Polarizability and basicity

The importance of solvent participation in the borderline mechanisms should be noted. Nucleophilic participation is minimized by high electronegativity, which reduces the Lewis basicity and polarizability of the solvent molecules. Trifluoroacetic acid and perfiuoro alcohols are among the least nucleophilic of the solvents used in solvolysis studies. These solvents are used to define the characteristics of reactions proceeding without nucleophilic solvent participation. Solvent nucleophilicity increases with the electron-donating capacity of the molecule. The order trifluoroacetic acid < trifluoroetha-nol <acetic acid < water < ethanol gives a qualitative indication of the trend in solvent nucleophilicity. More will be said about solvent nucleophilicity in Section 5.5. 

The position of aniline in the above reactivity order deserves special comment. Aniline is less basic than pyridine by a relatively small factor, 0.65 pA units, but is appreciably more polarizable it then seems likely that the inverted order of reactivity is caused by the polarizability term in accordance with Edwards equation. If this is correct, in the reactivity order piperidine > aniline > pyridine, inversion with respect to basicity appears to result from an abnormally high reactivity of aniline rather than from a particularly low reactivity of pyridine. This view differs from that based on relative steric requirements of the reagents, but other factors besides basicity and polarizability may well contribute to the quantitative experimental picture. 

One does not gain a clear indication of the dominance of any of the empirical parameters of the solvent from an examination of the dependence of the redox potential on those parameters, as the coefficients of the linear correlation are 0.72 (DN), 0.57 (ZiT(30)) and 0.69 (7i ), respectively. However, using equations which take simultaneously into account the acidity, basicity and polarizability of the solvent,11 one can obtain, for example, a linear variation of the redox potential according to the following equation ... 

Edwards has proposed an equation for the correlation of SN2 reaction rates that can be used in different types of solvent systems and that emphasizes the dependence of nucleophilicity on basicity and polarizability.52 The equation is... 

Our discussion has emphasized water solutions of these acids. Many organic compounds are soluble in numerous other solvents. The properties of acids and bases in other solvents depend on solvent properties such as polarity, acidity or basicity, and polarizability. 

Lithiated propynyltrimethylsilane has been used as a substitute for the anion HC=CCHf as a highly nucleophilic species (strongly basic and polarizable) it reacts smoothly with alkyl- and allyl halides to give predominantly acetylenic products [105] ... 

The deprotonations proceed with extreme ease, even at temperatures in the region of — 100°C.Most of the reactions reported are couplings with carbonyl compounds. The species have a low thermostability, and one may wonder whether their basicity (and polarizability) is sufficiently high to warrant good yields with electrophiles that are considerably less reactive than carbonyl compounds, e.g. alkyl halides. 

Nucleophilicity has also been correlated with basicity and polarizability (13) by... 

Basicity and Poiarizabiiity Another factor that affects the relative rates of E2 and Sn2 reactions is the relative basicity and polarizability of the base/nucleophile. Use of a strong, slightly polarizable base such as hydroxide ion, amide ion (NH2 ), or alkoxide ion (especially a hindered one) tends to increase the likelihood of elimination (E2). Use of a weakly basic ion such as a chloride ion (Cl ) or an acetate ion (CH3C02 ) or a weakly basic and highly polarizable one such as Br , l , or RS increases the likelihood of substitution (Sn2). Acetate ion, for example, reacts with isopropyl bromide almost exclusively by the Sn2 path ... 

Basicity and Polarizability Another factor that affects the relative rates of E2 and Sn2... 

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