L

labellingThe process of replacing a
stable atom in a compound with a ra-
dioisotope of the same element to
enable its path through a biological
or mechanical system to be traced by
the radiation it emits. In some cases
a different stable isotope is used and
the path is detected by means of a
mass spectrometer. A compound con-
taining either a radioactive or stable
isotope is called a labelled compound
and the atom used is a label. If a hy-
drogen atom in each molecule of the
compound has been replaced by a tri-
tium atom, the compound is called a
tritiated compound. A radioactive la-
belled compound will behave chemi-
cally and physically in the same way
as an otherwise identical stable com-
pound, and its presence can easily be
detected using a Geiger counter. This
process of radioactive tracing is
widely used in chemistry, biology,
medicine, and engineering. For ex-
ample, it can be used to follow the
course of the reaction of a carboxylic
acid with an alcohol to give an ester,
e.g.

CH 3 COOH + C 2 H 5 OH →

C 2 H 5 COOCH 3 + H 2 O

To determine whether the noncar-
bonyl oxygen in the ester comes
from the acid or the alcohol, the re-
action is performed with the labelled
compound CH 3 CO^18 OH, in which the
oxygen in the hydroxyl group of the
acid has been ‘labelled’ by using the

(^18) O isotope. It is then found that the
water product is H 218 O; i.e. the oxy-
gen in the ester comes from the alco-
hol, not the acid.
labileDescribing a chemical com-
pound in which certain atoms or
groups can easily be replaced by
other atoms or groups. The term is
applied to coordination complexes in
which ligands can easily be replaced
by other ligands in an equilibrium re-
action.
lactamsOrganic compounds con-
taining a ring of atoms in which the
group –NH.CO.– forms part of the
ring. Lactams can be formed by reac-
tion of an –NH 2 group in one part of
a molecule with a –COOH group in
the other to give a cyclic amide (see
illustration). They can exist in an al-
ternative tautomeric form, the lactim
form, in which the hydrogen atom
on the nitrogen has migrated to the
oxygen of the carbonyl to give
–N=C(OH)–. The pyrimidine base
uracil is an example of a lactam.

A

• Information about IUPAC nomenclature


























    

 



Lactams

lactateA salt or ester of lactic acid

(i.e. a 2-hydroxypropanoate).

lactic acid (2-hydroxypropanoic

acid)A clear odourless hygroscopic

syrupy liquid, CH 3 CH(OH)COOH, with

a sour taste; r.d. 1.206; m.p. 18°C; b.p.

122 °C. It is prepared by the hydroly-

sis of ethanal cyanohydrin or the oxi-