Hydride affinity of carbocation

Several criteria, including hydride affinities of carbocations (R+—H heterolytic BDE) and adiabatic ionization potentials of the corresponding free radicals (Table 1.30), indicate the order of decreasing stability of fluoromethyl carbocations to be CHF2+ > CH2F+ > CF3+ > CH3+ (Scheme 1.54) [1],... 

Hydride affinity of carbocations (21). Original data from Ref. 22. 

Table S3. Solution Hydride Affinity of Some Carbocations...

The heats of formation of the carbocations are then combined with the heats of formation of the corresponding hydrocarbons [21] and AH ° of H (139 kJ mol-1) [23] to yield the hydride affinity scale of Scheme 2. Although hydride affinities of larger carbocations (molecular formula a... 

It is known that for SnI ewaction (substitution-nucleophilic-unimolecular) [140-141] a key intermediate is a carbocation, therefore the more reactive substrate will be the one that can produce the most stable carbocation. The reactivity of methyl, ethyl, 2-propyl, and 2-methyl-2-propyl tosylates under SnI reaction conditions is inversely proportional to the calculated hydride affinity of the corresponding carbocations. The calculated values were in agreement with the experimental findings which were obtained through solvolysis rate measurement of these tosylates under SnI conditions [142, 143]. Correlation of the cation stability-hydride and affinity-solvolytic rate of the reaction under Sn 1 reaction conditions was observed for the allyl cation (allyl, 3-penten-2-yl, and 2-methyl-3-butene-2-yl cations)[144] and the benzyl cation (benzyl, 1-phenylethyl, and 2-phenyl-2-propyl cations) [145] series. The most reactive substrates were the ones that formed the carbocations with the lowest hydride affinity. 

One of the most important and general trends in organic chemistry is the increase in carbocation stability with additional alkyl substitution. This stability relationship is fundamental to understanding many aspects of reactivity, especially of nucleophilic substitution. In recent years, it has become possible to put the stabilization effect on a quantitative basis. One approach has been gas phase measurements which determine the proton affinity of alkenes leading to carbocation formation. From these data, the hydride affinity of the carbocation can be obtained. 

There is an excellent correlation between these data and the gas-phase data, in terms both of the stability order and the energy differences between carbocations. A plot of the gas-phase hydride affinity versus the ionization enthalpy gives a line of slope 1.63 with a correlation coefficient of 0.973. This result is in agreement with the expectation that the gas-phase stability would be somewhat more sensitive to structure than the solution-phase stability. The energy gap between tertiary and secondary ions is about 17kcal/mol in the gas phase and about 9.5 kcal/mole in the SO2CIF solution. 

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

Perhaps less obviously, the hydrocarbon also provides a reference for the carbocation. It is worthwhile examining the implications of such a reference, by considering briefly thermodynamic measurements of carbocation stabilities in terms of heats (enthalpies) or free energies of formation. Mayr and Ofial contrast our ability to measure the relative energies of tertiary and secondary butyl cations with the significant differences in relative stabilities of secondary butyl and isopropyl cations derived from different equilibrium measurements, namely, hydride, chloride, or hydroxide ion affinities. It is convenient to focus on this example and to assess the effectiveness of hydride affinities for comparing the stabilities of these three ions. 

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. 

Scheme 2 Hydride affinity scale of carbocations. (data from Ref. 23.) With //r°(H ) = 145 kJ mol-1 [20] the calculated hydride affinities are 6 kJ mol-1 greater than listed in this scheme.

The influence of cyclopropyl on the gas phase stability of carbocations as measured by ion cyclotron resonance is shown in Table 14, along with data for some reference compounds. The results are given as gas phase basicities, GB, and proton affinities, PA, defined as AG° and AH°, respectively, for dissociation of the protonated molecule, as in equation 11. In addition hydride affinities D(BH H ) for some cations defined as — AH° for equation 18 are included. For the gas phase basicities and proton affinities the products B are alkenes, amines, nitriles or carbonyl compounds, and thus for these values the stability of the cation is compared to a derivative where the substituent is conjugated with a carbon-carbon or carbon-oxygen double bond, or a nitrogen lone pair, whereas for hydride affinities the products are saturated. 

Carbocation stability in the gas phase can be measured by mass spectrometry and reported as hydride affinity, which is the enthalpy of the reaction ... 

As discussed in Section 3.4.1, carbocation stability can also be expressed in terms of hydride affinity. Hydride affinity values based on solution measurements can be... 

The relative reactivity of tertiary bridgehead systems toward solvolysis is well correlated with the increase in strain that results from conversion of the ring structure to a carbocation, as calculated by molecular mechanics. " This result implies that the increased energy associated with a nonplanar carbocation is proportional to the strain energy present in the ground state reactant. The solvolysis rates also correlate with bridgehead cation stability measured by gas phase hydride affinity andMP2/6-31 IG MO calculations. ... 

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. ... 

Figure 6.42 Stabilization and hydride affinities in primary, secondary, and tertiary carbocations. Evolution of...

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. 

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