Energy heterolytic bond dissociation energies

Table 1.4. Heterolytic Bond Dissociation Energies for Some C-H and C-Cl Bonds S...

Bromonium ion stabilities in the gas phase have also been measured in ion cyclotron resonance experiments by Beauchamp s group (Staley et al, 1977). The heterolytic bond dissociation energies shown in Table 3 are taken as a... 

With respect to the parent ethylenebromonium ion and expressed as heterolytic bond dissociation energies D (R +—Br"). Data from Staley et al. ( 1977). Calculations (Galland et al., 1990) indicate that this ion is not bridged but open. 

The next question which presents itself is whether we can explain why in some systems solvent co-catalysis occurs, whereas in others, apparently similar, it does not. Let it be said first that in fact there is very little experimental evidence on this point. From the thermochemical point of view one can say that alkyl halide co-catalysis is the more probable, the lower the heterolytic bond dissociation energy of the alkyl halide, the more stable the cation derived from the monomer, and the smaller the anion derived from the metal halide. It must, however, be remembered that the non-occurrence of alkyl halide co-catalysis may be due to a kinetic prohibition, i.e., an excessively high activation energy for a reaction which is thermodynamically possible. 

Table 1 Some heterolytic bond dissociation energies (all figures in kcal/mole) ...

Michael additions to benzotriazole-stabilized carbanions have been reviewed. review of the structural dependence of heterolytic bond dissociation energy of carbon-carbon a-bonds in hydrocarbons has summarized the synthesis and behaviour of molecules in which highly stable cationic and anionic hydrocarbon moieties have apparently been combined. 

The most widely studied reference acid is the proton. Proton affinity, PA(B), is defined for a base B as the heterolytic bond dissociation energy for removing a proton from the conjugated acid BH+ (equation 20). The homolytic bond dissociation energy D(B+—H) defined by equation 21 is related to PA(B) and the adiabatic ionization potentials IP(H) and IP(B) (equation 22) are derived from the thermochemical cycle shown in Scheme 6. 

An alternative view of the interaction of an alkali metal cation with a fluoride-containing anion is one of Lewis acid/base competition. The reactions discussed in the preceding section involved the reaction of an alkali fluoride salt with a Lewis acid with subsequent fluoride ion transfer to the Lewis acid. However, the alkali metal cation is a Lewis acid as well, and the degree of perturbation of the anion by the cation may be dependent on the differences in fluoride ion affinity of the Lewis acid and the alkali metal cation. The fluoride ion affinities for a variety of Lewis acids are well known from ICR (53,54,64) studies, while the fluoride ion affinities for alkali metal cations are the heterolytic bond dissociation energies of the gas phase alkali fluoride molecules... 

See also heterolytic bond-dissociation energy homolysis. 

The dehydration of aliphatic alcohols over silica-alumina was found to be zero order and a linear relationship between the activation energy and the heterolytic bond dissociation energy of the alcohols was obtained. 

Finally, the stabilization energies of ionic species may be related to some properties of the corresponding molecules such as their heterolytic bond dissociation energy (HBDE) and their gas-phase acidity (AH°dC,). 

The problem was again taken up in general terms at the Chemical Society Symposium on the Transition State (Maccoll, 1962). Here, the behaviour of carbonium ions in the mass spectrometer and in solvolytic reactions was examined in relation to the behaviour of the virtual carbonium ions postulated in the gas-phase elimination reaction. Such properties as the ease of formation of carbonium ions and their rearrangements were examined, and, where data were available, a linear relationship was found between the activation energy for elimination and the heterolytic bond dissociation energy. This relationship is shown in Fig. 6. 

A radical-based homodesmotic reaction gives a value of 30.4 kcal/mol, which compares with 29.1 kcal/mol for benzene by the same approach. " The gas phase heterolytic bond dissociation energy to form cyclopropenium ion from cyclopropene is 225 kcal/mol. This compares with 256 kcal/mol for formation of the allyl cation from propene and 268 kcal/mol for the 1-propyl cation from propane. It is clear that the cyclopropenyl cation is highly stabilized. 

In fast reactions, ionized amines show an even greater propensity to undergo the ubiquitous a-cleavage than do ionized alcohols and ethCTs. This distinction may reflect the superior stability of the immonium ions compared to their oxonium ion counterparts . Thus, the hydride anion affinities [D(R+ — H ), corresponding to the heterolytic bond dissociation energy of the conjugate compound] of immonium ions are ca 100 kJmol lower than those of the analogous oxonium ions [e.g. D(R+ — H ) of CH2=NH2" and CH2=0H+ are 912 and 1054 kJmol, respectively] . 

A fundamentally similar situation is obtained with the initiation of styrene by alkylaluminum halides or trialkylaluminum compounds. In the case of this monomer the demarcation between poor and good coinitiators is perhaps even more evident than with isobutylene. Fig. 2 shows the essential data obtained in a series of experiments in which various alkyl chlorides were added to styrene/Al(C2H5)2Cl charges in methyl chloride 24). Alkyl halides with high R—Cl heterolytic bond dissociation energies are very poor coinitiators compared with those with lower dissociation energies. Thus 3-chloro-l-butene, crotyl chloride, fert-butyl chloride, 1-chloroethylbenzene and diphenyl chloromethane are efficient because the carbocations which arise from these chlorides... 

The cr-substituent effects and solvent effects (extended Grunwald-Winstein equation) for solvolyses of / -X-substituted benzoyl chlorides (X = OMe, Me, H, and Cl) in 97% wt/wt hexafluoroisopropanol/water was explained by two competing pathways. Plots of log k versus a were consistent with a cationic path and explained by an 5 n2-5n1 spectrum of mechanism. Electron-donating groups favoured the reaction and values of p increased in the order AcOH < HCO2H < TEE < HFIP. A benzoyl cation intermediate was trapped in hexafluoroisopropanol. Ab initio calculations of heterolytic bond dissociation energies of various chloro- and fluoro-substituted and other benzoyl chlorides were correlated with log k for solvolyses... 

In contrast, the less strained four-7r-electron cyclopentadienyl cation is very unstable. Its p r+ has been estimated as --40, using an electrochemical cycle. The heterolytic bond dissociation energy to form the cation from cyclopentadiene is 258 kcal/mol, which is substantially more than for formation of an allylic cation from cyclopentene but only slightly more than the 252 kcal/mol for formation of an unstabilized secondary carbocation. " Solvolysis of cyclopentadienyl halides assisted by silver ion is extremely slow, even though the halide is doubly allylic. When the bromide and antimony pentafluoride react at -78°C, the EPR spectrum observed indicates that the cyclopentadienyl cation is a triplet. Similar studies indicate that pentachlorocyclopentadienyl cation is also a triplet, but the ground state of the pentaphenyl derivative is a singlet. 

Heterolytic bond dissociation energies for some C—H and C—Cl bonds... 

Fig. 4. Linear relationship between hydrogen bond energy —AHq,u for reaction X (HOH)= =X +HOH and gas phase basicity of X. Gas phase basicity of X expressed in terms of heterolytic bond dissociation energy Z)(H+—B ) corresponding to AH fov reaction HB=H++B in gas phase.

The standard enthalpy change for the reverse of equation 1 is the heterolytic bond dissociation energy for BH (equal by convention to the proton affinity of B). The homolytic bond dissociation energy for BH", i.e. the standard enthalpy change for reaction 5, has been obtained [17,18] for some bases from the proton affinity and the... 

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