New enthalpies of formation AHp(g) are provided for the series of orthoesters 2a-2f, 3 and 4 from their enthalpies of combustion AH; and their enthalpies of vaporization AH& or enthalpies of sublimation AH&b. From these data and from literature data the following new thermochemical increments were calculated [kJ . Table 4) are determined by the number of hydrogen atoms in the group and by the number of painvise 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.In recent publications we reported on quantitative enthalpic data of the anomeric stabilization of acetals and ketalsL21 obtained from their heats of formation AHfo(g). For this purpose AHfo(g) was divided into strain-free group increments for the atoms and groups of the compounds. This means that for strained structures the strain enthalpy H,, which was obtained by MM2 calculations for a reference compound was substracted from the experimental heat of formation AHf'(g).The anomeric stabilization is recognized from the increments of the carbon atoms which are flanked by the two oxygen ligands (see Table 3 kJ . mol-I. The introduction of the first alkoxy group into an alkane results in -16.9 and -16.6 kJ . mol-' stabilization, respectively, of the C or CH increment. The introduction of a second alkoxy group into the geminal position has an even higher stabilizing effect of -37.9 and -43.7 kJ .mol-' for the CH and the C increment, respectively. The difference of the stabilizing effects due to the introduction of the first and the second (gem.) alkoxy groups is, by definition, the anomeric stabilization. It is, accordingly -21.3 kJ . mot-' for acetals and -26.8 kJ . mol-' for ketalsc21 CH [02, C[02, C2] -61.9; C[02, C, Cpd provided that the stereoelectronic requirementsL31 are satisfied. There is evidence from force-field calculations that there is generally no steric inhibition for satisfying this requirement in the orthoesters and orthocarbarnates reported here[lb1.In the present paper we investigate in a similar way the anomeric effect of orthoesters, an effect discussed in the literature mainly with regard to the hydrolysis of orthoesters and for the reactivity in Grignard r e a c t i o n~[~~>~l .Only a few heats of formation of orthoesters have been reported so fad4]; they will be included in our analysis of the anomeric effect.Standard enthalpies of formation AHfo(g) obtained from the calorimetrically measured enthalpies of combustion AH,O(cr) and enthalpies of vaporization AH&,, or enthalpies of sublimation AH&b are reported for the eight orthoesters 2a-2f, 3, and 4. We prepared the 2,6,7-trioxabicyclo[2,2,2]octanes 2a-2f by the BF3 catalyzed rearrangement of the corresponding esters of 3-methyl-3-hydroxymethyl oxetanes l a -l f according to a procedure developed by CoreyL61. Purification was achieved by repeated crystallization, and, if possible, by s...
Enthalpies of formation ΔHf°(g) of ketals prepared from cyclopropanone, cyclobutanone, cyclopentanone, cyclohexanone, and 7‐norbornanone with methanol, ethylene glycol, and 2,2‐dimethyl‐1,3‐propanediol, as well as of acetals/ketals of acyclic aldehydes and ketones, have been determined by measuring their heats of combustion and their heats of sublimation/vaporization. Group increments defining the strain‐free energy level have been derived from the collected experimentally determined ΔHf°(g) values of unstrained acetals and ketals, and the anomeric stabilizations of these groups (given in brackets) have been calculated from a comparison of their group increments with those of ethers: CH2[2O, C] –64.9 (−20.2), CH[2O, C] –63.5 (−21.3), C[2O, 2C] –61.9 (−26.8), CH[2O, Ph] –58.4 (−16.2), C[2O, Ph, C] –56.3 (−21.2), C[2O, 2Ph] –67.1 (−32.0) kJ mol−1. Enthalpies of formation ΔHf°(g) of cyclic and spirobicyclic ketals have also been determined experimentally and compared with values obtained from molecular mechanics calculations (MM3). The close agreement of the results shows that the anomeric interactions, which are already integrated in the MM3 force field, are not dissimilar in the small‐ring cyclic and spirobicyclic ketals investigated in this study. The hybridization of the anomeric carbon atom apparently has no influence on the size of the anomeric effect detectable. The strain enthalpies of the cyclic and spirobicyclic ketals have therefore been calculated from their ΔHf°(g) values using the group increment scheme. Analysis and interpretation of all geminal interactions known in acetals/ketals is possible by means of a recently developed additivity scheme, and a single value of –38.6 kJ mol−1 has been obtained for the structural increment representing the inherent geminal O–C–O interactions.
The standard enthalpies of combustion AeH ~ of aliphatic diacetates 1 and aromatic diacetates 2 were measured calorimetrically. The enthalpies of vaporization Av~pH ~ or sublimation A~uhH ~ of 1 and 2 were obtained from the temperature function of the vapor pressure measured in a flow system. From AfH~ of 1 and 2 new values of group increments for the estimation of standard enthalpies of formation of these classes of compounds were derived. The geminal interaction energy between the geminal acyloxy groups shows no anomeric stabilization.KEY WORDS: Enthalpy of combustion; enthalpy of formation; enthalpy of vaporization; geminal interaction of substituents.
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