Exothermicity of reactions

This reaction exemplifies the important process of methyl removal, which becomes even more significant in the case of multiply branched paraffins. The rather large exothermicity of Reaction 11 results from the fact that a secondary carbonium ion is formed. The beta fission process can be illustrated using reactions in 8-ra-hexylpentadecane (compound 2) as an example. Table II shows that the ions formed by C-C fission at a branch point (Ci4+, m/e = 197 and Ci5+, m/e = 211) have intensities appreciably larger than the other alkyl ions in the same region... 

In a distillation column reactor (DCR), reaction and distillation occur simultaneously. This technology is also referred to as reactive distillation, or, if a catalyst is involved, as catalytic distillation. DCRs offer distinct advantages of exploiting the exothermicity of reactions, such... 

However, it should be noted that in some cases the existence of mass transfer limitations can be advantageously used to control the exothermicity of reactions. For... 

We immediately recognize this quantity E s) as equal to the exothermicity of reaction 17 ... 

The first compound of interest is aniline (14) itself. While drawings of its resonance structures permeate textbooks, the research literature acknowledges ambiguities as to its quantitation28. For example, there is a ca 38 kJ mol-1 spread of plausible resonance energies for aniline as defined by the exothermicity of reaction 12... 

Let us start with aliphatic species and sequentially move the amino groups apart. We are seemingly thwarted immediately. There is no enthalpy of formation reported for methylenediamine (diaminomethane) CH2(NH2)2 (21), a species we recognize as the simplest polyaminoalkane. As such the exothermicity of reaction 22... 

Let us assume that the difference in exothermicities between reactions 24 and 25 is the same as between 22 and 23. We thus conclude that reaction 22 will be exothermic by the exothermicity of reaction 23 and the difference of reactions 24 and 25. The net result is some 45 + 4 = 49 kJ mol-1. The enthalpy of formation of 21 is predicted39 to be ca -21 kJmol-1. 

The exothermicity of reaction (8.87) is sufficient to fragment the formyl radical and could be written as... 

The exothermicity of reaction 9.7 dictates a low temperature (450 °C or less) and use of a catalyst (Fe203/Al203, usually) if good yields are to be obtained in a reasonable time. Usually, the reaction is carried out in two steps. First temperatures of 450 to 500 °C are used and a substantial degree of conversion quickly results. Then the temperature is dropped to about 200 °C to optimize the yield. The C02 is removed by scrubbing the gas with water, in which CO2 is much more soluble than H2. 

Figure 20. Comparison of the early-time (35 fis) spectrum with three prior distributions (dashed curves) calculated for three values of the available energy for the O + FCO reaction (a) 392 kJ mol-1, the exothermicity of reaction (16) (6) 451 kJmol-1, the exothermicity of reaction (16) when the upper limit to the recommended range of values of AHf of FCO is used (c) 549kJ mol-1, the exothermicity of reaction (16) if FCO contains all of the energy released in its formation step, reaction (15).

We should not be surprised at the high exothermicity of Reaction 28-11. The peroxide is of high energy (thermochemically unstable) because it combines the strain-energy characteristics of small rings with the weakness of 0-0 bonds, whereas the product is a stable substance with a strong carbonyl bond. 

If equipartition is obtained, and the 138 kcal./mole exothermicity of reaction (31) is divided between two 02(3S ) molecules, the maximum vibrational energy possible would be 69 kcal./mole which corresponds to the experimental result. Reaction (32), 69 kcal./mole endothermic, would then be barely possible, but... 

This reaction enthalpy probes the superstrain associated with the two, trigonal, bridgehead carbons beyond that found in the individual rings. As may simply be concluded by looking at the molecular structure, the m = n = 1 species is clearly highly strained (quantum chemical calculations show the additional structural feature of non-planarity77). Experimental enthalpy-of-formation measurements of bicyclobutene show this compound to be accompanied by an exothermicity of reaction 18 of 42 42 kJ mol 1. 

