Ammonium compounds, eliminations

Diquat and paraquat are quaternary ammonium compounds largely used as contact herbicides and crop desiccants. When systemic absorption occurs, paraquat and diquat are rapidly distributed into the body. Paraquat primarily accumulates in the lungs and kidneys, while the highest diquat concentrations have been found in the gastrointestinal tract, liver, and kidneys (WHO, 1984). Urine is the principal route of excretion for both diquat and paraquat, which are primarily eliminated as unmodified compounds. Occupationally exposed workers can be monitored by measuring paraquat and diquat concentrations in urine samples (Table 6). Blood concentrations are useful to monitor acute poisoning cases. 

Hofmann elimination, quaternary ammonium compounds, 2 737 Hofmann-Gerhardt nomenclature,... 

Curare-like muscle relaxants act by blocking acetylcholine receptor sites, thus eliminating transmission of nerve impulses at the neuromuscular junction. There are two acetylcholine-like groupings in the molecules, and the drugs, therefore, probably span and block several receptor sites. The neurotransmitter acetylcholine is also a quaternary ammonium compound. The natural material present in curare is tubocurarine, a complex alkaloid that is a mono-quaternary salt. Under physiological conditions, the tertiary amine will be almost completely protonated (see Section 4.9), and the compound will similarly possess two positively charged centres. 

The quaternary ammonium derivatives of the belladonna alkaloids, as well as the synthetic quaternary ammonium compounds, are incompletely absorbed from the gastrointestinal tract. Consequently, greater amounts of these compounds are eliminated in the feces following oral administration. The blood-brain barrier prevents quaternary ammonium muscarinic blockers from gaining significant access to the CNS. 

Hoffman eliminations -ofquaternary ammonium compounds [QUATERNARY AMMONIUM COMPOUNDS] (Vol 20)... 

The action of quaternary ammonium compounds on the dyed fiber in many cases produces a change of shade. A further disadvantage is the possible elimination of aliphatic amines, e.g., in an alkaline medium in the presence of anionic substances or at excessive drying temperatures. This can cause an unpleasant (fishy) odor to arise. In such cases, an acid aftertreatment is necessary. 

When 88-1 was treated with phenyl- or methyllithium, no compound of type 90 containing pentacoordinated nitrogen could be obtained. The isolated products, however, could reasonably be attributed to a dehydrobenzene reaction sequence initiated by nucleophilic attack at a hydrogen atom ortho to the ammonium centerI07>. Elimination of triarylamine108> from 91, here in the form of an aryl-carbazole, led to the substituted aryne 92, which added the nucleophiles present to give 93a, 94a and, after hydrolysis, 93b, 94b, respectively. 

The intermediate is a charged quaternary ammonium compound that results from Sn2 substitutions on three CH3I molecules by the amine nitrogen. E2 elimination occurs because the neutral N(CH3)3 molecule is a good leaving group. 

In many cases it has been observed that reductive cleavage of quaternary ammonium compounds at lead is minor and that a Hofmann elimination is dominant this is brought about by an electrogenerated base. 

A common feature of the decompositions of many ammonium compounds is the identification of the first step in reaction as proton transfer. The consequent accumulation of protons with the residual oxy-anions may be followed by the elimination of water, accompanied by condensation (or continued condensation) of the anions, see chromates and phosphates above, ultimately leading to residual oxide formation. Alternatively the acid may be volatilized, many ammonium salts sublime... 

It is truly amazing under what conditions, using completely different methods, amino acids can be linked up with elimination of water. Thus, it has been known for years that micelles (see Sect. 10.2) can catalyse various types of reactions in aqueous media, e.g., hydrolysis or aminolysis. Micelles are aggregates which form in aqueous solution from tensides under certain conditions (temperature, concentration). Cetyltrimethylammonium bromide [Ci6H33N+(CH3)3Br ] micelles (this compound is a quaternary ammonium compound with tenside properties) are able to oligomerise relatively concentrated solutions of phosphoserine (50 mM), aspartic... 

Hofmann degradation usually occurs with -elimination (El reaction). Wittig and Polster85 86 discovered a variant in which quaternary ammonium compounds are converted by strong bases such as phenyl- or butyl-lithium into olefins, by way of ylides, with intramolecular cw-elimination 87... 

The magnitude of the effect for the elimination reactions indicates a high degree of bond formation between the base and the proton in the transition state. Proton transfer is extensive in the ammonium compound and the proton is just more than half transferred to the base for the sulphonium salt. These... 

However, it was suggested that if the residual anions could be completely removed from the organoclays, the primary degradation pathway would switch to an elimination-type mechanism [16]. The Hofmann elimination of ammonium compounds was most probably the source of additional amounts of vinyl-type unsaturation found in melt-processed OMMT-PE relative to both the polymer control and the Na+MMT-PE samples [28]. On the other hand, the presence of alkenes was also explained by three possible routes of decomposition (i) pyrolysis of alkanes derived from the major component of the organic part, i.e., hydrogenated tallow (HT) (ii) pyrolysis of the tallow (unsaturated fatty acids used for the preparation of the quaternary ammonium salt) and (iii) decarboxylation of RCOO and RCO radicals [17, 29]. 

New synthetic routes for the preparation of homologously pure dichloro DPX and tetrachloro DPX have been reported through the 1,6-Hofmaim elimination of chlotinated j -methy1ben2y1trimethy1 ammonium hydroxide. In the case of dichloro DPX, yields of 30% were reported (21). In the presence of ketone compounds, yields were iacreased to 50% (20). 

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