Pyridinium refers to the
cation
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...
. It is the
conjugate acid
A conjugate acid, within the Brønsted–Lowry acid–base theory, is a chemical compound formed when an acid donates a proton () to a base—in other words, it is a base with a hydrogen ion added to it, as in the reverse reaction it loses a ...
of
pyridine
Pyridine is a basic heterocyclic organic compound with the chemical formula . It is structurally related to benzene, with one methine group replaced by a nitrogen atom. It is a highly flammable, weakly alkaline, water-miscible liquid w ...
. Many related cations are known involving substituted pyridines, e.g. picolines, lutidines, collidines. They are prepared by treating pyridine with acids.
As pyridine is often used as an
organic base
An organic base is an organic compound which acts as a base. Organic bases are usually, but not always, proton acceptors. They usually contain nitrogen atoms, which can easily be protonated. For example, amines or nitrogen-containing heterocyclic ...
in
chemical reaction
A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and breaking ...
s, pyridinium salts are produced in many
acid-base reactions. Its
salts
In chemistry, a salt is a chemical compound consisting of an ionic assembly of positively charged cations and negatively charged anions, which results in a compound with no net electric charge. A common example is table salt, with positively c ...
are often
insoluble
In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution.
The extent of the solubi ...
in the organic solvent, so
precipitation
In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...
of the pyridinium leaving group complex is an indication of the progress of the reaction.
Pyridinium cations are
aromatic
In chemistry, aromaticity is a chemical property of cyclic ( ring-shaped), ''typically'' planar (flat) molecular structures with pi bonds in resonance (those containing delocalized electrons) that gives increased stability compared to satur ...
, as determined through
Hückel's rule
In organic chemistry, Hückel's rule predicts that a planar ring molecule will have aromatic properties if it has 4''n'' + 2 π electrons, where ''n'' is a non-negative integer. The quantum mechanical basis for its formulation was ...
. They are
isoelectronic
Isoelectronicity is a phenomenon observed when two or more molecules have the same structure (positions and connectivities among atoms) and the same electronic configurations, but differ by what specific elements are at certain locations in th ...
with
benzene
Benzene is an organic chemical compound with the molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar ring with one hydrogen atom attached to each. Because it contains only carbon and hydrogen atoms ...
.
''N''-Alkylpyridinium cations
When the acidic proton is replaced by
alkyl
In organic chemistry, an alkyl group is an alkane missing one hydrogen.
The term ''alkyl'' is intentionally unspecific to include many possible substitutions.
An acyclic alkyl has the general formula of . A cycloalkyl is derived from a cycloa ...
, the compounds are called ''N''-alkylpyridinium. A simple representative is
''N''-methylpyridinium (
5H5NCH3sup>+). These pyridinium intermediates have been used as electrophiles in synthetic organic chemistry to build dearomatized congeners called dihydropyridines, as demonstrated in one example from Smith in 2021. Earlier, the same research group also delineated the rules surrounding regioselectivities associated with adding nucleophiles to pyridinium electrophiles with varying substitutents.
From a commercial perspective, an important pyridinium compound is the
herbicide
Herbicides (, ), also commonly known as weedkillers, are substances used to control undesired plants, also known as weeds.EPA. February 201Pesticides Industry. Sales and Usage 2006 and 2007: Market Estimates. Summary in press releasMain page f ...
paraquat
Paraquat ( trivial name; ), or ''N'',''N''′-dimethyl-4,4′-bipyridinium dichloride ( systematic name), also known as methyl viologen, is an organic compound with the chemical formula C6H7N)2l2. It is classified as a viologen, a family of re ...
.
See also
*
Pyridinium chlorochromate
*
Pyridinium chloride
References
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