Carbanions proton affinity

The active site is viewed as an acid-base, cation-anion pair, hence, the basicity of the catalyst depends not only on the proton affinity of the oxide ion but also on the carbanion affinity of the cation. Thus, the acidity of the cation may determine the basicity of the catalyst. Specific interactions, i.e., effects of ion structure on the strength of the interaction, are likely to be evident when the carbanions differ radically in structure when this is likely the concept of catalyst basicity should be used with caution. 

A more general method for preparing carbenes often involves the a elimination of halides from carbanions.1-57 PAC can be used to examine the rates and energetics of the reverse reactions, the complexation of halides with carbenes (Fig. 5).58 Plots of A//com versus the proton affinities (PA) of the halides are linear for the two carbenes studied. Although the slopes of the plots are similar, complexation of the halides with phenylchlorocarbene is more exothermic than phenylfluorocarbene. This indicates that fluoro substitution stabilizes the carbene relative to the carbanion more than chloro substitution. The rate of complexation of carbenes with salts has also been examined by nanosecond absorption spectroscopy.59... 

Compounds with a high HOMO and LUMO (Figure 5.5c) tend to be stable to selfreaction but are chemically reactive as Lewis bases and nucleophiles. The higher the HOMO, the more reactive. Carbanions, with HOMO near a, are the most powerful bases and nucleophiles, followed by amides and alkoxides. The neutral nitrogen (amines, heteroaromatics) and oxygen bases (water, alcohols, ethers, and carbonyls) will only react with relatively strong Lewis acids. Extensive tabulations of gas-phase basicities or proton affinities (i.e., —AG° of protonation) exist [109, 110]. These will be discussed in subsequent chapters. 

TABLE 25. Proton affinity (PA) and relative stability with regard to the CH3 anion (AE of equation 20) given for cyclopropane and some other carbanions"... 

Froelicher and coworkers215 and Schleyer and coworkers216 calculated the proton affinity of several carbanions (Table 25). Especially, the HF/4-31+G results are in good agreement with experimental values. Calculated proton affinities are compared to that of CH3" with the help of the isodesmic reaction given in equation 20. 

A comparative study of gas-phase and liquid-phase CH acidity of a-substituted cyclopropanes has verified the correctness of an electrostatic model for the effect of solvation on equilibrium acidity 127 among variants of the LCAO MO method, only the semiempirical AMI method accurately predicted the proton affinity of the conjugate carbanions effects of solvation on protonation rates have been desegregated. 

Glidewell and Thomson79 calculated the proton affinities for a variety of silyl substituted carbanions. In the series H3SiCH2-, (II3Si)2CII and (II3Si)3C, stability increased by about 20 kcal mol-1 per silyl substituent. 

Carbanions have been less studied, apart from CH3, 167,168 but included in a more recent set of calculations169 on several carbanions (CH3-, C2H5-, and ethynyl anion) are calculations on C2H3-. For reliable calculations on this type of molecule, diffuse orbitals must be added to the basis set. Several different basis sets were used, but the geometry of the neutral parent molecule was used in some of the calculations. The main aim of this paper was to investigate the electron density and difference densities, electron affinities, and proton affinities. The inversion barrier in the vinyl anion was ca. 142 kJ mol-1 which was in good agreement with that found by Lehn et a/.148 in an earlier calculation. 

Carbanion Method AE Proton Affinity calc. exp. Reference... 

Accurate anion proton affinities are now available from theoretical calculations and this enables one to estimate the proton affinity as well as the relative energy of hypothetical carbanion configurations, as, for example, the cyclopropyl anion in the pyramidal and the planar C2y configuration. 

Because the nonbonding orbital is occupied, stability increases with s character, the converse of the situation for carbocations. The order of stability of carbanions is sp < sp < sp. The relative stability of gas phase carbanions can be assessed by the energy of their reaction with a proton, which is called proton affinity. The proton affinities of the prototypical hydrocarbons methane, ethene, and ethyne have been calculated at the MP4/6-31+G level/ The order is consistent with the electronegativity trends discussed in Section 1.1.5, and the larger gap between sp and sp, as compared to sp and sp, is also evident. The relative acidity of the hydrogen in terminal alkynes is one of the most characteristic features of this group of compounds. 

The stabilities of the C(6)-centred carbanions derived from 1,3-dimethyluracil, N-methyl-2-pyridone, and A-methyl-4-pyridone have been systematically investigated in the gas phase, DMSO (dimethyl sulfoxide) and in water solutions." The stabilities of the carbanions in the gas phase and DMSO were directly measured through their reactions with carbon acids with known proton affinity or pTTa values. The carbanions derived from the two pyridones were found to have the same stability, whereas the carbanion... 

Proton and electron affinities have been calculated for a series of 28 carbanions and radicals using MP2/6-31 and B3LYP methods there was poor correlation between proton affinity and % -character of C-H bonds and anions the importance of C—H and C-C hyperconjugative effects is apparent. A review of anions in the gas phase has referred to stability, structure, and energetic peculiarities of a wide range of anion types. ... 

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