elements present, IUPAC name gen-
eration, and viewing in RasMol.
A
- A limited version of ISIS/Draw at the
Elsevier MDL website (free registration
required)
iso-PreÜx denoting that a com-
pound is an *isomer, e.g. isopentane
(CH 3 CH(CH 3 )C 2 H 5 , 2-methylbutane) is
an isomer of pentane.
isobar 1.A curve on a graph indi-
cating readings taken at constant
pressure. 2.One of two or more nu-
clides that have the same number of
nucleons but different atomic num-
bers. Radium–88, actinium–89, and
thorium–90 are isobars as each has a
nucleon number of 228.
isocyanate See cyanic acid.
isocyanic acidSee cyanic acid.
isocyanideSee isonitrile.
isocyanide test A test for primary
amines by reaction with an alcoholic
solution of potassium hydroxide and
trichloromethane.
RNH 2 + 3KOH + CHCl 3 →RNC +
3KCl + 3H 2 OThe isocyanide RNC is recognized by
its unpleasant smell. This reaction of
primary amines is called the carby-
lamine reaction.
isoelectric pointThe point at
which a substance (such as a colloid
or protein) has zero net electric
charge. Usually such substances are
positively or negatively charged, de-
pending on whether hydrogen ions
or hydroxyl ions are predominantly
absorbed. At the isoelectric point the
net charge on the substance is zero,
as positive and negative ions are ab-
sorbed equally. The substance has its
minimum conductivity at its isoelec-
tric point and therefore coagulates
best at this point. In the case of hy-
drophilic substances, in which the
surrounding water prevents coagula-
tion, the isoelectric point is at the
minimum of stability. The isoelectric
point is characterized by the value of
the pH at that point. Above the iso-
electric pH level the substance acts as
a base and below this level it acts as
an acid. For example, at the isoelec-
tric point the pH of gelatin is 4.7. Pro-
teins precipitate most readily at their
isoelectric points; this property can
be utilized to separate mixtures of
proteins or amino acids.isoelectronic Denoting different
molecules that have the same num-
ber of electrons. For example N 2 and
CO are isoelectronic. The energy
level diagrams of isoelectronic mol-
ecules are therefore similar.
isoenzymeSee isozyme.isoindoleSee indole.isoleptic complexA metal com-
plex in which all the ligands are the
same.isoleucineSee amino acid.isomerismThe existence of chemi-
cal compounds (isomers) that have
the same molecular formulae but dif-
ferent molecular structures or differ-
ent arrangements of atoms in space.
In structural(or constitutional iso-
merism) isomerismthe molecules
have different molecular structures:
i.e. they may be different types of
compound or they may simply differ
in the position of the functional
group in the molecule. Structural iso-
mers generally have different physi-
cal and chemical properties. In
stereoisomerism, the isomers have
the same formula and functional
groups, but differ in the arrangement
of groups in space. Optical isomerism
is one form of this (see optical activ-
ity). Another type is cis–trans iso-
merism(formerly geometrical
isomerism), in which the isomers295 isomerism
i