Phase ammonium halides

In some cases, the Q ions have such a low solubility in water that virtually all remain in the organic phase. ° In such cases, the exchange of ions (equilibrium 3) takes place across the interface. Still another mechanism the interfacial mechanism) can operate where OH extracts a proton from an organic substrate. In this mechanism, the OH ions remain in the aqueous phase and the substrate in the organic phase the deprotonation takes place at the interface. Thermal stability of the quaternary ammonium salt is a problem, limiting the use of some catalysts. The trialkylacyl ammonium halide 95 is thermally stable, however, even at high reaction temperatures." The use of molten quaternary ammonium salts as ionic reaction media for substitution reactions has also been reported. " " ... 

The phase-transfer catalyst (Q+X-) is usually a quaternary ammonium halide (R4N+X ) such as tetrabutylammonium halide (CH3CH2CH2CH2)4N+X. ... 

Because of their high lipophobic character, compared with other ammonium salts, quaternary ammonium hydroxides are not readily prepared by liquidrliquid anion exchange. Only with quaternary ammonium hydrogen sulphates is it possible to transfer the ammonium hydroxide into the organic phase in any viable degree of concentration [30] and this procedure remains the cheapest and simplest procedure. Other methods include treatment of quaternary ammonium halides with silver oxide [31] and by anion exchange using polymer bound hydroxide [e.g. 32]. 

Lower molecular-weight quaternary ammonium halides, which partition across the two-phase system, transfer anions in measurable concentrations from the aqueous to the organic phase but, in contrast, many of the higher-molecular-weight quaternary ammonium halides with more than ca. 30 carbon atoms are virtually insoluble in aqueous media and their partition coefficients between aqueous and organic phases preclude the transfer of anions efficiently across the interface by the extraction process and yet catalysts, such as Aliquat 336 and Adogen 464, are extremely effective catalysts. 

The ability of quaternary ammonium halides to form weakly H-bonded complex ion-pairs with acids is well established, as illustrated by the stability of quaternary ammonium hydrogen difluoride and dihydrogen trifluorides [e.g. 60] and the extractability of halogen acids [61]. It has also been shown that weaker acids, such as hypochlorous acid, carboxylic acids, phenols, alcohols and hydrogen peroxide [61-64] also form complex ion-pairs. Such ion-pairs can often be beneficial in phase-transfer reactions, but the lipophilic nature of H-bonded complex ion-pairs with oxy acids, e.g. [Q+X HOAr] or [Q+X HO.CO.R], inhibits O-alkylation reactions necessitating the maintenance of the aqueous phase at pH > 7.0 with sodium or potassium carbonate to ensure effective formation of ethers or esterification [49,64]. 

It is noteworthy that benzyltriethylammonium chloride is a slightly better catalyst than the more lipophilic Aliquat or tetra-n-butylammonium salts (Table 5.2). These observations obviously point to a mechanism in which deprotonation of the amine is not a key catalysed step. As an extension of the known ability of quaternary ammonium halides to form complex ion-pairs with halogen acids in dichloromethane [8], it has been proposed that a hydrogen-bonded ion-pair is formed between the catalyst and the amine of the type [Q+X—H-NRAr] [5]. Subsequent alkylation of this ion-pair, followed by release of the cationic alkylated species, ArRR NH4, from the ion-pair and its deprotonation at the phase boundary is compatible with all of the observed facts. 

A compound whose addition to a two-phase organic water system helps to transfer a water soluble ionic reactant across the interface to the organic phase where a homogeneous reaction can take place is called a phase transfer catalyst. These catalysts enhance the rate of a reaction. A quaternary ammonium halide R4N+ X- e.g., tetrabutylammonium halide is phase transfer catalyst. It can cause the transfer of the... 

