Kinetic measurements indicate the existence of two different complexes of cucurbituril with 4-methylbenzylammonium ions: (i) an association complex, where the ammonium group binds to one of the polar portals of cucurbituril and the hydrophobic part extends into the solvent, and (ii) an inclusion complex, where the hydrophobic part extends into the cavity of cucurbituril. This complex is used as an indicator for the binding of simple monovalent cations to cucurbituril. Stability constants are reported for the 1 : 1 complexes of cucurbituril with different cations.

The kinetics of hydration of 17 different carbonyl compounds have been studied in acidic solution. The reactions have been found to be acid‐catalysed. For the aliphatic aldehydes propanal to hexanal the rate constants are k0 = (3.5 ± 1) · 10−3 s−1 and kH = (450 ± 30) dm3 mol−1 s−1 and for α‐keto acids they are in the ranges k0 = (0.13 to 0.40) s−1 and kH = (1 to 6) dm3 mol−1 s−1. For halogenated acetones the rate constants depend strongly on the substituents. The large negative value for the reaction entropy ΔS0 = −70 J mol−1 K−1 indicates that three water molecules are involved in the hydration process.

H.-J. Buschmann et al.: The Reversible Hydration of Carbonyl Compounds in Aqueous Solution, Part I 41 T h e same is true with respect to the theoretical approach concerning the stability of W/O microemulsions within the validity range of this model (wo > 20). The surprising validity of the applied equipartition principle as concluded from the excellent coincidence between measured and calculated points, appears noteworthy. T h e latter observation certainly points to a remarkable monodispersity of the discussed W/O microemulsions.The equilibrium constants of hydration (given in parenthesis) have been determined for the aliphatic aldehydes acetaldehyde (1.2), propionaldehyde (1.24), butyraldehyde (0.58), pentanal (0.34), and hexanal (0.41), for the halogenated acetones chloracetone (0.091), fluoroacetone (0.167), and l,l,l-trifluoroacetone (35), and for the 1,2-dicarbonyl compounds 2.3-butanedione (2.1) and 2,3-pentanedione (1.7). Comparing the constants shows, that the equilibrium of hydration of carbonyl compounds is determined mainly by the inductive effects of the substituents.Die Hydratations-Gleichgewichtskonstanten (in Klammern angegeben) wurden fiir die aliphatischen Aldehyde Acetaldehyd (1,2), Propionaldehyd (1,24), Butyraldehyd (0,58), Pentanal (0,34) und Hexanal (0,41), fur die halogenierten Acetone Chloraceton (0.091), Fluoraceton (0,167) und 1,l,l-Trifluoraceton (35) und fur die 1,2-Dicarbonylverbindungen 2,3-Butandion (2,l) und 2.3-Pentandion (1,7) bestimmt. EinVergleich der Konstanten zeigt, daD das Hydratationsgleichgewicht von Carbonylverbindungen vorwiegend durch den induktiven Effekt der Substituenten bestimmt wird. 0 OD 11 I I 00 KI J I + O i O c CHI-CH~--C-C-CHI .J i 1 i , , , , , 483 280 2.33 185 1MA 28, 162 (1974).Czech. Chem. Commun. 31, 388 (1966).

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