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2024年5月11日发(作者:)
PYRIDINE AND PYRIDINE DERIVATIVES Vol 20
PYRIDINE AND PYRIDINE DERIVATIVES
Since the early twentieth century, pyridine derivatives have been commercially important, but most prominently so during World War II and thereafter.
Many pyridines of commercial interest find application in market areas where bioactivity is important, as in medicinal drugs and in agricultural products
such as herbicides, insecticides, fungicides, and plant growth regulators. However, pyridines also have significant market applications outside the realm of
bioactive ingredients. For instance, polymers made from pyridine-containing monomers are generally sold on the basis of their unique physical properties
and function, rather than for any bioactivity. Pyridines can be classified as specialty chemicals because of a relatively lower sales volume than commodity
chemicals. They are most often sold in the marketplace as chemical intermediates used to manufacture final consumer products.
Pyridine compounds are defined by the presence of a six-membered heterocyclic ring consisting of five carbon atoms and one nitrogen atom. The
carbon valencies not taken up in forming the ring are satisfied by hydrogen atoms. The arrangement of atoms is similar to benzene except that one of the
carbon−hydrogen ring sets has been replaced by a nitrogen atom. The parent compound is pyridine itself (1). Substituents are indicated either by the
numbering shown, 1 through 6, or by the Greek letters, α, β or γ. The Greek symbols refer to the position of the substituent relative to the ring nitrogen
atom, and are usually used for naming monosubstituted pyridines. The ortho, meta, and para nomenclature commonly used for disubstituted benzenes is
not used in naming pyridine compounds.
Important commercial alkylpyridine compounds are α-picoline (2), βpicoline (3), γ-picoline (4), 2,6-lutidine (5), 3,5-lutidine (6),
5-ethyl-2-methylpyridine (7), and 2,4,6-collidine (8). In general, the alkylpyridines serve as precursors of many other substituted pyridines used in
commerce. These further substituted pyridine compounds derived from alkylpyridines are in turn often used as intermediates in the manufacture of
commercially useful final products.
Kirk-Othmer Encyclopedia of Chemical Technology (4th Edition) 1
PYRIDINE AND PYRIDINE DERIVATIVES Vol 20
As is the case with most specialty organic compounds, pyridine sales are generally not publicized, and industrial processes for their manufacture
are either retained as trade secrets or patented (see P
ATENTS AND TRADE SECRETS
). Up to about 1950, most pyridines were isolated from coal-tar fractions;
however, after 1950 synthetic manufacture began to take an ever-increasing share of products sold. By 1988, over 95% of all pyridines were produced by
synthetic methods.
Pyridine was first synthesized in 1876 (1) from acetylene and hydrogen cyanide. However, α-picoline (2) was the first pyridine compound reported
to be isolated in pure form (2). Interestingly, it was the market need for (2) that motivated the development of synthetic processes for pyridines during
the 1940s, in preference to their isolation from coal-tar sources. The basis for most commercial pyridine syntheses in use can be found in the early work
of Chichibabin (3). There are few selective commercial processes for pyridine (1) and its derivatives, and almost all manufacturing processes produce (1)
along with a series of alkylated pyridines in admixture. The chemistry of pyridines is significantly different from that of benzenoids. Pyridines undergo
some types of reaction that only highly electron-deficient benzenoids undergo, and do not undergo some facile reactions of benzenoids, such as
Friedel-Crafts alkylation and C-acylation, for example.
Physical Properties
The physical properties of pyridines are the consequence of a stable, cyclic, 6- π-electron, π-deficient, aromatic structure containing a ring nitrogen atom.
The ring nitrogen is more electronegative than the ring carbons, making the two-, four-, and six-ring carbons more electropositive than otherwise would
be expected from a knowledge of benzenoid chemistries. The aromatic π-electron system does not require the participation of the lone pair of electrons
on the nitrogen atom; hence the terms weakly basic and π-deficient used to describe pyridine compounds. The ring nitrogen of most pyridines undergoes
reactions typical of weak, tertiary organic amines such as protonation, alkylation (qv), and acylation.
Liquid pyridine and alkylpyridines are considered to be dipolar, aprotic solvents, similar to dimethylformamide or dimethyl sulfoxide. Most
pyridines form a significant azeotrope with water, allowing separation of mixtures of pyridines by steam distillation that could not be separated by simple
distillation alone. The same azeotropic effect with water also allows rapid drying of wet pyridines by distillation of a small forecut of water
azeotrope.
Pyridine.
Many physical properties of pyridine are unlike those of benzene, its homocyclic counterpart. For instance, pyridine has a boiling
point 35.2°C higher than benzene (115.3 vs 80.1°C), and unlike benzene, it is miscible with water in all proportions at ambient temperatures. The much
higher dipole moment of pyridine relative to benzene is responsible, in significant part, for the higher boiling point and water solubility. Benzene and
pyridine are aromatic compounds having resonance energies of similar magnitude, and both are miscible with most other organic solvents. Pyridine is a
weak organic base (
pK
a
=5:22
), being both an electron-pair donor and a proton acceptor, whereas benzene has little tendency to donate electron pairs or
accept protons. Pyridine is less basic than aliphatic, tertiary amines. Table 1 lists some physical properties of pyridine, and Table 2 compares physical
properties of pyridine to some alkyl- and alkenylpyridine bases.
Table 1. Physical Properties of Pyridine
Physical property
b
enthalpy of fusion at
¡41:6
±
C
, kJ/mol
b
enthalpy of vaporization, kJ/mol
at 25°C
115.26°C
Value
8.2785
40.2
35.11
Kirk-Othmer Encyclopedia of Chemical Technology (4th Edition) 2
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