The use of mixtures of sodium hydroxide and benzyltrimethylammonium chloride or tetrabutylammonium bromide failed to enhance the DPGE alkylation of HEC by the in situ formation of the corresponding quaternary ammonium hydroxide phase transfer catalyst. These quaternary ammonium halides are too soluble in aqueous /-butyl alcohol and are preferentially extracted into the organic phase. Mixtures of benzyltrimethylammonium hydroxide and sodium acetate were also ineffective in enhancing the DPGE alkylation of HEC for the same reason, namely preferential solubility of benzyltrimethylammonium acetate in the organic phase. 

Substituted NH ions Whalley, 28> discussed the spectroscopic effects of orientional disorder about one axis (in contrast to the disorder about three axes as described by Whalley and Bertie 03) and Bertie and Whalley 129> in the a-phases of the methylammonium halides. In principle, all vibrations of an orientational disordered crystal are spectroscopically active, but if the disorder is only about one axis, some restrictions operate, the symmetric bands are sharp in the one-dimensional disordered case, but the bands due to asymmetric vibrations (E) are broad. Whalley use the infra-red results of Sandorfy et al. 130>131> 0f the CH3 -ammonium halides to illustrate the effect which is predicted from interionic coupling of the E-modes. No such effect is visible in the spectrum of the methoxyammonium ion CH3ONH3 reported by Nelson, 32>. 

Alkylpyridinium or ammonium halides constitute a group of cationic detergents with well known surface properties. The kinetics of emulsion breaking or in general coagulation, comprises flocculation and coalscence. It has been reported (1) that cations and anions at low concentrations retard the coalescence and association of dispersed phase of emulsions. 

When the pseudo-spherical ammonium ion is mostly replaced by a truly spherical ion the complex sequence of phase changes found in the pure ammonium halides is suppressed. The mixed potassium ammonium halide salts retain their NaCl cubic structure down to the lowest temperatures. The alkali metal ions support the structure leaving the ammonium ions as free to rotate at 1 K as at 300K [13]. The INS spectrum of this system is quite different from the pure salt and there are no sharp features in any region of the spectrum. We shall analyse the impact that this freedom has on the internal modes about 1400 cm. ... 

C.H. Perry R.P. Lowndes (1969). J. Chem. Phys, 51, 3648-3654. Optical phonons and phase transitions in the ammonium halides. 

Phase-transfer agents are the most popular catalysts for the Halex reaction with alkaline fluorides. All types of transfer agents have been claimed tetraalkyl-ammonium halides (refs. 34, 48), Aliquat 336 (ref. 49), branched pyridinium halides (eventually supported on a polymer) (refs. 50 to 53), tetraalkylphosphonium chlorides (refs. 42, 54 - 57) or bromides (ref. 12), crown-ethers (refs. 58, 59) eventually associated with Ph4PBr (refs. 43, 60), tris-(dioxa-3,6-heptyl)amine (TDA-1) (ref. 61) or polyethyleneglycols (PEG) (ref. 62). 

In the two-phase alkylation using alkali hydroxides as bases and quaternary ammonium halides as catalysts (20) a reverse order (RCl > RBr > RI) of the customary alkylator reactivity prevails. Quaternary ammonium iodides exhibit little catalytic effect (21). Because the effective concentration of R4N Nu is low and limited by the initial amount of catalyst, the nucleophile cannot be expected to compete successfully for the alkylating agent with I , if its softness lies in between OH and I . 

Halide salts of quaternary ammonium and phosphonium cations, and those derived from heterocyclic bases (both aromatic and alicyclic), have long been known (Table 11.14). Most are relatively high melting (and would not be considered ionic liquids), though the structural characteristics of quaternary ammonium halides have been studied by those with interests in plastic crystals and so-called rotator phases [7]. Many deconpose at or near their melting temperatures, a characteristic that would limit interest in them as reaction media [667]. Indeed, it is probably as a result of the view, widely held in the past [668], that such materials may be intrinsically unstable that the onset of the recent massive interest in ionic liquids was delayed. 

---