where R is a hydrocarbon group or
hydrogen. Enamines can be produced
by condensation of an aldehyde or
ketone with a secondary amine.
enantiomeric pairA pair of mol-
ecules consisting of one chiral mol-
ecule and the mirror image of this
molecule. The molecules making up
an enantiomeric pair rotate the plane
of polarized light in equal, but oppo-
site, directions.enantiomersSee optical activity.
enantiomorphSee optical activ-
ity.enantiomorphismSee optical ac-
tivity.enantiotopicDenoting 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 Cabcd.
enantiotropy See allotropy.endo-PreÜx used to designate a
bridged ring molecule with a sub-
stituent on the ring that is on the
same side as the bridge. If the sub-
stituent is on the opposite side the
compound is designated exo-.endocannabinoidsSee cannabi-
noids.ENDORElectron-nuclear double res-
onance. A magnetic resonance tech-
nique involving exitation of both
electron spins and nuclear spins. Two
sources of radiation are used. One is
aÜxed source at microwave fre-
quency, 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 elec-
tron spins by hyperÜre coupling, in-
creasing the relaxation time of the
excited electron spins and increasingthe signal strength. The technique,
which is usually done at low temper-
atures, is used to investigate para-
magnetic molecules. See also
electron paramagnetic resonance.endothermicDenoting a chemical
reaction that takes heat from its sur-
roundings. Compare exothermic.end pointThe point in a titration
at which reaction is complete as
shown by the *indicator.
energyA measure of a system’s
ability to do work. Like work itself, it
is measured in joules. Energy is con-
veniently 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 in-
cludes 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 (rela-
tivistic). The rotational kinetic energy
of a body having an angular velocity
ωis 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 bandsA range of energies
that electrons can have in a solid. In
a single atom, electrons exist in dis-
crete *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 ap-
proach to energy levels in solids is
often known as the band theory.
Each band represents a large numberenantiomeric pair 204e