Admittedly, not all polycyclic tetrasubstituted olefins can be so simply constructed or described as bicyclo[m.n.O]alkenes. Gas-phase ion-molecule experiments (combined with some judicious but non-experimentally measured numbers) allowed the derivation of80 the formal enthalpy of hydrogenation of cubene (27a), i.e. the exothermicity of reaction 19 to form the saturated cubane (27b). 

Accepting the archival enthalpy of formation of gaseous cubane81, we thus deduce the enthalpy of formation of cubene80 to be 1000 17 kJ mol-1. It is hard to have an intuitive feel for such a large, positive enthalpy of formation. One possible probe of the energetics of cubene is the exothermicity of reaction 20... 

However, an alternative probe of conjugation energy in alkyl phenyl ketones is to identify this additional stabilization with the exothermicity of reaction 47,... 

The reasons for this dramatic drop in the rate of reaction on going from the gas phase to aqueous solution have been discussed above. It may be recalled that the difference in the exothermicity of reactions (23) and (26) is as much as 38 kj mol"1. The relatively small effect of the thermochemistry on the rate can be rationalized by invoking the polar effect in the transition state (Russell 1973 Pross et al. 1991 Roberts 1996). Quantum mechanical studies on the solvent effect on the rate of the CH3 plus H202 system can reproduce the dramatic drop upon going from the gas phase to aqueous solutions and indicate that the major reason is the difference between the solvation energies of H202 and H02- in water (Delabie et al. 2000). 

The comparison of free energies and exothermicity of reactions with constants k4 and 5 (Figure 5.8) shows that formaldehyde formation by the reaction (k5) displays higher exothermicity (512.6 kJ/mol compared with — 288.7 kJ/mol for the reaction (k4)) and is accompanied by a significant reduction of the Gibbs energy (—487.92kJ/mol). 

Surfaces for X + M2 and M + X2.—The potential energy surfaces for the two exothermic reactions considered immediately above, F + H2 and H + F2, are repulsive in the nomenclature of Polanyi,14-15 i.e. a surface in which most of the exothermicity is released as the products separate. The other possible extreme of behaviour of a potential energy surface for an exothermic reaction is denoted by characterizing the surface as being of the early downhill or attractive type.14-15 In these systems, most of the exothermicity of reactions appears as vibrational excitation of the newly formed bond. The alkali metal (M), halogen (X) reactions of the type... 

Much as benzene and phenyl are the simplest aromatic hydrocarbon or arene and aryl group respectively, methane and methyl are the simplest aliphatic hydrocarbon or alkane and alkyl group respectively. The resonance energy of the various halobenzenes may be identified as the negative of the exothermicity of reaction 34. 

Formation of C02 from C1C(0)02 may also occur by reaction with atomic oxygen (equation 85) or chlorine (equation 86). Furthermore, the chloroformylperoxy radical is reduced by nitric oxide (equation 87), like its fluorine analogue. However, the resulting chloroformyloxy radical C1C(0)0 is very unstable, and the exothermicity of reaction 87 would cause dissociation (equation 88) into C02 and atomic Cl. 

But remember that H+H is the simplest of all reactions. Moving more in the direction of true chemistry, consider next a reaction for which only two nuclei are hydrogens (instead of three) — the F+H reaction. This reaction is over 1 eV exothermic in going from the reactant valley, over a small (1 kcal) barrier, to the product valley. The exothermicity of reaction means that there are several energetically accessible (open) vibrational channels for this system even at the threshold for reaction. If we include all the rotational levels with each vibration, and the proper (2j+l) rotational degeneracies, we have an unthinkably large number of coupled equations to solve — over 1200 channels. (See Fig. 10.) To solve this problem, we must... 

For the three-ring compounds, AE s in Table I show a decrease in the exothermicity of reaction 1 in the sense IH > 2H > 3H, and also in the series 4H > > 6H. According to these results, the calculated relative ease of... 

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