Page 165 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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

Heterocyclic Chemistry

Charles M. Marson
University College London

I. General Aspects of Heterocyclic Systems
II. Physical Properties
III. Chemical Properties
IV. Synthesis of Heterocycles
V. Major Classes of Heterocyclic Compounds

GLOSSARY Enantioselective A reaction that affords predominantly
one enantiomer. Enantiomers of a compound are
Alkaloid Cyclic organic compound distributed in living related to each other as nonsuperimposable mirror
organisms and containing nitrogen. Many alkaloids are images.
bases of significant pharmacological activity. Pi-excessive Heterocycle that contains a greater density
Annelation Attachment, by synthesis, of a fragment to a of electronic charge than does a carbon atom of ben-
structure of one or more rings to produce a bi- or poly- zene. The converse holds for a pi-deficient heterocycle.
cyclic molecule. It can also refer to electronic changes Mesoionic Five-membered heteroaromatic mesomeric
that may occur when a structure is annelated. betaine that cannot be satisfactorily represented by one
Betaine Zwitterion of net neutral charge containing non- polar structure and that possesses a sextet of electrons
adjacent anionic and cationic sites. A hydrogen atom associated with the heterocyclic ring.
is not bonded to the cationic site. Nucleophile Electron-pair donor, such as RO or R 3 N.
−
Cyclodehydration Ring-forming reaction that proceeds Protonation Formal addition of a proton (H ).
+
with loss of water. Quaternization Alkylation of a tertiary amine to give a
Degradation Procedure by which a molecule is cleaved quaternary ammonium salt.
to fragments, which are often identified, thereby assist- Resonance structures Two or more structures of the
ing structural elucidation of the parent molecule. same compound that differ only in the pairing arrange-
Delocalizationenergy Calculatedadditionalbondingen- ment of the electrons. These structures have no differ-
ergy that results when the assumption that electrons ing chemical or physical properties; their identity is
are constrained to isolated double bonds is removed. depicted by a double-headed arrow (↔) inserted be-
Corresponds to the resonance energy in valence bond tween them. Resonance structures are nearly synony-
theory. mous with canonical forms.
Electrophile Electron-pair acceptor, such as H , AlCl 3 , Tautomerism Structural isomers of different energies
+
+
or Br . that are interconvertible by surmounting a low-energy

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