Tetra-alkyl ammonium ions

We can go beyond the case of fast-ion conductors and apply these principles to understand the role of templating species in inorganic and organic synthesis. A number of examples of zeolite frameworks have been shown in Chapter 2 to follow closely IPMS. The crystallisation of zeolites invariably requires the presence of templating species. A wide variety of templates have been used, from sodium ions, tetra-alkyl ammonium ions to crown ethers. 

As has been discussed in Sec. II.B, hydrophobicity of supporting cations is expected to enhance the apparent complexation of weak acidic polyelectrolytes, and it is of interest to study how the metal complexation equilibria are affected by the addition of hydrophobic supporting cations, such as tetra-alkyl ammonium ions. Representative plots obtained by a potentiometric titration study on Ca2+/PAA in the presence of excess TMA+C1 salt [43] are shown in Figure 15. By comparison of the log(Arca)app vs. a plots of the systems of Ca2+/PAA/Na+ with Ca2+/PAA/TMA+, it is apparent that Ca2+ complexation is highly enhanced by the addition of TMA+ at any salt con-... 

Consequently, an ISE for nitrate for example, a strongly hydrophilic ion, must have a strongly hydrophobic ion-exchanger ion. This conclusion has been demonstrated experimentally for a series of NO3 ISEs based on tetra-alkyl-ammonium salts with long alkyl chains [161] (see fig. 7.2). It was found that, in the studied series of substances, the tetradodecylammonium ion which is... 

As ammonium ions give a reduction wave in tetra-alkyl-ammonium hydroxide solutions, a method for the determination of nitrogen was devised. The method consists of mineralization by the conventional Kjeldahl method used in the semimicro or micro modification. Because waves at very negative potentials (e.g. of ammonium ions) are usually less reproducible than waves at more positive potentials, the accuracy of this method is somewhat lower than that for ordinary polarographic analysis. Other types of polarographic determinations of ammonium ions could be applied, e.g. using the anodic waves corresponding to the formation of salts with mercury or the condensation with phthalic dialdehyde (p. 142). 

ILs— tetraethyl and tetrabutyl ammonium nitrate— were used as additives for protein refolding. Pure liquid tetra-alkyl ammonium nitrates were utilized to denature the protein. EAN is a colorless to slightly yellow-colored IL having no characteristic odor and works as an amphoteric solvent. EAN is a liquid electrolyte at room temperamre and involves dissociable protons thus, it is also called as protic IL [79-82], which can be used as medium electrolytes for fuel cells [83] and polymer membrane separators [84]. The properties and apphcations of EAN were recently reviewed in the literature [85], EAN is miscible with water to form mixtures at aity composition, and both the component ions favorably form hydrogen bonds with water [86]. 

Two approaches have been adopted for pC02 sensing, namely (1) wet sensors and (2) plastic (solid-state) sensors. A wet sensor consists of a pH-sensitive probe dissolved in aqueous bicarbonate buffer solution, which is separated from the gaseous or liquid test medium by a gas-permeable membrane [9]. In plastic sensors, a polar pH-probe is immobilised in a thin polymer film. The probe is usually ion-paired with a lipophilic base such as a tetra-alkyl ammonium hydroxide. This ion-pair combination facilitates compatibility between the two components, whilst simultaneously eliminating the need for aqueous buffers due to the associated water of hydration. This makes it possible for these sensors to maintain their sensitivity to CO2, although they may show some humidity dependence. The requisites of a CO2 probe are a suitable a significant change in absorption or emission characteristics upon... 

