where R is a hydrocarbon group or
hydrogen. Enamines can be produced
by condensation of an aldehyde or
ketone with a secondary amine.
enantiomeric pair A pair of molecules consisting of one chiral molecule and the mirror image of this
molecule. The molecules making up
an enantiomeric pair rotate the plane
of polarized light in equal, but opposite, directions.
enantiomers See optical activity.
enantiomorph See optical activity.
enantiomorphism See optical activity.
enantiotopic Denoting a ligand a,
attached to a *prochiral centre, in
which the replacement of this ligand
with a ligand d (which is different
from the ligands a, b, and c attached
to the prochiral centre) gives rise to a
pair of enantiomers C abcd .
enantiotropy See allotropy.
endo- PreÜx used to designate a
bridged ring molecule with a substituent on the ring that is on the
same side as the bridge. If the substituent is on the opposite side the
compound is designated exo-.
endocannabinoids See cannabinoids.
ENDOR Electron-nuclear double resonance. A magnetic resonance technique involving exitation of both
electron spins and nuclear spins. Two
sources of radiation are used. One is
a Üxed source at microwave frequency, which partially saturates the
electron spins. The other is a variable
radiofrequency source, which excites
the atomic nuclear spins. Excitation
of the nuclear spins affects the electron spins by hyperÜre coupling, increasing the relaxation time of the
excited electron spins and increasing
the signal strength. The technique,
which is usually done at low temperatures, is used to investigate paramagnetic molecules. See also
electron paramagnetic resonance.
endothermic Denoting a chemical
reaction that takes heat from its surroundings. Compare exothermic.
end point The point in a titration
at which reaction is complete as
shown by the *indicator.
energy A measure of a system’s
ability to do work. Like work itself, it
is measured in joules. Energy is conveniently classiÜed into two forms:
potential energy is the energy stored
in a body or system as a consequence
of its position, shape, or state (this includes gravitational energy, electrical
energy, nuclear energy, and chemical
energy); kinetic energy is energy of
motion and is usually deÜned as the
work that will be done by the body
possessing the energy when it is
brought to rest. For a body of mass m
having a speed v, the kinetic energy
is mv
2 /2 (classical) or (m – m 0 )c
2 (relativistic). The rotational kinetic energy
of a body having an angular velocity
ω is Iω
2 /2, where I is its moment of
inertia.
The *internal energy of a body is
the sum of the potential energy and
the kinetic energy of its component
atoms and molecules.
energy bands A range of energies
that electrons can have in a solid. In
a single atom, electrons exist in discrete *energy levels. In a crystal, in
which large numbers of atoms are
held closely together in a lattice,
electrons are inÛuenced by a number
of adjacent nuclei and the sharply
deÜned levels of the atoms become
bands of allowed energy; this approach to energy levels in solids is
often known as the band theory.
Each band represents a large number
enantiomeric pair
204
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hydrogen. Enamines can be produced
by condensation of an aldehyde or
ketone with a secondary amine.
enantiomeric pair A pair of molecules consisting of one chiral molecule and the mirror image of this
molecule. The molecules making up
an enantiomeric pair rotate the plane
of polarized light in equal, but opposite, directions.
enantiomers See optical activity.
enantiomorph See optical activity.
enantiomorphism See optical activity.
enantiotopic Denoting a ligand a,
attached to a *prochiral centre, in
which the replacement of this ligand
with a ligand d (which is different
from the ligands a, b, and c attached
to the prochiral centre) gives rise to a
pair of enantiomers C abcd .
enantiotropy See allotropy.
endo- PreÜx used to designate a
bridged ring molecule with a substituent on the ring that is on the
same side as the bridge. If the substituent is on the opposite side the
compound is designated exo-.
endocannabinoids See cannabinoids.
ENDOR Electron-nuclear double resonance. A magnetic resonance technique involving exitation of both
electron spins and nuclear spins. Two
sources of radiation are used. One is
a Üxed source at microwave frequency, which partially saturates the
electron spins. The other is a variable
radiofrequency source, which excites
the atomic nuclear spins. Excitation
of the nuclear spins affects the electron spins by hyperÜre coupling, increasing the relaxation time of the
excited electron spins and increasing
the signal strength. The technique,
which is usually done at low temperatures, is used to investigate paramagnetic molecules. See also
electron paramagnetic resonance.
endothermic Denoting a chemical
reaction that takes heat from its surroundings. Compare exothermic.
end point The point in a titration
at which reaction is complete as
shown by the *indicator.
energy A measure of a system’s
ability to do work. Like work itself, it
is measured in joules. Energy is conveniently classiÜed into two forms:
potential energy is the energy stored
in a body or system as a consequence
of its position, shape, or state (this includes gravitational energy, electrical
energy, nuclear energy, and chemical
energy); kinetic energy is energy of
motion and is usually deÜned as the
work that will be done by the body
possessing the energy when it is
brought to rest. For a body of mass m
having a speed v, the kinetic energy
is mv
2 /2 (classical) or (m – m 0 )c
2 (relativistic). The rotational kinetic energy
of a body having an angular velocity
ω is Iω
2 /2, where I is its moment of
inertia.
The *internal energy of a body is
the sum of the potential energy and
the kinetic energy of its component
atoms and molecules.
energy bands A range of energies
that electrons can have in a solid. In
a single atom, electrons exist in discrete *energy levels. In a crystal, in
which large numbers of atoms are
held closely together in a lattice,
electrons are inÛuenced by a number
of adjacent nuclei and the sharply
deÜned levels of the atoms become
bands of allowed energy; this approach to energy levels in solids is
often known as the band theory.
Each band represents a large number
enantiomeric pair
204
e
www.AzShimi.ir www.AzShimi.com