing highly reactive species which enter into parallel or competitive reactions [12 ]. ... 

Purely parallel reactions are e.g. competitive reactions which are frequently carried out purposefully, with the aim of estimating relative reactivities of reactants these will be discussed elsewhere (Section IV.E). Several kinetic studies have been made of noncompetitive parallel reactions. The examples may be parallel formation of benzene and methylcyclo-pentane by simultaneous dehydrogenation and isomerization of cyclohexane on rhenium-paladium or on platinum catalysts on suitable supports (88, 89), parallel formation of mesityl oxide, acetone, and phorone from diacetone alcohol on an acidic ion exchanger (41), disproportionation of amines on alumina, accompanied by olefin-forming elimination (20), dehydrogenation of butane coupled with hydrogenation of ethylene or propylene on a chromia-alumina catalyst (24), or parallel formation of ethyl-, methylethyl-, and vinylethylbenzene from diethylbenzene on faujasite (89a). 

The criticism of those deactivation functions appeared very soon (S), 6). Froment et al. propose to consider coke formation as a reaction in competition with cracking (parallel or successive), it can be observed experimentally. It is... 

The theoretical models discussed above are frequently employed in the description of the kinetics of gas-phase reactions, especially reactions of atoms and free radicals. This class of reactions is of interest in a broader scientific context, and a better understanding of their mechanism is of primary importance for the development of chemical modeling. Free atoms and radicals are very reactive species, which occur in and take part in many different reaction systems. Therefore, a radical reaction usually proceeds in competition with a few parallel or subsequent processes. The kinetic behavior of the reaction system may be very complicated and difficult for quantitative description. Theoretical investigations of the reaction kinetics provide information useful for a better understanding and correct interpretation of experimental findings. Results of ab initio calculations are employed to evaluate the rate constant in terms of the computational methods of the reaction rate theory. 

Parallel coupling constitutes another type of coupled enzyme reactions. This includes the competition of two enzymes for a common substrate as well as the conversion of alternative substrates and the competitive binding of a substrate and an inhibitor to an enzyme. Thus, analytes become measurable even though they cannot be converted to readily detectable products. Coupled enzyme reactions can also be used to eliminate disturbances of the enzyme or transducer reaction caused by constituents of the sample. Compounds interfering with the signal transduction can be transformed into inert products by reacting them with an (eliminator) enzyme which can be coimmobilized with the analyte-converting (indicator) enzyme in the vicinity of the transducer. On the other hand, constituents of the sample which are at the same time intermediate products of coupled enzyme reactions and will thus... 

Parallel reactions (also called competitive or side reactions),... 

Identical reactants can undergo simultaneous reactions leading to different products. These kinds of reactions are called parallel, competitive, or side reactions. 

It must be stressed that this result depends essentially on the coupling between sequences. A study of reaction (1) alone and of reaction (2) alone with the condition that rates of (1) and (2) thus measured should be equal at the maximum, would lead to erroneous results. In all systems where more than one stoichiometrically simple reaction takes place, coupling between sequences must be taken into account, whether the sequences are arranged in parallel or in series, whether they are catalytic os chain reactions. Selectivity, product composi-tion and overall rates may be affected substantially by the competition for active centers and the chain transfer steps. In other words, if two rate functions, for two reactions taking place separately, are of the form ... 

Some years ago it was claimed that the [Fe(fz)3] anion reacted with oxalate by a bimolecular mechanism. Nucleophilic attack of oxalate at metal or ligand here seemed equally unlikely, and a reinvestigation of this reaction, with a parallel study of reactions of this complex with edta and related ligands, indicates a dissociative mechanism, with ligand competition causing the observed kinetic pattern. ... 

The reactivity of thiophen has also been compared with that of seleno-phen and the relative reactivities in five electrophilic substitutions have been determined by kinetic or competitive procedures. The results have been compared with those available in the literature for furan. In all the reactions examined, selenophen exhibited a reactivity intermediate between those of furan and thiophen. p-Constants for electrophilic substitution of substituted thiophens are usually smaller than in the benzene series. A comparison of the trifluoroacetylation of a series of substituted thiophens and furans yielded p-values of — 7.4 and — 10.7 respectively. The observed order of substrate selectivity in the trifluoroacetylation (furan > thiophen) thus parallels the positional selectivity in electrophilic substitution, the oi ratio always being larger in furans than in thiophens. The relative importance of primary steric effects in benzene and thiophen has been investigated by determination of the isomer distributions in the acetylations of 2- and 3-methylthiophen, 2- and 3-t-butylthiophen, and toluene and t-butylbenzene. Steric hindrance is less significant in the thiophen series owing to the more favourable geometry. - ... 

When there is only one undesired reaction a distinction is made between competitive reactions (also called parallel reactions), where the undesired product is formed from a reactant or from an intermediate product, and consecutive reactions (also called reactions in series), where the undesired product is formed from the desired product. We can summarize these two situations by the following reaction schemes ... 

When a consecutive reaction has to be minimized, the liquid volume should be small (section S.4.2.2), and a liquid-in-gas dispersion or a parallel flow arrangement are the best choices. When a competitive reaction (of the transferred reactant) has to be minimized, the gas-phase mass transfer coefficient should be as high as possible (section S.4.2.2, eq. (5.45)), and again a liquid-in-gas dispersion or parallel flow are to be preferred. 

Reaction systems (1) and (3) represent cases in which all reactions are of first order. For reaction systems (2) and (4), it is, however, characteristic that reactions of different orders occur in the consecutive or composite (parallel) reaction scheme. Scheme (5) represents mixed reactions (consecutive-competitive reactions) that commonly occur in an industrial context. 

Is the reaction competitive-consecutive or competitive-parallel See Chapter 13. 

Successive reactions take place in steps, with the formation of intermediate products. The steps may be irreversible the starting material and the different intermediates react in one direction to give the product of the reaction. Alternatively, one or more steps may be reversible, or again parallel, with formation of several products (competitive reactions). 

---