2:1. For instance, ethanol passed over
hot pumice undergoes dehydration
to ethene:
C 2 H 5 OH – H 2 O →CH 2 :CH 2Substances such as concentrated sul-
phuric acid, which can remove H 2 O
in this way, are known as dehydrat-
ing agents. For example, with sul-
phuric acid, methanoic acid gives
carbon monoxide:
HCOOH – H 2 O →COdehydrogenase Any enzyme that
catalyses the removal of hydrogen
atoms (dehydrogenation) in biologi-
cal reactions. Dehydrogenases occur
in many biochemical pathways but
are particularly important in driving
the electron-transport-chain reac-
tions of cell respiration. They work
in conjunction with the hydrogen-
accepting coenzymes NAD and
FAD.
dehydrogenation A chemical re-
action in which hydrogen is removed
from a compound. Dehydrogenation
of organic compounds converts sin-
gle carbon–carbon bonds into double
bonds. It is usually effected by means
of a metal catalyst or – in biological
systems – by *dehydrogenases.
dehydrohalogenation A type of
chemical reaction in which a hydro-
gen halide is removed from a mol-
ecule with formation of a double
bond. A simple example is the forma-
tion of ethene from chloroethane
using alcoholic potassium hydroxide:
CH 3 CH 2 Cl + KOH →CH 2 = CH 2 +
KCl +H 2 O.deionized waterWater from
which ionic salts have been removed
by ion-exchange. It is used for many
purposes as an alternative to distilled
water.
deliquescenceThe absorption of
water from the atmosphere by a hy-
groscopic solid to such an extent that
a concentrated solution of the solid
eventually forms.delocalizationThe spreading of
valence electrons over two or more
bonds in a chemical compound. In
certain compounds, the valence elec-
trons cannot be regarded as re-
stricted to deÜnite bonds between
the atoms but move over several
atoms in the molecule. Such elec-
trons are said to be delocalized. Delo-
calization occurs particularly when
the compound contains alternating
(conjugated) double or triple bonds,
the delocalized electrons being those
in the pi *orbitals. The molecule is
then more stable than it would be if
the electrons were localized, an ef-
fect accounting for the properties of
benzene and other aromatic com-
pounds. The energy difference be-
tween the actual delocalized state
and a localized state is the delocaliza-
tion energy. Another example is in
the ions of carboxylic acids, contain-
ing the carboxylate group –COO–. In
terms of a simple model of chemical
bonding, this group would have the
carbon joined to one oxygen by a
double bond (i.e. C=O) and the other
joined to O–by a single bond (C–O–).
In fact, the two C–O bonds are identi-
cal because the extra electron on the
O–and the electrons in the pi bond
of C=O are delocalized over the three
atoms. Delocalization of electrons is
a feature of metallic bonding. The de-
localization energy of molecules can
be calculated approximately using
the *Hückel approximation, as was
done originally by Hückel. However,
modern computing power enables
delocalization energy to be calculated
using *ab-initio calculations, even for
large molecules. See also localization.delta bondingChemical bonding
involving delta(δ) orbitals. A δorbital
is so called because it resembles a167 delta bonding
d