吡啶及其衍生物(Pyridine_and_Pyridine_Derivates)

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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carbon−hydrogen ring sets has been replaced by a nitrogen atom.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

本文标签： 衍生物吡啶