Formal reactions

It has to be emphasized that these formal reaction schemes of Figures 3-13, 3-15, and 3-16 have the potential to discover novel reactions. Application of these bond- and electron-shifting schemes to specific molecules and bonds may correspond to a known reaction but may also model a completely novel reaction. 

NO according to the following formal reaction equations, thus limiting the maximum NO conversion ... 

But a price has to be paid for this potential advantage, because the number of reactions that could be obtained by applying a formal reaction generator scheme could be very high. Furthermore, most of the suggestions could be chemically mean-... 

Naturally, there must be differences in the two types of search procedures. However, they do not originate in the mechanisms for generating reactions the latter is achieved for both types of searches via the formal reaction schemes. Instead, the differences come from the way in which the reactions are evaluated and selected. The various... 

Fig. 10. Forward and retrosynthetic search by formal reaction generators...

EROS is based on formal reaction generators which regard reactions as bond-and electron-shifting processes ... 

Fig. 13. Reactions described by application of the formal reaction schemes.

Section). Clearly the ideal situation is to employ as many evaluations of the first type as possible, as these detect those bonds that actually take part in a reaction. A preselection is thus possible, guiding the reaction generation phase in the right direction. In the second type of evaluation, the reaction products must first be generated, with the possibility that they are ultimately evaluated as not being feasible for some reason. The formal reaction generation step would then be wasted time. 

The values of reaction enthalpies in a forward search can be of use in predicting the products of a reaction the more exothermic a reaction, the more it should be preferred. The situation will be different in a retrosynthetic search where retroreactions should be calculated to be endothermic to some degree. This underlines the point previously made (Sect. 2, Fig. 9) that the differences between a forward and a retrosynthetic search do not reside in the way reactions are generated — in both cases in EROS by the formal reaction schemes — but in the way they are evaluated. 

Olefins containing at least one allylic hydrogen atom react with 02 to form an allylic hydroperoxide. The analogous formal reaction of such alkenes with ethylene is known as ene-reaction. The tricyclic lactone peroxypartheno-lide has been prepared by such a reaction (6.13)619). 

We would likewise deduce that the formal reaction in equation 6... 

We start with a discussion of allene (propadiene), the simplest diene of all. Its gas phase enthalpy of formation is 190.5 1.2 kJmol-1. We wish to compare this quantity with that of related monoenes. The first comparison addresses the relative stability of one and two double bonds in a 3-carbon chain. Conceptually, this may be expressed as the enthalpy of the formal reaction 9... 

Consider now other bismethylenecycloalkanes. We start with 1,2-dimethylenecyclo-pentane, 63, and acknowledge there are no accompanying thermochemical data for its 1,3-isomer, 64. We can write the formal reaction 30... 

From enthalpy of formation data of 63 from Roth, and for the other species from Pedley, we find reaction 30 is exothermic by 6 kJ mol 1. Consider now the isomeric 1,3-and 1,4-dimethylenecyclohexane, 65 and 66 no thermochemical data for its 1,2-isomer are seemingly available. We can write the related formal reactions 31a and 31b. 

In contrast, L FeMnOg behaves more as expected for nearly-stoichiometric oxides on which adsorption of CO is frequently limited, with the formal reaction order (0.4-0.6) being in a range frequently found for oxide catalysts (9). 

Another interesting comparison involves Ar,Ar,Ar/,Ar/-tetramethyl-2-butyne- 1,4-diamine (25) for no hydrogen bonding is expected in any phase. The experimentally measured enthalpy of formation data are for the liquid. The following formal reaction can be written ... 

The final comparison we will make in this section considers the formal reaction... 

There is also the possibility that the enthalpy of combustion is in error. Because there is no possibility of intramolecular hydrogen bonding, the 1,4-disubstituted species, 45/48, is expected to be the most normal . Consider the formal reaction... 

The majority of instrumental mass fractionation that takes place during TIMS occurs on or around the filaments used to produce ions. As a simple example, the ionization of Ca at the surface of a Ta filament may be represented by the formal reactions ... 

Figure 1. Formal reaction scheme with examples...

There are thermochemical data for only one nonmethyl aliphatic hydroxylamine, N,N-diethylhydroxylamine . The enthalpy of formation difference between it and A-methyl-hydroxylamine is 71.6 kJ mol . This is very nearly the same as the difference of 79.7 kJ mol between the corresponding primary and secondary alcohols, ethanol and 3-pen-tanol, where the N of the hydroxylamine is replaced by a CH. Thus, the formal reaction enthalpy of equation 3 is only 8.1 kJmol . 

Similarly, removing only the NH and thus comparing the resulting phenols with hydroxy-lamines, as in equation 5, the formal reaction enthalpy is 13.3 kJ mol using the enthalpies of fusion in Reference 5. 

In our previous review, we attempted to establish some simple relationships among peroxides, ethers, alcohols and alkanes using a limited peroxide data set and a sometimes less than adequate data set for the comparison classes of compounds. The analysis is extended in this volume. The formal reactions, equations 2-5 and 6-9, that illustrate some typical comparisons are shown in Schemes 1 and 2. 

For the comparison of hydroperoxides with methyl ethers (equation 2), we find there is enthalpy of formation data only for dimethyl ether, isopropyl methyl ether and t-butyl methyl ether (again ignoring the ethyl and propyl hydroperoxides). The enthalpies of formal reaction 2 for R = Me, i-Pr and f-Bu (two gas phase enthalpies of formation for f-BuOOH) are —53.1, —54.9 and —37.6 or —48.6 kJmoU, respectively, in the gas phase. In the liquid phase, the enthalpies of reaction are —7.4, —35 (from the estimated enthalpy of formation of isopropyl hydroperoxide) and —20.0 kJmoU, respectively. Because the enthalpy of formation deviations from linearity for dimethyl ether and methyl hydroperoxide might not be identical, the reaction enthalpy might not be consistent with those... 

For reaction 3 to replace an oxygen with a methylene group to form a primary alcohol, there are enthalpies of formation for only seven alcohols to compare with the nineteen hydroperoxides, almost all of them only for the liquid phase. The enthalpies of the formal reaction are nearly identical, —104.8 1.1 kJmol, for R= 1-hexyl, cyclohexyl and ferf-butyl, while we acknowledge the experimental uncertainties of 8.4 and 16.7 kJmol, respectively, for the enthalpies of formation of the secondary and tertiary alcohols. We accept this mean value as representative of the reaction. For R = 1- and 2-heptyl, the enthalpies of reaction are the disparate —83.5 and —86.0 kJmol, respectively. From the consensus enthalpy of reaction and the enthalpy of formation of 1-octanol, the enthalpy of formation of 1-heptyl hydroperoxide is calculated to be ca —322 kJ mol, nearly identical to that derived earlier from the linear regression equation. The similarly derived enthalpy of formation of 3-heptyl hydroperoxide is ca —328 kJmol. The enthalpy of reaction for R = i-Pr is only ca —91 kJmol, and also suggests that there might be some inaccuracy in its previously derived enthalpy of formation. Using the consensus enthalpy of reaction, a new estimate of the liquid enthalpy of formation of i-PrOOH is ca —230 kJmoU. ... 

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