2:1. For instance, ethanol passed over
hot pumice undergoes dehydration
to ethene:
C 2 H 5 OH – H 2 O → CH 2 :CH 2
Substances such as concentrated sulphuric acid, which can remove H 2 O
in this way, are known as dehydrating agents. For example, with sulphuric acid, methanoic acid gives
carbon monoxide:
HCOOH – H 2 O → CO
dehydrogenase Any enzyme that
catalyses the removal of hydrogen
atoms (*dehydrogenation) in biological reactions. Dehydrogenases occur
in many biochemical pathways but
are particularly important in driving
the *electron-transport-chain reactions of cell respiration. They work
in conjunction with the hydrogenaccepting coenzymes *NAD and
*FAD.
dehydrogenation A chemical reaction in which hydrogen is removed
from a compound. Dehydrogenation
of organic compounds converts single 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 hydrogen halide is removed from a molecule with formation of a double
bond. A simple example is the formation of ethene from chloroethane
using alcoholic potassium hydroxide:
CH 3 CH 2 Cl + KOH → CH 2 = CH 2 +
KCl +H 2 O.
deionized water Water from
which ionic salts have been removed
by ion-exchange. It is used for many
purposes as an alternative to distilled
water.
deliquescence The absorption of
water from the atmosphere by a hygroscopic solid to such an extent that
a concentrated solution of the solid
eventually forms.
delocalization The spreading of
valence electrons over two or more
bonds in a chemical compound. In
certain compounds, the valence electrons cannot be regarded as restricted to deÜnite bonds between
the atoms but move over several
atoms in the molecule. Such electrons are said to be delocalized. Delocalization 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 effect accounting for the properties of
benzene and other aromatic compounds. The energy difference between the actual delocalized state
and a localized state is the delocalization energy. Another example is in
the ions of carboxylic acids, containing 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 identical 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 delocalization 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 bonding Chemical bonding
involving delta (δ) orbitals. A δ orbital
is so called because it resembles a
167
delta bonding
d
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hot pumice undergoes dehydration
to ethene:
C 2 H 5 OH – H 2 O → CH 2 :CH 2
Substances such as concentrated sulphuric acid, which can remove H 2 O
in this way, are known as dehydrating agents. For example, with sulphuric acid, methanoic acid gives
carbon monoxide:
HCOOH – H 2 O → CO
dehydrogenase Any enzyme that
catalyses the removal of hydrogen
atoms (*dehydrogenation) in biological reactions. Dehydrogenases occur
in many biochemical pathways but
are particularly important in driving
the *electron-transport-chain reactions of cell respiration. They work
in conjunction with the hydrogenaccepting coenzymes *NAD and
*FAD.
dehydrogenation A chemical reaction in which hydrogen is removed
from a compound. Dehydrogenation
of organic compounds converts single 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 hydrogen halide is removed from a molecule with formation of a double
bond. A simple example is the formation of ethene from chloroethane
using alcoholic potassium hydroxide:
CH 3 CH 2 Cl + KOH → CH 2 = CH 2 +
KCl +H 2 O.
deionized water Water from
which ionic salts have been removed
by ion-exchange. It is used for many
purposes as an alternative to distilled
water.
deliquescence The absorption of
water from the atmosphere by a hygroscopic solid to such an extent that
a concentrated solution of the solid
eventually forms.
delocalization The spreading of
valence electrons over two or more
bonds in a chemical compound. In
certain compounds, the valence electrons cannot be regarded as restricted to deÜnite bonds between
the atoms but move over several
atoms in the molecule. Such electrons are said to be delocalized. Delocalization 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 effect accounting for the properties of
benzene and other aromatic compounds. The energy difference between the actual delocalized state
and a localized state is the delocalization energy. Another example is in
the ions of carboxylic acids, containing 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 identical 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 delocalization 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 bonding Chemical bonding
involving delta (δ) orbitals. A δ orbital
is so called because it resembles a
167
delta bonding
d
www.AzShimi.ir www.AzShimi.com