degenerate chemical reaction SeeIDENTITY REAC-
TION.
degenerate rearrangement A MOLECULAR REAR-
RANGEMENTin which the principal product is indistin-
guishable (in the absence of isotopic labeling) from the
principal reactant. The term includes both “degenerate
INTRAMOLECULARrearrangements” and reactions that
involve INTERMOLECULARtransfer of atoms or groups
(“degenerate intermolecular rearrangements”): both are
degenerate ISOMERIZATIONs. The occurrence of degen-
erate rearrangements may be detectable by isotopic
labeling or by dynamic NUCLEAR MAGNETIC RESO-
NANCE(NMR) techniques. For example, consider the
[3,3]SIGMATROPIC REARRANGEMENTof hexa-1,5-diene
(Cope rearrangement),
Synonymous but less preferable terms are automer-
ization, permutational isomerism, isodynamic transfor-
mation, topomerization.
See alsoFLUXIONAL; MOLECULAR REARRANGEMENT;
VALENCE ISOMER.
dehydration reaction(condensation reaction) A
chemical reaction in which two organic molecules
become linked to each other via COVALENT BONDs with
the removal of a molecule of water; common in synthe-
sis reactions of organic chemicals.
dehydrogenase An OXIDOREDUCTASE that catalyzes
the removal of hydrogen atoms from a SUBSTRATE.
delocalization A quantum mechanical concept usu-
ally applied in organic chemistry to describe the pi
bonding in a CONJUGATED SYSTEM. This bonding is not
localized between two atoms: instead, each link has a
“fractional double-bond character” or BOND ORDER.
There is a corresponding “delocalization energy,”
identifiable with the stabilization of the system com-
pared with a hypothetical alternative in which formal
(localized) single and double BONDs are present. Some
degree of delocalization is always present and can be
estimated by quantum mechanical calculations. The
effects are particularly evident in AROMATICsystems
and in symmetrical MOLECULAR ENTITIES in which a
lone pair of electrons or a vacant p-orbital is conju-
gated with a double bond (e.g., carboxylate ions, nitro
compounds, enamines, the allyl cation). Delocalization
in such species can be represented by partial bonds or
as RESONANCE (here symbolized by a two-headed
arrow) between CONTRIBUTING STRUCTUREs.
The examples on the next page also illustrate the
concomitant delocalization of charge in ionic conju-
gated systems. Analogously, delocalization of the spin
of an unpaired electron occurs in conjugated RADICALs.
See alsoMESOMERISM.demodulation The process of retrieving information
(data) from a modulated carrier wave, the reverse of
modulation.denaturation In DNA denaturation, two strands of
DNA are separated as a result of the disruption of the
hydrogen bonds following exposure to extreme condi-
tions such as high temperature, chemical treatment, pH
changes, salt concentration, and others. Denaturation
in proteins by heat, acids, bases, or other means results
in the change in the three-dimensional structure of the
protein so that it cannot perform its function and
becomes biologically inactive.denatured A commercial term used to describe
ethanol that has been rendered unfit for human con-
sumption by the addition of harmful ingredients.denitrification The reduction of nitrates to nitrites,
including nitrogen monoxide (nitric oxide), dinitrogen
oxide (nitrous oxide), and ultimately dinitrogen cat-
alyzed by microorganisms, e.g., facultative AEROBICsoil
bacteria under ANAEROBICconditions.72 degenerate chemical reaction