elements present, IUPAC name generation, and viewing in RasMol.
A
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Elsevier MDL website (free registration
required)
iso- PreÜx denoting that a compound 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 indicating readings taken at constant
pressure. 2. One of two or more nuclides that have the same number of
nucleons but different *atomic numbers. Radium–88, actinium–89, and
thorium–90 are isobars as each has a
*nucleon number of 228.
isocyanate See cyanic acid.
isocyanic acid See cyanic acid.
isocyanide See 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 O
The isocyanide RNC is recognized by
its unpleasant smell. This reaction of
primary amines is called the carbylamine reaction.
isoelectric point The point at
which a substance (such as a colloid
or protein) has zero net electric
charge. Usually such substances are
positively or negatively charged, depending 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 absorbed equally. The substance has its
minimum conductivity at its isoelectric point and therefore coagulates
best at this point. In the case of hydrophilic substances, in which the
surrounding water prevents coagulation, 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 isoelectric pH level the substance acts as
a base and below this level it acts as
an acid. For example, at the isoelectric point the pH of gelatin is 4.7. Proteins 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 number of electrons. For example N 2 and
CO are isoelectronic. The energy
level diagrams of isoelectronic molecules are therefore similar.
isoenzyme See isozyme.
isoindole See indole.
isoleptic complex A metal complex in which all the ligands are the
same.
isoleucine See amino acid.
isomerism The existence of chemical compounds (isomers) that have
the same molecular formulae but different molecular structures or different arrangements of atoms in space.
In structural (or constitutional isomerism) isomerism the 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 isomers generally have different physical 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 activity). Another type is cis–trans isomerism (formerly geometrical
isomerism), in which the isomers
295
isomerism
i
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A
• A limited version of ISIS/Draw at the
Elsevier MDL website (free registration
required)
iso- PreÜx denoting that a compound 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 indicating readings taken at constant
pressure. 2. One of two or more nuclides that have the same number of
nucleons but different *atomic numbers. Radium–88, actinium–89, and
thorium–90 are isobars as each has a
*nucleon number of 228.
isocyanate See cyanic acid.
isocyanic acid See cyanic acid.
isocyanide See 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 O
The isocyanide RNC is recognized by
its unpleasant smell. This reaction of
primary amines is called the carbylamine reaction.
isoelectric point The point at
which a substance (such as a colloid
or protein) has zero net electric
charge. Usually such substances are
positively or negatively charged, depending 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 absorbed equally. The substance has its
minimum conductivity at its isoelectric point and therefore coagulates
best at this point. In the case of hydrophilic substances, in which the
surrounding water prevents coagulation, 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 isoelectric pH level the substance acts as
a base and below this level it acts as
an acid. For example, at the isoelectric point the pH of gelatin is 4.7. Proteins 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 number of electrons. For example N 2 and
CO are isoelectronic. The energy
level diagrams of isoelectronic molecules are therefore similar.
isoenzyme See isozyme.
isoindole See indole.
isoleptic complex A metal complex in which all the ligands are the
same.
isoleucine See amino acid.
isomerism The existence of chemical compounds (isomers) that have
the same molecular formulae but different molecular structures or different arrangements of atoms in space.
In structural (or constitutional isomerism) isomerism the 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 isomers generally have different physical 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 activity). Another type is cis–trans isomerism (formerly geometrical
isomerism), in which the isomers
295
isomerism
i
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