ISSN:
1572-9001
Keywords:
Thermodynamic stability
;
acetals
;
13C NMR substituent effects
;
cis-trans isomers
;
ab initio calculations
Source:
Springer Online Journal Archives 1860-2000
Topics:
Chemistry and Pharmacology
Notes:
Abstract The spatial structures of a number of mono- and disubstituted 1,1-dimethoxycyclohexanes (cyclohexanone dimethyl acetals) were studied by 13C NMR spectroscopy. In the monosubstituted acetals, substituents (Me, Et, i-Pr, and MeO) on C-2 are axially oriented, contrary to their normal, equatorial orientation on C-3 and C-4. Besides the spectroscopic study, the relative thermodynamic stabilities of the cis-trans isomers of a few 2,X-dialkyl (X = 3, 4, 5, or 6) derivatives of the parent cyclohexanone dimethyl acetal were determined by acid-catalyzed chemical equilibration in MeOH solution. In the most stable isomeric form, the 2-substituent is axial and the other equatorial. In the less stable isomer, both substituents are equatorial, excluding the cis-2,6-dimethyl derivative, where the 13C NMR shift data point to a predominance of the diaxial form. In general, the enthalpy difference between the isomeric forms is ca. 9 kJ mol−1, while the entropy term favors the less stable isomer by 4 to 16 J K−1 mol−1. In the 2,6-dimethyl derivatives, however, the trans form is favored by only 0.8 kJ mol−1 in ΔG m ⊖ at 298.15 K. The main findings of the experimental work are in good agreement with ab initio calculations.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1023/A:1022416030994
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