Hammett bases

Taken from Reference 170. The values are averaged ones obtained by U V and N MR methods in H2S04 or HC104. Bunnett-Olsen s values, 0.4-0.6 in equation 29. This suggested that sulphoxides are not Hammett bases (J. F. Bunnett and F. P. Olsen, Can. J. Chem., 44, 1899 (1966))... 

We used DFT to optimize the geometries of various Hammett bases on cluster models of zeolite Brpnsted sites. For p-fluoronitrobenzene and p-nitrotoluene, two indicators with strengths of ca. -12 for their conjugate acids, we saw no protonation in the energy minimized structures. Similar calculations using the much more strongly basic aniline andogs of these molecules demonstrated proton transfer from the zeolite cluster to the base. We carried out F and experimental NMR studies of these same Hammett indicators adsorbed into zeolites HY and HZSM-5. 

The above values can only represent a rough measure of basicity, since it has been found23,43 that cyclopropenones do not behave as Hammett bases. A refined treatment minimizing the NMR effects extraneous to the protonation of interest gave H0 values of —5.0 for methyl cyclopropenone and —1.5 for dimethyl cyclopropenone23. ... 

Oxadiazoles are weak Hammett bases. The basicity constants of 2,5-diphenyl-1,3,4-oxadiazole (pK -2.49) and of 2-(4-methylphenyl)-5-phenyl-l,3,4-oxadiazole (pAa -1.15) were measured by the method of Yates and MacClelland in an aqueous solution of sulfuric acid in the range from pH 7 to H0 -10. Both compounds exhibited luminescence properties depending on the acid concentration <1996SAA1875>. 

Furoxans are very feeble bases, even when an amino group is attached to the ring. 4-Amino-3-methylfuroxan behaves as a Hammett base [15], as does benzofuroxan (2, R=H) [7]. By contrast, the two methylfuroxan carboxylic acid isomers show equal high dissociation constants [9]. 

The pKa s of protonated benzoic acid and its derivatives are readily measured by conventional spectrophotometric methods. Data for over forty compounds have been obtained by Stewart and Granger34 and Stewart and Yates35 and the results have been summarized by Arnett36. Substituted benzoic acids are well-behaved Hammett bases, with slopes of the logarithm of the ionization ratio plots close to 1.0. But, as explained by Arnett36, this is only true, for benzoic itself t any rate, in its region of protonation. In more dilute acid a sudden sharp change in activity coefficient occurs, which means that the observed... 

Aliphatic esters do not behave even approximately as expected for Hammett bases. Plots of the logarithms of the ionization ratios against — H0 are linear, but have a slope of 0.62 for simple acetates37. Lane et al.3t< have found a similar slope, 0.65, for ethyl acetate38. 

This equation takes into account that benzaldehyde is a Hammett base and F = [F]/ 7V]. Equation 16 can be used to obtain pKa for benzaldehye, because a linear relation should be observed between lg F and Ho with a slope equal to unity. 

UV spectroscopy of adsorbed Hammett bases has also been used to estimate the acidity of solids such as zeolites.38... 

Table 1.1. Selected pA, .n Values for Extended Hammett Bases...

The natural clay minerals are hydrous aluminum silicates with iron or magnesium replacing aluminum wholly or in part, and with alkali or alkaline earth metals present as essential constituents in some others. Their acidic properties and natural abundance have favored their use as catalysts for cracking of heavy petroleum fractions. With the exception of zeolites and some specially treated mixed oxides for which superacid properties have been claimed, the acidity as measured by the color changes of absorbed Hammett bases is generally far below the superacidity range. They are inactive for alkane isomerization and cracking below 100 °C and need co-acids to reach superacidity. 

The pJ a values of phosphine oxides have been measured. Their deviation from Hammett base behaviour, their Ho dependencies, and their sites of protonation have been studied. The acidifying effects of phosphoryl, phosphinyl, and thiophosphinyl groups have been studied. Ionization constants have been used to examine substituent effects in alkanephosphonic acids and phosphinyl-carboxylic acids, structural correlations in various phosphorus acids, and solvent effects on the properties of thiophosphorus acids. The linear free-energy relationships, which are based mainly on p a data, have been analysed and reviewed. ... 

Although the ultraviolet spectrum of benzofuroxan undergoes an extensive, and reversible, change on solution in fairly concentrated sulfuric acid, suggesting that protonation is occurring, the site of proton attachment is not known. Benzofuroxans behave approximately as Hammett bases (Gr. (logio[BH+] — logio [B])/Hq= 1), with pK s —8.3+ 0.1 (unsubst.), —8.4+ 0.1 (4-Me), —8.2+ 0.1 (5-Me) (cf. — 8.4 + 0.1 for benzofurazan). C-Protonation (as 35) would probably require the protonation to follow a steeper acidity function (cf. azulenes ), and therefore N- (36) or 0- (37) protonation is in-... 

Protonation of Hammett Bases in Water, 96% H2S04, and HS03F... 

The various species present in nitration media of different acidities are presented in Figure 1. The forms of the oxy-aclds of the N(III) and N(V) oxidation states of nitrogen are shown as a function of acidity also shown is the point of significant protonation of 2,4-DNT. The scale used at the bottom of the figure is the Hammett acidity function, used here for convenience, since the function spreads the acid regions of interest, even though the species involved are not necessarily Hammett bases. 

There are indications that cyclohexadienones are not Hammett bases 3M-226) that in many cases their protonation is better described by the amide acidity function Therefore the basicity of cyclohexadienones is generally characterized either by the value of pK in the scale or by that of (H )ip f e value of the Hammett acidity function at which the cyclohexadienone is half converted into a conjugate acid — the hydroxybenzenium ion. The available data on the basicity of cyclohexa-2,5-dienones are summarized in Table 4. In a comparison is drawn between the baadty of cyclohexa-2,5-dienones and that of cyclohexa-2-enones. 

Failure to obey Hammett s postulate may be the result of inadequate treatment of the experimental data (see III-A) to correct for spectral solvent shifts, or it may result from the base in question actually having different activity coefficient behavior from a true Hammett base. Clearly recognized examples of the latter condition... 

Fig. 1. Extrapolation of indicator data for various types of weak bases to standard state of infinite dilution in water to give (supposedly) thermodynamic pKa s. (1) Theoretical plot for ideal Hammett Base with pK =—6.0. Solid lines represent observable region of indicator change and dotted lines represent extrapolation. All ideal Hammett indicators should lie on line 1 of this plot with appropriate change of left-hand scale, to correspond to their pKa s. (2) Theoretical plot of ideal weak base that gives slope of 1.10. Note that it does not extrapolate to standard state in water (pH = 7) and so observed value of pHa does not refer to same standard state as ideal Hammett base. (3) Actual behavior of benzoic acid (half-protonated at Ht, = —7.38). Note that although it follows ideal Hammett base behavior in observable region of indicator change, it departs sharply from it at lower acidities as shown by activity measurements. At present there is no way of telling how many supposedly ideal Hammett bases behave in this way over part of the acidity region and what standard state they finally extrapolate to.

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