Pyridinium salts comes under the category of cationic surfactants. These are unsaturated heterocyclic compounds having different functional groups present either on Pyridine ring or at nitrogen atom. Initially, they were used as effective germicides. Several researches have been carried out on these salts, making them important for various applications. Pyridinium salts are easily accessed and versatile precursors to nitrogen containing eterocycles. Irradiation of pyridinium salts with UV in presence of base gives unique bicyclic aziridine compounds.
The base of Pyridinium salts which is Pyridine is a basic heterocyclic organic compound with the chemical formula C5H5N. It is structurally related to benzene, with one methine group (=CH-) replaced by a nitrogen atom. The pyridine ring occurs in many important compounds, including azines and the vitamins niacin and pyridoxal. The chemical structure of pyridine ring is as follows:-
Pyridine is harmful if inhaled, swallowed or absorbed through the skin. Effects of acute pyridine intoxication include dizziness, headache, lack of coordination, nausea, salivation, and loss of appetite. They may progress into abdominal pain, pulmonary congestion and unconsciousness.
Pyridine might also have minor neurotoxic, genotoxic, and clastogenic effects. Exposure to pyridine would normally lead to its inhalation and absorption in the lungs and gastrointestinal tract, where it either remains unchanged or is metabolized. Minor amounts of pyridine are released into environment from some industrial processes such as steel manufacture, processing of oil shale, coal gasification, coking plants and incinerators.
Pyridinium Salts such as Butyl Pyridinium Chloride,Cetyl Pyridinium Chloride and Lauryl Pyridinium Chloride are formed by effecting a direct reaction between the corresponding alkyl chloride and pyridine.
Commonly Encountered Pyridine Derivatives
1. Methylpyridines (Picolines)
These have the trivial generic name picolines; on oxidation they give the appropriate acids; pyridine-2- and -4-carboxylic acids are called 2- and 4-picolinic acids, respectively . Pyridine-3-carboxylic acid is better known as nicotinic acid.
2. Pyridones (Hydroxypyridines)
2-Hydroxy- and 4-hydroxypyridines are in tautomeric equilibrium with isomers bearing a carbonyl group (Scheme 2.22). These are called 2- and
4-pyridones, respectively. The pyridone forms are favoured in ionic solvents and also in the solid state.
3. Aminopyridines
All three aminopyridines are known, but although the 2-and 4-aminopyridines are potentially tautomeric with imino forms, they seem to exist as the amino tautomers.
4. Pyridinium Salts
Nucleophilic addition readily takes place with pyridinium salts; attack is normally easier at the C-2(6) position, since the inductive effect of the positively charged. nitrogen atom is greatest here. When the sites adjacent to the nitrogen are blocked, however, attack occurs at C-4. The products are dihydropyridines.
The Applications of Pyridinium Salts:-
- They are frequently used in analytical and physical chemistry.
- Some of the pyridinium salts are used as disinfectants in various medical applications (e.g., eye drops, solutions, foams).
- Some pyridinium salts (C 12 and C 16 ) were also used as solubilizers for water insoluble compounds in Analytical Chemistry.
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