Trifluoroacetic acid, basicity

For solutes that are strong acids and bases a third component is often required in the mobile phase. In general, acidic solutes require an acidic additive (like trifluoroacetic acid). Basic solutes require a basic additive (like dimethylethylamine). These additives are mixed with the modifier at 0.1-0.5%. Polar solutes are separated on polar stationary phases (silica, amino, diol, cyano, ethyl pyridine, etc.)... 

Chemically, 2,2,2-trifluoroethanol behaves as a typical alcohol. It can be converted to trifluoroacetaldehyde [75-90-1] or trifluoroacetic acid [76-05-1] by various oxidi2iag agents such as aqueous chlorine solutions (51) or oxygen ia the preseace of a vanadium pentoxide catalyst (52). Under basic conditions, it adds to tetrafluoroethylene and acetylene to give, respectively, 1,1,2,2-tetrafluoroethyl 2/2/2 -trifluoroethyl ether [406-78-0] (53) and... 

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 basic polymer appears to be a hydroxylated polyether to which octadecyl chains have been bonded and so it behaves as a reverse phase exhibiting dispersive interactions with the solutes. An example of the separation of a series of peptides is shown in figure 15. The column was 3.5 cm long, 4.6 mm i.d. The solutes shown were (1) oc-endorphin, (2) bombesin, (3) y-endorphin, (4) angiotensin, (5) somatostatin and (6) calcitonon. The separation was carried out with a 10 min linear program from water containing 0.2% trifluoroacetic acid to 80% acetonitrile. 

If the analyte contains either an acidic or a basic functionality, adjusting the pH of the extraction solvent to make the analyte either ionic or nonionic may be advantageous. To make an analyte that contains an acidic or basic functionality nonionic for extraction into a nonpolar solvent, a small amount (5% or less) of an organic acid (such as acetic acid or trifluoroacetic acid) or organic base (triethylamine) along with methanol (about 10%) can be added to diethyl ether or ethyl acetate. Conversely, buffered solutions can be used to control the pH precisely in such a way as to control the charge on an analyte and thus improve its extraction efficiency into polar solvents. 

Solubility - The oxidized polymer (VIII) has a greater solubility than the original polymer (VII). It was found to be soluble in acetone, chloroform, benzene, DMF and DMSO. Unlike the polymer (VII), (VIII) was not soluble in formic acid or trifluoroacetic acid that was expected since the pyrrole moiety is less basic than pyrrolidine. In the oxidized polymer, the pair of unshared electrons on the nitrogen atom are contributing to the pyrrole ring aromaticity, therefore, unavailable for protonation as in the case of polymer (VII). A comparison of the solubilities is given in Table I. 

Similar treatment of a trifluoroacetic acid solution of p-tolualdehyde with triethylsilane gives only a 20% yield of /7-xylene after 11 hours reaction time followed by basic workup. Use of 2.5 equivalents of dimethylphenylsilane enhances the yield to 52% after only 15 minutes. This reaction proceeds stepwise through the formation of a mixture of the trifluoroacetate and the symmetrical ether. These intermediates slowly form the desired /7-xylene product along with Friedel-Crafts side products under the reaction conditions (Eq. 192).73 Addition of co-solvents such as carbon tetrachloride or nitromethane helps reduce the amount of the Friedel-Crafts side products.73... 

In order to produce the ions necessary for analysis, the pH of the mobile phase often has to be modified. Volatile organic acids (formic acid, acetic acid, and trifluoroacetic acid) and volatile bases (ammonium hydroxide) are used to provide this modification. With the analysis of basic compounds, a lower pH mobile... 

FIGURE 5 Differences in selectivity obtained on Chiralcel OD-H, OJ and Chiralpak AD columns under NPLC conditions. Chromatographic conditions hexane/ethanol 90/10 (v/v) with 0.1% (v/v) diethylamine or trifluoroacetic acid for basic or acidic compounds, respectively. Flow rate I mL/min. 

Purine [93] is analogously protonated on N-1 (Cobum et al., 1965). With a pT a Value of 2 30, it has an enhanced basicity compared with that of pyrimidine (pA a " T23). In the nmr spectrum in trifluoroacetic acid, the resonance of the captured proton is not observable owing to exchange, but it can be observed in... 

Pyrrolo[l,2-a]benzimidazoles [181] (R = H, Me, Ph R = Me, CH2Ph), unsubstituted at the 1- and 3-positions, protonate in trifluoroacetic acid exclusively on C-1 (Alekseeva et al., 1972b). A methyl substituent in the 1-position leads to mixtures of C-1 and C-3 protonated forms, the relative amounts depending on the presence and nature of substituents at C-3. Without a C-3 substituent, the extent of protonation at the position is 81%, but decreases to 18% in the 3-methyl- and 3-phenyl derivatives the basicity of the derivatives increases simultaneously. 

Gutmann acceptor numbers were determined in the "usual" way via the chemical shift variation of triphenylphosphine oxide by Osteryoung et al. [26]. While, again, the donor numbers were concentration- and composition-independent for basic melts, the acidic melts showed a strong composition dependence. Nonetheless, the acidity range was comparably small and was found around 100 (which compares to the acidity of trifluoroacetic acid). The donor number for basic melts was found to be 98, which was, of course. 

In contrast to X -phosphorins, X -phosphorins can be protonated. The basicity is very much influenced by the nature of the substituents R and R at the phosphorus. 1.1-Dialkyl or 1.1-diaryl-X -phosphorins are even protonated by aqueous HCl the salts are deprotonated by aqueous NaOH. Strong acids in organic solvents, e. g. trifluoroacetic acid in hexane or benzene, (see p. 106), are required to proto-nate 1.1-dialkoxy-X -phosphorins. Addition of tert-butoxide deprotonates the salt. By studying the NMR spectra of 1. l-dimethoxy-2.4.6-tris-pentadeuterophenyl-X -phosphorin 185 in benzene solutions containing H and D-trifluoroacetic acid Stade could show that two different protonation products are formed in a ratio of 3 1. One product is the result of C—2 protonation 186 the other of C-4 protonation 187 (Fig. 38). Similar results were observed in the case of 1.1-bis-dimethylamino-2.4.6-triphenyl-X -phosphorin... 

Compound 59 is weakly basic, insoluble in dilute mineral acids but soluble in concentrated sulfuric acid, from which it is precipitated on addition of water.1156 NMR has shown 332b to undergo protonation at C-7 in trifluoroacetic acid, even though this destroys the aromaticity, rather than N-protonation.392 Perchlorate salts of 332a were isolated but their NMR spectra showed that C-7 protonation had not occurred.3346 N-Protonation may be involved in this case. 

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