Quaternary Ammonium compounds Hofmann elimination

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

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. 

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 Hofmann fission of quaternary ammonium salts displays some unusual features in the steroid field. The elimination is normally of E2 character, and in acyclic structures gives olefinic products according to the familiar Hofmann rule [ 14]. The behaviour of 3-trimethylammonium steroidal salts is unexceptional [32, 3 ] the axial 3u compounds in the 5a-series giving A -olefins (5), or in the case of compounds already unsaturated at C-C 6) (6), the A -dienes (7). The 3j8-trimethylammonium-5a compounds are stable to bases, or... 

A Pyrrolidine is a secondary amine and reacts with two moles of methyl iodide to form a quaternary ammonium salt, compound A. The analytical data for compound B shows that it has one double bond equivalent which is present as an alkene. Consequently, compound B has been formed from compound A by a Hofmann elimination ... 

In E2 elimination with bases like KOH and CH30Na, most alkyl halides give Saytzeflf orientation. Certain other compounds (quaternary ammonium salts, Sec. 23.5, for example) give Hofmann orientation. Alkyl sulfonates fall in between. With each kind of compound, orientation is affected—sometimes drastically—by the choice of base and solvent, and by stereochemistry. (The percentage of 1-hexene from 2-hexyl chloride, for example, jumps from 33% in CH ONa/ CH3OH to 91% in /-BuOK//-BuOH, evidently for steric reasons.) In all this, we should remember that orientation is a matter of relative stabilities of competing transition states these stabilities are determined by electronic factors—alkene character and carbanion character—with superimposed conformational factors. 

The stereochemistry of Hofmann elimination is commonly anti but less so than was formerly believed. Syn elimination is important for certain cyclic compounds, and can be made important even for open-chain compounds by the proper choice of base and solvent. Quaternary ammonium ions are more prone to syn elimination than alkyl halides and sulfonates. Electronically, anti formation of the double bond is favored in eliminations but when the alkene character of the transition state is slight—as here—other factors come into play conformational factors, it has been postulated. 

Hofmann degradation of 5of-cholestan-4a- and -4/3-yI trimethylammonium salts closely resembled that of the C-6 quaternary ammonium salts the 4j -compound producing solely the 4-ene by Saytzeff elimination whilst the 4a-epimer gives largely demethylated 4a-dimethylamino-steroid, with some 3-ene. ... 

For a quaternary ammonium ion to undergo an elimination reaction, the counterion must be hydroxide ion because a strong base is needed to start the reaction by removing a proton from a j8-carbon. Since halide ions are weak bases, quaternary ammonium halides cannot undergo a Hofmann elimination reaction. However, a quaternary ammonium halide can be converted into a quaternary ammonium hydroxide by treating it with silver oxide and water. The silver halide precipitates, and the halide ion is replaced by hydroxide ion. The compound can now undergo an elimination reaction. 

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 ... 

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]. 

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