Which ions are specifically adsorbed. It depends, of course, on the metal, but detailed and accurate data available for mercury show that anions, which are generally not hydrated, tend to be specifically adsorbed. This includes most of the anions, but not F". Also, some highly symmetrical anions such as CIO4, BF7, and PFg are not specifically adsorbed on mercury. In contrast, most cations are not specifically adsorbed on mercury. Cesium, which was found to be specifically adsorbed to some extent, is an exception. Also, large organic cations of the tetra-alkyl ammonium type (for example [(C2Hs)4N] ) are found to be specifically adsorbed. While this information has been most valuable for the understanding of the double-layer structure, it should be borne in mind that mercury is a rather inert, non-catalytic surface. Thus, the structure of the double layer at the surface of catalytic metals could be quite different, and should be considered for each solid metal separately. 

Quaternary ammonium salts are generally stable under neutral or acidic conditions up to 150°C, but decomposition can occur with the quaternary ammonium ion acting as an alkylating agent in its reaction with anions (Scheme 1.1). Soft nucleophiles, such as RS, are more reactive with tetra-n-butylammonium bromide and benzyltriethylammonium chloride, although the latter salt also C-benzylates phenyl-acetonitrile under basic conditions [46], These side reactions are considerably slower than the main catalysed reactions with, for example, a haloalkane and the amount of unwanted impurity in the final alkylated product is never greater than the amount of catalyst used (i.e. generally > 2%). Harder anions, e.g. R2N and RO, rarely react with the ammonium salts. 

Similarly, the reactivity of phenolate ions as the tetra- -butylammonium salt has been shown to be 3 10" times higher than that of the corresponding potassium salt in the Sn2 alkylation reaction with 1-halobutanes, carried out in 1,4-dioxane [340], Whereas the rate of alkylation of the potassium salt increases by a factor of ca. 10 on going from 1,4-dioxane (fir = 2.2) to iV,iV-dimethylformamide (e, = 36.7), the alkylation rate of the quaternary phenolate is essentially insensitive to the same solvent change. Obviously, the phenolate ion combined with the larger ammonium ion is already very reactive because of the relatively weak cation-anion interaction in the ion pair. In such cases, dissociation to a truly free anion does not seem to be required in order to explain the high reactivity [340]. 

Organomontmorillonites are prepared by exchanging tetra alkyl quaternary ammonium ions onto the surface of the clay. The existing counterion of the montmorillonite is a metal ion (sodium, potassium, etc.). The positive charged quat displaces the positively charged metal ion to produce the organoclay and the metal halide salt (sodium chloride, potassium chloride, etc.). The salt is washed away from the organoclay... 

The ammonium catalyst can also influence the reaction path and higher yields of the desired product may result, as the side reactions are eliminated. In some cases, the structure of the quaternary ammonium cation may control the product ratio with potentially tautomeric systems as, for example, with the alkylation of 2-naph-thol under basic conditions. The use of tetramethylammonium bromide leads to predominant C-alkylation at the 1-position, as a result of the strong ion-pair binding of the hard quaternary ammonium cation with the hard oxy anion, whereas with the more bulky tetra-n-butylammonium bromide O-alkylation occurs, as the binding between the cation and the oxygen centre is weaker [11], Similar effects have been observed in the alkylation of methylene ketones [e.g. 12, 13]. The stereochemistry of the Darzen s reaction and of the base-initiated formation of cyclopropanes under two-phase conditions is influenced by the presence or absence of quaternary ammonium salts [e.g. 14], whereas chiral quaternary ammonium salts are capable of influencing the enantioselectivity of several nucleophilic reactions (Chapter 12). 

The interfacial mechanism provides an acceptable explanation for the effect of the more lipophilic quaternary ammonium salts, such as tetra-n-butylammonium salts, Aliquat 336 and Adogen 464, on the majority of base-initiated nucleophilic substitution reactions which require the initial deprotonation of the substrate. Subsequent to the interfacial deprotonation of the methylene system, for example the soft quaternary ammonium cation preferentially forms a stable ion-pair with the soft carbanion, rather than with the hard hydroxide anion (Scheme 1.8). Strong evidence for the competing interface mechanism comes from the observation that, even in the absence of a catalyst, phenylacetonitrile is alkylated under two-phase conditions using concentrated sodium hydroxide [51],... 

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