Decomposition, quaternary ammonium salts

When potassium fluoride is combined with a variety of quaternary ammonium salts its reaction rate is accelerated and the overall yields of a vanety of halogen displacements are improved [57, p 112ff. Variables like catalyst type and moisture content of the alkali metal fluoride need to be optimized. In addition, the maximum yield is a function of two parallel reactions direct fluorination and catalyst decomposition due to its low thermal stability in the presence of fluoride ion [5,8, 59, 60] One example is trimethylsilyl fluoride, which can be prepared from the chloride by using either 18-crown-6 (Procedure 3, p 192) or Aliquot 336 in wet chlorobenzene, as illustrated in equation 35 [61],... 

Normally, persulfate (41) can only be used to initiate polymerization in aqueous or part aqueous (emulsion) media because it has poor solubility in most organic solvents and monomers. However, it has been reported that polymerizations in organic solvent may be initiated by crown ether complexes of potassium persulfate.234 237 Quaternary ammonium persulfates can also serve as useful initiators in organic media. 4 The rates of decomposition of both the crown ether complexes and the quaternary ammonium salts appear dramatically... 

Toxic intermediate products are produced during hydrolysis. Approximate half-life in water at 25°C is 1.3 min. Decomposition comes through slow change into quaternary ammonium salts. Decomposition point is below 94°C. 

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. 

Alternative procedures for the generation of dichlorocarbene and dibromocarbene under phase-transfer catalysed conditions are also available. Where the reactive substrate is labile under basic conditions, the thermal decomposition of solid sodium trichloroacetate or bromoacetate under neutral conditions in an organic solvent is a valuable procedure [10-12], The decarboxylation is aided by the addition of a quaternary ammonium salt, which not only promotes dissolution of the trihaloacetate anion in the organic solvent, but also stabilizes the trihalomethyl anion. Under optimum reaction conditions, only a catalytic amount of the quaternary ammonium salt is required, as a large amount of the catalyst causes the rapid generation of the dichlorocarbene with resultant side reactions. 

Base-catalysed quaternary ammonium salts give alkenes and 3° amines. This reaction is known as Hofmann elimination or Hofmann degradation. Amines can readily be converted to quaternary ammonium salt by the treatment of excess primary alkyl halides, and then Ag20 and H2O. Quaternary ammonium salts undergo E2 elimination, when heated with NaOH to give alkenes and tertiary amines. Thermal decomposition of a quaternary ammonium salt by NaOH to an alkene is known as Hofmann elimination. 

Furthermore, investigations of template decomposition in zeolites upon calcination of the as-synthesized materials have received much attention by researchers (248-250). However, the decomposition mechanisms of quaternary ammonium salts, which are commonly used as templates for zeolite synthesis, are still not well understood. Thus, MAS NMR investigations of the decomposition of quaternary ammonium ions in zeolites bear the potential of significantly improving the understanding of these processes (243,251). For this purpose, a general method was developed for the synthesis of... 

A vigorously stirred suspension of 0.2 to 1 mole of sodium amide in 200 ml of xylene, in which were dissolved 0.1 mole of a,a-diphenyl-7-hexamethyleneimino butyronitrile was boiled for 12 hours. Thereupon the excess of sodium amide was decomposed with water and the xylene layer was separated, washed with water and extracted with hydrochloric acid. This acidic extract was made strongly alkaline with concentrated lye and the separated base was extracted with ether. After drying, the ether was evaporated and the l,l-diphenyl-3-hexamethyleneimino propane distilled in vacuo. The boiling point was 170-174°C/1 mm, the refractive index nD20 = 1.56 36, and the density d420 = 1.009. From the oil obtained several acid additions and quaternary ammonium salts can be obtained by reaction with acids containing a non-toxic anion or esters thereof. The hydrochloric acid salt, for instance, melts at 189-192°C, the methiodide at 174-177°C under decomposition. 

Other special injection devices have been applied in combination with derivatization only rarely, if the reaction is carried out directly in the injection port. The method of thermal degradation of quaternary ammonium salts has already been mentioned. This can be performed in a sealed capillary directly in the injection port of the apparatus. The port should then be modified in order that when the reaction is finished, the capillary can be crushed and the products swept into the column. Another method of modification of the injection port in the thermal decomposition of hydrazones by a-ketoglutaric acid is shown in Fig. 4.3 (p.77). 

The decomposition of quaternary ammonium salts can also be achieved by the action of alkyl halides (Scheme 4.5). 

The evidence suggests that 2-propanamine interacts with the protons associated with the framework Fe atoms to form 2-propyl ammonium cations which maintain 2-propanamine in the zeolite to high temperatures. Above approximately 600K, the decomposition rate for these cations to form propene and ammonia becomes appreciable. The decomposition reaction is very similar to the Hofmann elimination reaction found for quaternary ammonium salts and provides indirect evidence that ammonium ions are involved in the reaction. When Fe is removed from the framework of the molecular sieve, the associated proton site is lost, along with the capability for forming the ammonium ion and carrying out the reaction at that site. 

