Does the Anomeric Effect in Ketals Depend on the Hybridization of the Central Carbon Atom?

Abstract

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.

Calorimetric and Computational Thermochemical Study of 3,3-Tetramethyleneglutaric Acid, 3,3-Tetramethyleneglutaric Anhydride, and 3,3-Tetramethyleneglutarimide

“…In that glutaric acid and the related one-ring species, glutaric anhydride and imide, have been studied, we have now proceeded to the experimental and computational study of 3,3-tetramethyleneglutaric acid (cyclopentane-1,1-diacetic acid, here abbreviated 3,3-tmg acid), 3,3-tetramethyleneglutaric anhydride (8-oxaspiro[4.5]decane-7,9-dione, spiro[tetrahydropyran [4,1′]cyclopentane]-2,6-dione, likewise 3,3-tmg anhydride), and 3,3-tetramethyleneglutarimide (8-azaspiro[4.5]decane-7,9-dione, spiro[piperidine[4,1′]cyclopentane]-2,6-dione, likewise 3,3-tmg imide). We additionally note that these are spiro compounds containing heterocyclic rings, and as such surprises may be expected for this class of such multi-ring species, − relatively understudied at least by thermochemists interested in enthalpies of formation.…”

Enthalpy of Formation and Strain of Norbornane from Thermochemical Measurements and from ab Initio Calculations

“…Norbornane is a key compound in structural chemistry, and knowledge of its geometry and energetics reveals how structural parameters respond to the presence of substantial strain. , Disappointingly, the data for enthalpy of formation of norbornane in the crystalline state are in disarray. Although Δ f H m °(cr) has been reported for norbornane by several authors, the agreement between these values is poor.…”

“…For the definition of the symbols see ref 18: Th ) 298.15 K; V(bomb) ) 0.2664 dm3 ; p i (gas) ) 3.04 MPa; m i (H2O) ) 0.78 g; ∆U(ign) ) 1.46 J; m(Pt) ) 12.18 g. b Masses obtained from apparent masses…”