Hydride ion affinities

All alkyl ions tested demonstrate a comparable behaviour independent of the sign of their charges. The decrease of the reaction enthalpies AH (11) with the change from the methyl to the ethyl cation (AAH (ll) = 165 kJ mol-1) and from the ethyl to the but-2-enyl cation (AAH°(11) = 117 kJ mol-1) corresponds to the increase of stability of these carbenium ions, which are expressed by the difference of their heats of formation (AAH f = —118 and AAHj = —42 kJ mol-1 90)) and of their hydride ion affinity (AHIA = 176 and 126 kJ mol-1 91)), respectively. 

This reaction corresponds to the basic process during the initiation of cationic polymerizations by RX/MtXn and when reversed is the termination reaction. It will be handled more in detail in part 4.2. When X = H, the reaction enthalpy of the previous equation is equal to the hydride ion affinity (HIA) which is shown for various relevant... 

That means The higher the proton affinity of the monomer, the lower the hydride ion affinity of its cation, that is, the more stable this cation (see part 4.1.3). 

Simple linear equations could also be developed for the other three systems of Figure 4, PA of aldehydes and ketones(4e), and their hydride ion affinities, both of the neutral (4f) and protonated forms (4g), However, in addition to effective polarizability and electronegativity, hyperconjugation had also to be used as a parameter, as p-orbitals carrying a partial positive charge are involved in the reactions 4e to 4g (26),... 

The observed order clearly contradicts the results of recent calculations of hydride ion affinity as carried out by the MINDO/3 method, indicating thiophene to be more reactive than furan.190... 

A further popular scale in the gas phase is hydride ion affinity (HIA)2,37 for which X =H. ... 

Free energies of formation, hydride ion affinities, and pKR. Is there an optimum measure of carbocation stability ... 

The group contributions apply only to alkyl cations and are of limited practical value. However, apart from illustrating the application of group additivity contributions to energies of formation of carbocations, they offer a significant insight into comparisons of stability based on hydride ion affinities (HIAs) and pAlR values. 

By employing hydride ion affinity (HIA, the enthalpy change for the reaction RH -> R-hH ) [13] of a carbonyl group as a useful theimochemical correlate, the extent to which the complexation of CH3CHO to BF3 makes the carbonyl electrophilic in the gas phase was also clearly demonstrated. 

Use of the AMI method to calculate proton and hydride ion affinities for 4- and 5-substituted 2-methylimidazoles (9) and attempting to relate these to model benzyl systems has demonstrated excellent correlations between 4-substituted compounds and w-toluenes and the 5-isomers and p-toluenes. The substituents appear to exert only minimal effects on the change in charges on the annular nitrogens as the anionic or cationic species are formed (Scheme 1) <93JOC7336>. 

Unfortunately, since a true experimental value for the heat of formation of thiazole is not known, it is not possible to evaluate the accuracy of the calculated values. However a close practical estimation of an experimental AHf° for thiazole can be obtained (see Section 3.06.4). Dewar s AMI semiempirical MO method has also been used by Bean <93JOC7336> to calculate the heats of formation and charges in the 4- and 5-substituted-2-methyl thiazoles and their benzyl-like anions. This study afforded also proton and hydride ion affinities of the benzyl-like anion and cation of 2-methylthiazoles (see Section 3.06.7.3.2). 

The hydride ion affinity of the cation (R + H —RH) relative to that of the vinyl cation (taken from a list in Ref. 67). 

Figure 5.10 Absoiute (data below the structures) and relative (data near the arrows) gas phase hydride ion affinities for selected carbocations. All energies are in kcal/mol and taken from ref 21.

Figure 5.12 (a) Gas phase hydride ion affinities for carbocations with a-heteroatoms. All energies are in kcal/mol and taken from ref 26. (b) Cost of rehybridization at a heteroatom X is included (as a penalty) in stabilization provided to a carbocation by the lone piiir of X. The different trend for compounds of Group V originates from the higher cost of planarization for the heavier elements (Norbital size of X increases. ... 

One fundamental aspect of carbocation chemistry is the large dependence of the energy of the cation on the substituents attached to the positively charged carbon atom. This dependence is most evident in the gas phase. Table 5.4 shows thermodynamic data for selected carbocations. The hydride ion affinity, HIA(R ), is defined as the negative of the AH for the attachment of a hydride ion to the cation in the gas phase. That is, the greater the H1A(R ), the more endothermic is the removal of a hydride ion from an alkane. We expect the trends to be the same for heterolytic dissociation of alkyl halides or other species that produce carbocations. 

A common and very valuable measure of gas phase carbocation stability is the hydride ion affinity (HI A), defined as AH° for the reaction in Eq. 2.15. This is simply the heterolytic analogue of the homolytic cleavage associated with the bond dissociation energy (BDE) just discussed. Just as a larger BDE implies a less stable radical, a larger HIA implies a less stable carbocation. And, just as with the BDE, the usefulness of the HIA is that it provides a number that can be compared directly for cations of dissimilar structure. This is less true of other measures of cation stability. 

Gas Phase Hydride Ion Affinities (HIA, in kcal/mol) for Selected Carbocations (in kcal/mol) ... 

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