Geminal substituent effects, 11. The anomeric effect in orthoesters — the concept of geminal pairwise interactions for the interpretation of standard enthalpies of formation

Abstract

Abstract New enthalpies of formation Δ H (g) are provided for the series of orthoesters 2a ‐ 2f , 3 and 4 from their enthalpies of combustion Δ H and their enthalpies of vaporization Δ H or enthalpies of sublimation Δ H . From these data and from literature data the following new thermochemical increments were calculated [kJ · mol −1 ]: CH[O 3 ] – 106.4, C[O 3 , C] ‐103.7, and C[O 3 , Ph] ‐124.5. In addition the increment C[O 4 ] – 167.1 was calculated from literature data for Δ H (g) of the orthocarbonates 9a and 9b . The anomeric stabilization derived from these results and from previous results for acetals and ketals are reported in Table 3. – A new concept of additive structural increments for the quantitative analysis of the group increments of hydrocarbons, ethers, acetals, and ketals is proposed. It is shown that the increments for CH 3 , CH 2 , CH, and C groups with their different neighboring atoms (see Table 4) are determined by the number of hydrogen atoms in the group and by the number of pairwise geminal interactions between all atoms attached to the central carbon atom. – Using this procedure for orthoesters and orthocarbonates, deviations from additivity for the geminal interaction between oxygen atoms, i.e. the anomeric effect, are observed.