Titration curve, tetrabutylammonium

The addition of tetrabutylammonium chloride to H solutions of [68] and [69] in deuteriated acetonitrile resulted in remarkable nmr shifts of the respective proton signals of both receptors. Of particular note were the substantial downfield shifts of the amide protons (AS = 1.28 ppm for [68] and 1.52 ppm for [69]) on addition of one equivalent of chloride. These results suggest that a significant —CO—NH-Cr hydrogen-bonding interaction contributes to the overall anion complexation process. Subsequent nmr titration curves suggesting 1 1 stoichiometry with anion complexes of [68] and [69] were found in all cases. Negligible shifts were observed under identi-... 

Figure 11-10 shows a titration curve for a mixture of five acids titrated with 0.2 M tetrabutylammonium hydroxide in methyl isobutyl ketone solvent. This solvent is not protonated to a great extent by any of the acids. We see that perchloric acid is a stronger acid than HCI in this solvent as well. 

Upon addition of tetrabutylammonium fluoride in CD2C12, it was found that the H NMR titration curve produced when following the amide NH resonance chemical shift for 11, shifted downfield until one equivalent of fluoride had been added (Figure 9). Following this an upfield shift in the resonance was observed to reach a plateau after two equivalents and then remaining constant at approximately 9.3 ppm. This behaviour appeared indicative of a initially a binding event followed by a deprotonation process. Support for this hypothesis was provided by treatment of receptor 11 with one equivalent of tetrabutylammonium hydroxide to yield the tetrabutylammounium salt of 11. The chemical shift of the amide NH of the salt was found to be the same chemical shift as that observed when the free receptor was treated with two equivalents of fluoride. 

Figure 5.11. Titration curves in nonaqueous solvents A Titration of 2,3,5-trimethylphenol with tetrabutylammonium hydroxide in toluene. B Titration of Na in NHa with dimethyl sulfide.

Figure 3, Titration curve for tetrabutylammonium hydrogen silicate hydrate in aqueous suspension. Key , SiOSi + H O SiOH + HOSi (g SiO + H SiOH , SiOH + OH SiO + and , SiOSi + OH SiO + H05/.

Anomalous titration curves for PLL in pure water are observed as illustrated by Figure 1 which shows the effect of various counterions on the potentiometric behaviour of PLL [12, 14]. A conformational change between a very compact form (stable at low a values) and an extended coil-like form clearly appears. Stabilization of the compact conformation is found to be more pronounced as the length of apolar chains of tetraalkylammonium counterion chains increases. The transition between the two forms, which is initiated at a critical oc of 0.15-0.20 (in the presence of Na" or K ), is shifted to higher values of a in the presence of tetraalkylammonium counterions attaining an of about 0.7 with bulky (and hydrophobic) N-tetrabutylammonium ions (Bu4N ). 

The preferred titrant used by many people in recent years has been tetrabutylammonium hydroxide, also in benzene—methanol. This titrant has two major advantages over sodium and potassium salts. The first is that the products of titration are always soluble. With sodium and potassium you frequently get very gelatinous precipitates that are very disturbing. The second advantage of this titrant is that you can get excellent potentiometric curves using the glass electrode. With sodium and potassium tit-rants, the titration curves frequently do not tell the whole story. For example, you might get a titration curve with a very small break and yet when you add an indicator, the end point is extremely sharp. With tetrabutylammonium hydroxide you do not have this difficulty. 

It remains to be mentioned that contrary to what we believed together with others, the solutions of quaternary alkoxides appear to be quite stable at room temperature when well protected from carbon dioxide and humidity. We kept solutions of tetrabutylammonium ethoxide or iso-propoxide in ethanol—benzene 1 10 in automatic burets at 25 to 30°C during a rather humid summer season and after two months they were practically unchanged, giving the same titration curves as before. 

Fig. 3. Titration curves of 0.05 N phenol with 0.3 N tetrabutylammonium ethoxide in different solvents.

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