Group trends formation constants

Examining a host of formation constant data, Moeller et al. (18) have divided the trends in the formation constants qualitatively into three groups. We find that the trends are better represented by five curves as shown in Fig. 3. Smooth curves are drawn through the points to emphasize the general profiles. 

As a comparison of the enthalpies of formation of isomeric aldehydes and ketones shows, the disubstituted carbonyl compounds (ketones) are more stable than the monosubstituted carbonyl compounds (aldehydes), analogous to the stability order for the corresponding 1,1-disubstituted ethenes and the 1-n-alkenes. For the three aldehyde/methyl ketone isomeric pairs (nc = 3-5), initially it seems that the enthalpies of isomerization are fairly constant. However, if the interpolated value for pentanal is used, the trend is clearly that of more negative enthalpy of isomerization with increasing c(g) —31.7, —33.9, —34.5 kJmol-1. The non-constant enthalpies are expected because the slopes, ag, in Table 4 are quite different for aldehydes and ketones. The greater contribution to the isomerization enthalpy differences comes from the methyl ketone which is more stabilized than the aldehyde by an additional —CH2— group. The enthalpies of isomerization of the corresponding alkenes are much less exothermic (—6.1 0.8 kJmol-1) than those of the carbonyl compounds. 

Increasing the hydrophobic part of the surfactant molecules favours micelle formation (see Table 4.3). In aqueous medium, the c.m.c. of ionic surfactants is approximately halved by the addition of each CH2 group. For non-ionic surfactants this effect is usually even more pronounced. This trend usually continues up to about the C16 member. Above the C18 member the c.m.c. tends to be approximately constant. This is probably the result of coiling of the long hydrocarbon chains in the water phase50. 

Vibrational force constants have been calculated for the XBF3 group in the adducts Mc2X,BF3 (X = O, S, or Se). Comparison with values for the separated fragments showed that the expected trends were occurring on adduct formation. Thus, in Me20,Bp3, F(CO) is 4.54 x 10 dyncm (5.38 x 10 in free Me20), and F(BP) is ca. 5 x 10 dyn cm (7.840 in free BF,). 

The equiiibrium constants for addition of alcohols to carbonyl compounds to give hemiacetals show the same response to structural features as the hydration reaction. Equilibrium constants for addition of methanol to acetaldehyde in both water and chloroform solution are near 0.8 The structural effects of the alcohol group have been examined. " Steric effects result in an order of CH3 C2H5 > (CH3)2CH > (013)30 for acetaldehyde hemiacetals. EWG substituents in the alcohol disfavor hemiacetal formation and this trend is believed to reflect the decreasing n tt hyperconjugation (anomeric effect, see Topic 1.2) as the substituents become more electron withdrawing. 

The separation of metals into distinct classes was based on empirical thermodynamic data, namely, trends in the magnitude of equilibrium constants that describe the formation of metal-ion/ligand complexes. On the basis of these criteria, metal ions can be divided into three groups hard, soft, and borderline. The partition of a particular ion in each group is shown in Figure 7.1 (Nieboer and Richardson 1980 see also Morgan and Stumm 1991). 

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