ISSN:
1434-193X
Keywords:
Acetals
;
Small-ring ketals
;
Strain
;
Thermochemistry
;
Chemistry
;
General Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
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.
Additional Material:
1 Ill.
Type of Medium:
Electronic Resource
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