Base strength correlation with

Lewis base strengths correlate well with other measures of basicity such as the p. a [7]. They are given for a variety of anions in Table 10.2 and References [7, 38]. 

Acids (and bases) are classified by their strength, the amount of (or OH ) produced per mole of substance dissolved, in other words, by the extent of then-dissociation into ions (see Table 4.2). Because acids and bases are electrolytes, then-strength correlates with electrolyte strength strong electrolytes dissociate completely, and weak electrolytes dissociate slightly. 

Even acetone shows appreciable shifts following contact with metal halides - the magnitude of which correlates with the expected strength of the Lewis acid-base interaction. 

Table 5.8 summarizes the NBO descriptors of the net charge transfer from Lewis base to Lewis acid (Qcf), change in covalent-bond polarization (A/Ah), and (P)NBO overlap of n0 with bond (Sna) and antibond (Sna>) orbitals of the Lewis acid. The entries in Table 5.8 show the unfavorable diminution of l/w /5nal and reduced charge transfer as the Lewis acid changes from polar HF to apo-lar CH4. These NBO descriptors can also be closely correlated with quantities in Table 5.7, showing their mutual dependence on the strength of n-a donor-acceptor interaction. 

The earliest LFER, advanced by Bronsted, correlates the acid dissociation constant and base strength (1/A h) of species with its effectiveness as a catalyst in general acid (At h) and base (Atgl-catalyzed reactions respectively. The relationships take the form... 

In addition, rearranging Eq. (21) and combining with Eq. (19) illustrates that i = k" y. Therefore, a two parameter (one term) model would require that a plot of the enthalpies of adduct formation of one acid versus the enthalpies of adduct formation of another acid for the same series of bases be linear with a zero intercept. The enthalpies of adduct formation for 12 and phenol with a wide series of bases does not give rise to such a plot as can be seen in Fig. 5. These acids have very different C/E ratios and their enthalpies of adduct formation cannot be correlated by a one term model. Furthermore, a one term model could riot incorporate systems in which reversals in donor-acceptor strength are observed 32). However, it is possible to correlate enthalpies of adduct formation for acids with very similar C/E ratios such as hydrogenbonding acids using a one term equation. Correlations restricted to one particular type of acid are, of-course, only a subset of the overall E and C correlation. 

These expectations have been borne out. We have confirmed the anticipated correlations of experimental aqueous acid and base strengths with /s,min ° A base becomes stronger, and its p (b lower, as the /s,mm of the basic site decreases. An acid becomes stronger, and its pK lower, as the Is,mm of the conjugate base s basic site increases. To assess basicity and predict p Tb therefore, /s,mm should be computed for the basic site itself to assess acidity and predict pK, /s,mm must be obtained for the conjugate base, i.e. A for an acid HA. 

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