The ease of acetonide formation for some 1,2-and 1,3-glycols is mzasured and in the discussion of the equilibrium data (tables I and 11). obtained by quantitative evaluation as a function of temperature, emphasis is placed upon the pronounced entropy decrease, and its consequence in the interpretation. All cyclizations are found to be favoured by enthalpy decreases but are governed by entropy changes.The studies on the conductivity of boric acid in the presence of glycols, and the studies on chemical equilibration of these glycols with acetone by Boeseken [l] may be considered as the start of conformational information on heterocyclic compounds. Since then, these and other results, have been used to correlate constitution and conformation with reactivity, and such aspects were of a fundamental nature for the study of natural polyhydroxylic compounds. It is therefore surprising to find rather scarce information with respect to detailed thermodynamic parameters which govern the acetalization. It is obvious, that in order to obtain substantial information, it does not suffice to rely upon observed equilibrium states obtained at one temperature only.We began the study of acetal cyclization by a reinvestigation of the acetonide formation of several glycols and the determination of the equilibrium states as a function of temperature by quantitative nmr spectroscopy. The data obtained for mixtures of acetone and glycols are gathered in tables I and 11. DISCUSSION OF THE TABLES General Aspects1. Although all the reactions are favoured by a decrease of enthalpy, they all are characterized by unfavorable entropy changes. This is not unexpected because during cyclization a diol molecule is replaced by a more associated water molecule.Therefore the gross portion of the observed entropy decrease must be attributed to the concurrent decrease in translational freedom. Those rare reactions which occur almost isoentropically are probably characterised by an increase of rotational and/or vibrational freedom. All discussions on entropy changes in this paper are of comparative nature, and cannot be done on an absolute basis. One of the most striking features, especially when one limits the discussion to a homologous series, is the nearly constant ratio of AH" versus ASo (tableII, nosl,

Thermodynamic parameters for the cis-trans equilibrium of 2,4-disubstituted 1,3-dioxolanes were evaluated using gaschromatographic data. The equilibrium is always close to 60:40 (cistrans), and only very slightly dependent on the nature of the substituents. The r . butyl group substituted in dioxolanes has no anancomeric (biasing) effect.The cis-trans equilibrium of 1,3-dioxolanes in function of the nature of substituents on C-2 and C-4 was determined by gaschromatographic analysis.Using a glass capillary column of 60 m coated with Carbowax, it was possible in most cases to separate the cis and trans-2,4-disubstituted epimers obtained by equilibration in acidic medium. These equilibria were obtained at several temperatures, starting from a molar concentration of one of the pure epimers in carbon tetrachloride solution, containing 0.005 N trifluoroacetic acid. The equilibrium, when attained, was fixed by neutralization of the catalyst with potassium carbonate. Starting from a mixture containing mainly the cis-resp. trans isomer, the real equilibrium was allowed to be reached.The 1,3-dioxolanes were prepared by mixing the corresponding aldehydes with 1,Zpropanediol (FLUKA), or by mixing with 3,3-dimethyl-l,2-butanediol in benzene with p. toluenesulfonic acid as the catalyst and removing water by azeotropic distillation. 3,3-Dimethyl butanediol was prepared following the literature [2]. After preparative separation by GC on a 20 m x 9 mm EYTMOcoated column whereever possible, the cis and trans isomers were characterised by their nmr and I.R. spectrum.The more stable cis isomer has always the smallest retention time, except for 2,4-di t.Bu-1 ,fdioxolane. The relative shift values in the pmr spectrum were used for the assignment of the configuration [3]. At that time [3] the cis-2,4-disubstituted-l,3-dioxolanes were supposed to be the less stable isomers, but clearly this statement must be reversed [4], so thus the advanced [3] criteria. In accordance with the extended von Auwers-Skita rule [5], the more stable isomer has the lower density and refraction index, even when the retention time is found to be reversed (2,4-di t.Bu-l,3-dioxolane). Table 1 gives the amount of the more abundant (cis) isomers at equilibrium for several temperatures, as obtained by electronic integration of the corresponding peak areas in the chromatograms. These data are mean values of several (up to 15) evalua-

The PMR shift of aldehydic protons (8CHO) of substituted aromatic aldehydes varies with the nature and the position of the substituent (I), as is the case for other functional groups (2). No clear relation between the observed shift and Hammett's (J value is however found for the aldehydes (I), but precise shift data have not been published. Table I collects those, obtained at 56.4 Mc for infinite dilutions (extrapolated from solutions of 1 to 1/20 in molarity).The failure of the free energy relationship is somewhat more pronounced for the meta derivatives, suggesting additional disturbance caused by magnetic anisotropy, not to be neglected in one of the possible rotamers of the formyl group. Correlation with (J, on, BR, GI, o m or (J' was tried without success, although the best linear relation was found for para substitution :

Für die Dioxolane (I) und (II) wird gezeigt, daß das Epimerenverhältnis weitgehend von den 2‐ und 4‐Substituenten unabhängig ist und ca. 60/40 (cis/ trans) beträgt.

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