Basic strength aromatic bases

Armarego109 has determined the pAa values of indolizines and aza-indolizines from their ultraviolet spectra. He found that indolizine itself has the same basic strength as a-naphthylamine (pAa 3.9) and that a methyl substituent increases the pAa by two to three units. Mason and Smith,110 studying the fluorescence spectra of various aromatic nuclei including indolizine, found no evidence to support the prediction that acid-base properties of aromatic hydrocarbons in the first electronic excited state should differ from those in the ground state. 

The effects of substituents on the basic nature of imidazole are summarized in Section 4.02.1.3.4. Tables 1 and 2 list the basic pKa values of some representative imidazoles and benzimidazoles. From these it can be seen that methyl and other alkyl substituents exert a weak, base-strengthening inductive effect which is additive. Aromatic substituents decrease basic strength, while groups attached to these aryl rings exert their normal behavior. In fact, Hammett studies have permitted prediction with reasonable accuracy and correlation of p/Ca values for a variety of imidazoles. In a study of meta- and para-substituted 1-phenylimidazoles Pozharskii <70CHE194) has shown not only that 1-phenylimidazole is a... 

A striking example of a correlation between rate and the acid-base properties of the substrate is provided by recent work of Kresge on tritium exchange between hydronium ion and seven hydroxy- and alkoxy-benzenes. The basic strengths of these aromatic species can be determined by spectroscopic measurements in aqueous perchloric acid, and by com-... 

In addition to the determination of total base, it is also possible to titrate mixtures. This can be done in two ways. One is to titrate mixtures based on the type of amines present. For example, one can distinguish between primary, secondary, and tertiary amines. This is done simply. Acetylate the primary and secondary amines in the mixture with acetic anhydride. They are converted to amides which are only weakly basic. Tertiary amines are not affected and titrate very well. A further differentiation can be made, however. The primary amine can be reacted with salicyl aldehyde to form a Schiff base. The secondary and tertiary amines are unaffected as far as basic strength is concerned, so that one can titrate the sum of secondary and tertiary amines. By these two titrations plus a determination of total amine, one can resolve the mixture. This approach works well for aliphatic amines, but not for aromatic amines. 

Another method of analyzing a mixture of bases is to utilize the difference in the basicity of its components. As an example, let us take a mixture of aromatic and aliphatic amines. Since aliphatic amines are more strongly basic, one would expect to get a titration curve with two breaks, one for the aliphatic amine and one for the aromatic amine. However, you must not use glacial acetic acid for this titration because you will get a curve similar to curve B in Fig. 1. In other words, you get one potentiometric end point for the sum of the two. The reason is that glacial acetic acid reacts with aliphatic amines to form the acetate ion, which has about the same basic strength as the aromatic amine. Glacial acetic acid levels these two amines to the same strength. What you have to do is employ a nonaqueous solvent like acetonitrile and titrate with perchloric acid dissolved in dioxane. If you do this. 

Utilization of zeolite acidic strength in Cg-aromatics (xylens) systems is illustrated next. In the presence of strong acids, xylene isomers have varying basicity (Table 6.5), with m-xylene being the most basic and p-xylene the least basic among the Cg-aromatics [27]. Based on the basicity of the xylenes, the acidity of each zeolite can be properly adjusted to selectively adsorb m-xylene or p-xylene. As demonstrated in Figure 6.4, a more acidic zeolite such as NaY will selectively adsorb m-xylene from other Cg-aromatics [28, 29], In contrast. Figure 6.5 shows that a weaker acidic zeolite such as KY will selectively adsorb p-xylene from other Cg-aromatics [30, 31]. In both systems, toluene was used as the desorbent. 

Alkanamines have acid strengths corresponding to Ka values of about 10 33, which means that their conjugate bases are powerfully basic reagents. Therefore they are very effective in causing elimination reactions by the E2 mechanism (Section 8-8) and aromatic substitution by the aryne mechanism (Section 14-6C). The following example illustrates this property in a useful synthesis of a benzenamine from bromobenzene ... 

Substitution by an aromatic group has a marked effect on solution base strength. One might be tempted to attribute the low basicity of aniline, di- and triphenylamines, and phenol compared with reference compounds (Table 3.17) to partial delocalization of the nonbonded electron pair on the nitrogen or oxygen into the n orbital system of the ring. But gas-phase results indicate the basicity... 

Colpa et al. (1963) calculated p/if(S1 )-values for a series of aromatic hydrocarbons, but could not detect fluorescence changes in the regions of acidity indicated by the Forster cycle, although fluorescence spectra attributed to proton complexes of 3,4-benzopyrene and 1,2-benzanthracene were observed in some solutions containing only the neutral molecule in the ground state. Flurry and co-workers (1963, 1966, 1967) have carried out theoretical and Forster cycle calculation on the excited state basicities of poly-methylbenzenes and Kuz min et al. (1967) have also calculated p/ (Sj)- and p/ (Tj)-values for polycyclic aromatic hydrocarbons increases in base strength of from 7 to 30 powers of ten were derived for Sj. 

Base strengths. The pKa values of pyrroles and benzopyrroles are given in Table 2. These basicities are lower than those of enamines in consequence of the loss of aromaticity which accompanies protonation on the ring... 

Compounds which are vanishingly weakly basic in H20, such as nitro-anilines, nitro-aromatic or halo-nitro-aromatic compounds are protonated to a greater or less extent in superacids, allowing them to be used as indicators in Hammett Acidity Function measurements. Cl- which has essentially zero base strength in H20 is protonated by the superacids to HC1, which being monomolecular is expelled as a gas from the highly associated solvents. This provides a commonly-used important route to synthesis of anhydrous fluorides, fluorosulfates and triflates. 

The chemistry of amines is dominated by the lone-pair electrons on nitrogen, which makes amines both basic and nucleophilic. The base strength of arylamines is generally lower than that of aliphatic amines because the nitrogen lone-pair electrons are delocalized by interaction with the aromatic tt system. Electron-withdrawing substituents on the aromatic ring further weaken the basicity of a substituted aniline, while electron-donating substituents increase basicity. 

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