Hydroxylic solvents polarity

We consider first the Sn2 type of process. (In some important Sn2 reactions the solvent may function as the nucleophile. We will treat solvent nucleophilicity as a separate topic in Chapter 8.) Basicity toward the proton, that is, the pKa of the conjugate acid of the nucleophile, has been found to be less successful as a model property for reactions at saturated carbon than for nucleophilic acyl transfers, although basicity must have some relationship to nucleophilicity. Bordwell et al. have demonstrated very satisfactory Brjinsted-type plots for nucleophilic displacements at saturated carbon when the basicities and reactivities are measured in polar aprotic solvents like dimethylsulfoxide. The problem of establishing such simple correlations in hydroxylic solvents lies in the varying solvation stabilization within a reaction series in H-bond donor solvents. 

Majetich and Hicks <96SL649> have reported on the epoxidation of isolated olefins (e.g., 61) using a combination of 30% aqueous hydrogen peroxide, a carbodiimide (e.g., DCC), and a mildly acidic or basic catalyst. This method works best in hydroxylic solvents and not at all in polar aprotic media. Type and ratios of reagents are substrate dependent, and steric demand about the alkene generally results in decreased yields. 

For reactions that are traditionally performed in hydroxylic solvents or in polar aprotic solvents, PTC has the following advantages no need for expensive aprotic solvents, shorter reaction time and/or lower reaction temperature, use of aqueous alkali hydroxides instead of other expensive bases. Several examples are given in Section 4.2.2. 

So far as actual changes of mechanistic pathway with change of solvent are concerned, increase in solvent polarity and ion-solvating ability may (but not necessarily will) change the reaction mode from SN2— SN1. Transfer from hydroxylic to polar, non-protic solvents (e.g. DMSO) can, and often do, change the reaction mode from SN1 — Sn2 by enormously increasing the effectiveness of the nucleophile in the system. 

Other preparative snags also occur in the addition of HHal to alkenes. Thus in solution in H20, or in other hydroxylic solvents, acid-catalysed hydration (p. 187) or solvation may constitute a competing reaction while in less polar solvents radical formation may be encouraged, resulting in anti-Markownikov addition to give 1-bromopropane (MeCH2CH2Br), via the preferentially formed radical intermediate, MeCHCH2Br. This is discussed in detail below (p. 316). 

It may seem curious to make specific mention of this matter, but in a biological context, it is crucial. The nuclei of porphyrins and related systems are hydrophobic it appears to be generally desirable that the photosensitizer has some degree of intermediate polarity (i.e., amphiphilic properties Section 9.22.3) and the incorporation of a metal, such as zinc(II), which can take on an axial ligand (e.g., H20 in aqueous media, or RNH2 in a biological fluid), or magnesium(II), which can take on two, is expected to enhance solubility in hydroxylic solvents, and shift the... 

However, for hydroxylic solvents such as methanol or water, specific solvent effects exist, the dielectric continuum result in eq. 17 is no longer applicable, and variations in XQ are appreciable. Even so, eq. 14 still applies in that for a series of excited states like (bpy)0sL 2+ > plots of lnknr vs. Eem remain linear and have the same slope as the plots for polar organic solvents. The difference is that the lines are parallel but offset, because the term appears in the intercept and xo is non-negli-gible for the hydroxylic solvents. 

Another important effect observed when reactions take place in the liquid phase is associated with the solvation of the reactants. Theoretical comparison showed that the collision frequencies of the species in the gas and liquid phases are different, which is due to the difference between the free volumes. In the gas phase, the free volume is virtually equal to the volume occupied by the gas species (FfwT), while in the liquid phase, it is much smaller than the volume of the liquid species (V < V). Since the motion and collision of the species occur in the free volume, the collision frequency in the liquid is higher than in the gas by the amount (V/Vf)U3 [32,33]. The activation energies for the reactions of radicals and atoms with hydrocarbon C—H bonds in the gas and the liquid phases are virtually identical, and that in the liquid is independent of the solvent polarity. This also applies to the parameter bre, which can be seen from the following examples referring to the interaction of the hydroxyl radical with hydrocarbons [30] ... 

It is more difficult to interpret micellar effects upon reactions of azide ion. The behavior is normal , in the sense that k /kw 1, for deacylation, an Sn2 reaction, and addition to a carbocation (Table 4) (Cuenca, 1985). But the micellar reaction is much faster for nucleophilic aromatic substitution. Values of k /kw depend upon the substrate and are slightly larger when both N 3 and an inert counterion are present, but the trends are the same. We have no explanation for these results, although there seems to be a relation between the anomalous behavior of the azide ion in micellar reactions of aromatic substrates and its nucleophilicity in water and similar polar, hydroxylic solvents. Azide is a very powerful nucleophile towards carboca-tions, based on Ritchie s N+ scale, but in water it is much less reactive towards 2,4-dinitrohalobenzenes than predicted, whereas the reactivity of other nucleophiles fits the N+ scale (Ritchie and Sawada, 1977). Therefore the large values of k /kw may reflect the fact that azide ion is unusually unreactive in aromatic nucleophilic substitution in water, rather than that it is abnormally reactive in micelles. 

The dependence of the fluorescence quantum yields and lifetimes of these stabilizers on the nature of the solvent suggests that the excited-state, non-radiative processes are affected by solvation. In polar, hydroxylic solvents, values of the fluorescence quantum yield for the non proton-transferred form are significantly lower, and the fluorescence lifetimes are shorter, than those calculated for aprotic solvents. This supports the proposal of the formation, in alcoholic solvents, of an excited-state encounter complex which facilitates ESIPT. The observed concentration dependence of the fluorescence lifetime and intensity of the blue emission from TIN in polymer films provides evidence for a non-radiative, self-quenching process, possibly due to aggregation of the stabilizer molecules. 

The reactions discussed in the following sections take place in aprotic solvents, and reference to known or estimated thermodynamic basicities will relate to DM SO unless otherwise noted, since DM SO is the polar aprotic solvent in which most thermodynamic acidities have been measured [55-58]. Values of pK determined in DM SO can usually be assumed to parallel values in DMF [59, 60], MeCN, and other polar aprotic solvents whereas pK values (and relative pK values) related to water and other hydroxylic solvents can be very different. 

The structure of [Ni(ttas)X2] [ttas = bis-(o-dimethylarsinophenyl)methyl-arsine, X = Cl, Br, I, or SCN] is very sensitive to the nature of X and the solvent. Five-co-ordinate monomers are favoured in polar solvents, whereas hydroxylic solvents favour disproportionation to [Ni(ttas)2] and either NiX - or its solvolysis products. The co-ordination geometry of [Ni(ttas)2Y2] (Y = CIO4, NO3, or I) is not certain. ... 

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