Cyclopropane proton affinity

In contrast to non-cyclic alkanes, reaction of H3O+ with cyclic alkanes is exothermic, even for cyclopropane (proton affinity = 750 kJ moP ). Reaction is by simple non-dissociative proton transfer in the case of cyclopropane, but larger cycUc alkanes can undergo dissociative channels. For example methylcyclohexane reaction is dominated by H2 ejection (90% of the products are produced in this reaction channel) [18]. 

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"... 

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. 

The proton affinities of alicyclic carboxylic acids are identical within 5 kj mol-1, with cyclohexane >cyclopropane >cyclopentane >cyclobutane [205]. This quite surprising situation seems not to be fully explained yet. 

Cubane has had an interesting place in the discussion of the correlation between C-H acidity and carbon hybridization. Its acidity was measured by the H exchange NMR technique and found to be about 6.6 x 10 as reactive as benzene. An experimental gas phase measurement of the proton affinity (PA) as 404kcal/mol is available. (See Tables 3.14 and 3.38 for comparable data on other hydrocarbons.) Both of these values indicate that cubane is somewhat more acidic than expected on the basis of the carbon hydridization. There appears to be unusual hybridization of the anion in this case. An AIM analysis suggests that the C—C bond paths in the anion are less than 90°, suggesting that the bonds bend inward toward the center of the ring. Sauers also noted an increase in s character on going from the hydrocarbon to the anion. Of the 28 deprotonations he examined, only cyclopropane and bicyclo[l.l.l]pentane also showed increased s character in the anion. 

The participation of a-bonds is by no means always weaker than that for it- and n-donors. Schleyer notes that the neighbouring group ability should mirror basicity. The proton affinity of the strained a-bonds of cyclopropane is 179 kcal/mole... 

From the limited amount of experimental data in Table 1 each additional carbon atom increases the proton affinity by approximately 10 kcal mol Cyclopropane has a higher proton affinity than propane by ca 20 kcal mol" indicating considerable relief of strain on protonation. The calculated proton affinity of cyclopropane is at a very high level of theory and is 1.9 kcal mol lower than the experimental value, leading to the suggestion that perhaps the experimental value is slightly too high". ... 

Like protons, transition-metal ions are strongly acidic and they can, in principle, add to both the C—H and C—C bonds of alkanes. As already noted in the section on proton affinities (Table 1) strained cycloalkanes are intrinsically more basic than open-chain alkanes, and the reaction of cyclopropane with Pt((II) to form a platinacyclo-butane (equation 14) was the first reaction of a formally saturated hydrocarbon with a transition-metal ion . The driving force in this reaction is relief of the strain associated with the small ring. The resulting metallacyclobutane is essentially free of ring strain. Many low-valent transition-metal complexes have been found to react with cycloalkanes. Metal ions convet the strained hydrocarbons quadricyclane , cubane , bicyclo-[2.1.0]pentanes , bicyclo[3.1.0]hexanes , bicyclo[4.1.0]heptanes and bicyclo-butanes into less strained isomers (usually cyclohexanes). 

The gas-phase heat of formation of protonated cyclopropane and methyl-cyclopropane, and their proton affinities, have been measured mass-spectro-metrically. The value for protonated cyclopropane (Aiff = 199.8 2.2 kcal moP ) is between the values for 1- and 2-propyl cations so it is a distinct... 

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