isoprene A colourless liquid diene,
CH 2 :C(CH 3 )CH:CH 2. The systematic
name is 2-methylbuta-1,3-diene. The
isoprene structure is the fundamen-
tal structural unit in *terpenes and
natural *rubber. The compound itself
is used in making synthetic rubbers.
isoquinolineSee quinoline.isotactic polymerSee polymer.
isotherm1.A line on a map or
chart joining points or places of
equal temperature. 2.A curve on a
graph representing readings taken at
constant temperature (e.g. the rela-
tionship between the pressure and
volume of a gas at constant tempera-
ture).isothermal process Any process
that takes place at constant tempera-
ture. In such a process heat is, if nec-
essary, supplied or removed from the
system at just the right rate to main-
tain constant temperature. Compare
adiabatic process.isotonicDescribing solutions that
have the same osmotic pressure.isotopeOne of two or more atoms
of the same element that have the
same number of protons in their nu-
cleus but different numbers of neu-
trons. Hydrogen (1 proton, no
neutrons), deuterium (1 proton, 1
neutron), and tritium (1 proton, 2
neutrons) are isotopes of hydrogen.
Most elements in nature consist of a
mixture of isotopes. See isotope sepa-
ration.
isotope separationThe separa-
tion of the *isotopes of an element
from each other on the basis of slight
differences in their physical proper-
ties. For laboratory quantities the
most suitable device is often the
mass spectrometer. On a larger scale
the methods used include gaseous
diffusion (widely used for separating
isotopes of uranium in the form ofthe gas uranium hexaÛuoride), distil-
lation (formerly used to produce
heavy water), electrolysis (requiring
cheap electrical power), thermal dif-
fusion (formerly used to separate ura-
nium isotopes, but now considered
uneconomic) and centrifuging. Vari-
ous laser methods (involving the
excitation of one isotope and its
subsequent separation by electro-
magnetic means) have also been
employed.isotopic isomersSee isotopomers.
isotopic number (neutron excess)
The difference between the number
of neutrons in an isotope and the
number of protons.
isotopologuesSubstances that dif-
fer only in isotopic composition, e.g.
CH 3 OH, CH 2 DOH, CHD 2 OH, CD 3 OH,
CD 3 OD.isotopomers(isotopic isomers)
Molecules that have the same num-
bers of each type of isotopic atom
but in different arrangements in the
molecule. For example, CH 2 DOH and
CH 3 OD are isotopomers.isotropic Denoting a medium
whose physical properties are inde-
pendent of direction. Compare
anisotropic.isozyme(isoenzyme)One of several
forms of an enzyme in an individual
or population that catalyse the same
reaction but differ from each other
in such properties as substrate
afÜnity and maximum rates of en-
zyme–substrate reaction (see
michaelis–menten curve).itaconic acidA product of the fer-
mentation of theÜlamentous fungus
Aspergillus niger. Itaconic acid is used
commercially in the production of
adhesives and paints.
IUPAC International Union of Pure
and Applied Chemistry. An interna-isoprene 298i