Thermal decomposition of quaternary ammonium salts and bases is most valuable in structural investigations of amines, particularly heterocyclic secondary amines (Hofmann exhaustive methylation). The course of the elimination (A or B) is determined by the nature of the four alkyl groups on the nitrogen atom. The reaction has found little use in the synthesis of pure olefins. The yields are low even when three of the alkyl groups are methyl radicals. Carbon-skeleton rearrangement does not occur. Thus, the only olefin obtained by pyrolysis of pinacolyltri-methylammonium hydroxide, (CHj),CCH(CHj)N(CHj)j OH, is /-butylethylene (50%). ... 

Cyclopropene (533) has been prepared by Hofmann elimination of the quaternary ammonium salt (532) (equation 139). Decomposition of 532at 90-100°C was... 

Scheme 6.42, diisopropylidene mannitol was treated with A, A -dimethylformamide dimethyl acetal followed by methyl iodide to effect this transformation [74,75]. The reaction proceeds through initial acetal transfer generating the illustrated mannitol acetal. Methyl iodide then alkylates the nitrogen, producing a quaternary ammonium salt. Loss of trimethylamine, followed by ring opening and decomposition, gives the 3,4-olefinic mannitol derivative shown. 

Hofmann rule The principal alkene formed in the decomposition of quaternary ammonium hydroxides that contain different primary alkyl groups is always ethylene, if an ethyl group is present. Originally given in this limited form by A.W. Hofmann, the rule has since been extended and modified as follows When two or more alkenes can be produced in a P-elimination reaction, the alkene having the smallest number of alkyl groups attached to the double bond carbon atoms will be the predominant product. This orientation described by the Hofmann rule is observed in elimination reactions of quaternary ammonium salts and tertiary sulfonium salts, and in certain other cases. 

The reductive cleavage of quaternary ammonium salts to give a tertiary amine and a hydrocarbon by reaction with sodium amalgam in hydroxylic solvents is called the Emde degradation. However, saturated hydrocarbons are not cleaved under these conditions. Grovenstein suggested that the reason for this unreactivity is due to the fact that the sodium reacts with the alcohol much faster than with the ammonium salts. However, by operating in dioxane or dioxane-alcohol mixtures, the dealkylation reaction could be accomplished. The products of the decomposition of either tetramethylammonium chloride or bromide are methane, ethylene, trimethylamine, and dimethylethylamine. 

In the reactions of sodium in liquid ammonia with quaternary ammonium salts with different alkyl groups the methyl group is always cleaved first (except for the 2-butyl group). This behavior is similar to LAH reductions 33> and to thermal decompositions of halide salts 2h... 

Recently Wilson and Joule 158> studied the demethylation of a variety of quaternary ammonium acetates in aprotic solvents. Earlier Lawson and Collie 92> showed that the decomposition of solid tetramethylammonium acetate at 180—200 °C gives the methyl ester in good yield. However, the use of an aprotic solvent significantly lowers the temperature (60—140 °C) required for decomposition. This reaction is most useful for the demethylation of aromatic quaternary ammonium salts that are soluble in benzene or benzene-chloroform mixtures. Its application to aliphatic quaternary ammonium salts requires a longer time, although N,N-dimethylpiperidinium acetate was demethylated at 100 °C in xylene-... 

Another interesting comparison was reported by von Braun 150> in which various quaternary ammonium hydroxides were decomposed under Hofmann conditions and in the presence of glycerol. These results also show that the displacement reaction is favored in the presence of a hydroxylic solvent. It was also noted by Hanhart and Ingold 57> that displacement reactions are favored over elimination reactions in thermal decompositions of quaternary ammonium salts as the nucleophilicity of the anion is increased from chloride to acetate. 

Path 5 has not been confirmed in the ylid, but products which can be attributed to this path are noted in the decomposition of the fluor-enylid and in alkoxy-substituted quaternary ammonium salts. Although ethylene and polymethylene are observed in the decomposition of the ylid, it has been suggested that these products are formed via a stepwise alkylation reaction. However, the ylid may be regarded as a carbenoid, with the N+(CH3)3 group behaving as a pseudohalogen, which reacts either as the free ylid or as the lithium halide complex. 

Zerda, J., R. Neumann, and Y. Sasson, Hofmarm Decomposition of Quaternary Ammonium Salts Under Phase-Transfer Catalytic Conditions, J. Chem. Soc. Perkin Trans., 2, 823 (1986). 

The importance of the anion in determining the decomposition pathway of quaternary ammonium salts was recognized by Ingold in 1927 (7). At 135°-140°C, tetramethylammonium (TTMA) hydroxide decomposes... 

Once the fact that compound A is a tertiary amine is recognized, the reactions that lead to the formation of compound B should be seen as the formation of a quaternary ammonium salt with subsequent decomposition by heat (Hofmann elimination). The formation of quaternary ammonium salts can be generalized as follows ... 

When B is triethyl amine, a quaternary ammonium salt is formed. Since triethyl amine is volatile at the temperatures usually used for polyaddition, the catalyst would be depleted from the reaction mixture as the reaction progresses. In the case of a catalyst such as benzyl triethyl ammonium chloride, which would be expected to undergo thermal decomposition into benzyl chloride and triethyl amine, such a mechanism would predict why not only... 

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