Hydrogen bonding solvation effects

Finally, since besides the inductive effect of the sulfoxide and the sulfone functional groups, hydrogen bonding, field effects and steric effects to solvation may or may not work in the same direction, the pKx values can be useful in assigning configurations of suitable pairs of stereoisomeric sulfoxide and sulfone carboxylic acids291. 

The conformation that a furanoid compound will adopt in the crystal is not readily predicted. A furanoid compound can adopt one of four envelope conformations in which C-2 or C-3 is endo or exo, namely, 2E, E2, 3E, and E3 less commonly, it may adopt a twist conformation. The energy difference between the four common envelope conformations may be so slight that other effects predominate in determining the conformation that the compound will adopt, both in the crystal and in solution. Such effects include those caused by hydrogen bonding, solvation, and close contacts with an aglycon. 

In cases where there is strong solvation of the carbanion, as for example hydrogen bonding solvation of enolate or nitronate ions in hydroxylic solvents, the intrinsic barrier is increased further because the transition state cannot benefit significantly from this solvation. This is the reason why AG for the deprotonation of nitroalkanes in water is particularly high, i.e., much higher than in dipolar aprotic solvents, see, e.g., entry 11 versus 15 and entry 13 versus 16 in Table 1. These solvation effects will be discussed in more detail below. 

Solvation can have a large effect on intrinsic barriers or intrinsic rate constants, especially hydrogen bonding solvation of nitronate or enolate ions in hydroxylic solvents. Table 4 reports intrinsic rate constants in water and aqueous DMSO for a number of representative examples.19,20,23 25,40,54 56 Entries 1-4 which refer to nitroalkanes show large increases in ogka when... 

There has been considerable interest in the factors that control the stereoselectivity of cyclobutanol formation. Three main factors were identified quite early pre-existing conformational preferences due to steric effects or to internal hydrogen bonding solvation of the OH group and variable rotational barriers for cyclization. More recently Griesbeck has proposed that orbital orientation favoring soc produces another form of conformational preference in triplet biradicals [55], These factors have different importance depending upon the molecule. 

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