Page 166 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

P1: GOL Final Pages
Encyclopedia of Physical Science and Technology EN007F-314 July 6, 2001 16:59

322 Heterocyclic Chemistry

barrier. Isomerization involves the migration of a group A convenient method of depicting organic structures
or atom (especially H, referred to as proton tau- uses polygonal forms in which the carbon atoms reside at
tomerism). apices and are bonded to the appropriate number of hy-
Ylid 1,2-Dipolar species, of net neutral charge, compris- drogen atoms if no other symbols (e.g., O for oxygen) are
ing a negatively charged carbon atom bonded to a pos- present. Replacement of a CH 2 group in cyclohexane by
itively charged heteroatom.
Zwitterion Species of net neutral charge that contains H 2
C
separate cationic and anionic sites. H 2 C CH 2
H 2 C CH 2
N N
H H
piperidine
HETEROCYCLIC COMPOUNDS possess a cyclic
structure with at least two different kinds of atoms in NH (nitrogen is usually trivalent) gives a heterocyclic
the ring. By far the most common type, and that exclu- compound called piperidine. It is often useful to compare
sively considered here, contains carbon together with one a heterocycle to its carbocyclic analog; conversely, any
or more heteroatoms. The most common heteroatoms are number of heterocycles can be drawn by replacing the
nitrogen, oxygen, and sulfur, but many other elements can carbon atoms in carbocyclic compounds.
act as heteroatoms, such as phosphorus, tin, and silicon.
Heterocyclic compounds are extensively distributed in
benzene
nature; many play crucial roles in the biochemistry of liv-
ing organisms. For example, the genetic material deoxyri-
bonucleic acid (DNA) contains a sequence of nitrogen
heterocycles held together by hydrogen bonds across the pyridine
heterocyclic rings. Many sugars exist in the form offive- or N N N
six-membered oxygen heterocycles. Most members of the
vitamin B group are nitrogen heterocycles. Many naturally Removal of hydrogen (H 2 ) from a CH CH bond results
occurring pigments, antibiotics, and alkaloids are hetero- in the formation of a C C bond, an unsaturated linkage
cyclic compounds. Owing to great advances in organic referred to as olefinic. An aromatic structure contains al-
ternating single and double bonds in a ring, such as ben-
synthesis, many dyes, plastics, pharmaceuticals, pesti-
zene. Formal replacement of a CH in benzene by N gives
cides, and herbicides contain heterocycles synthesized in
the heterocycle known as pyridine. In general, the proper-
the laboratory and not necessarily found in nature.
ties of aromatic and heteroaromatic compounds are quite
different from those of their saturated (or partially satu-
rated) counterparts. That is because a special stability is
I. GENERAL ASPECTS OF associated with those pairs of electrons (the so-called pi
HETEROCYCLIC SYSTEMS electrons) in a ring. Consequently, aromatic compounds
are usually very stable and do not suffer addition of other
A. Comparison with Carbocyclic Compounds atoms across the ring, unlike most other unsaturated com-
pounds. A circle placed within a ring is used to denote a
Molecules of organic chemical compounds are built up cyclic and delocalized array of six electrons in pi orbitals.
from a framework of carbon (C) atoms and associated Saturated or partly unsaturated heterocycles usually re-
hydrogen (H) atoms, to which oxygen (O) or other het- semble the acyclic (noncyclic) analogs closely in physi-
eroatoms may or may not be attached. Because each car- cal and chemical properties. Thus, piperidine has much
bonatomnormallyhasfourbondstootheratoms,carbonis in common with the acyclic amine diethylamine. Sim-
said to be tetravalent. Hydrocarbons, which consist solely ilarly, ethyl vinyl ether has much in common with
of carbon and hydrogen atoms, may be linear, branched, 2,3-dihydrofuran.
or cyclic. In the last case, such compounds are referred to
as carbocyclic.
Cyclohexane (C 6 H 12 ) is a typical carbocycle. N
H
diethylamine
H 2
C
H 2 C CH 2 4 3
H 2 C CH 2 5 O 2 O
C 1
H 2
cyclohexane 2,3-dihydrofuran ethyl vinyl ether