tem. For example, the sequence for
some common ligands is I, Br, Cl,
SO 3 H, OCOCH 3 , OCH 3 , OH, NO 2 , NH 2 ,
COOCH 3 , CONH 2 , COCH 3 , CHO,
CH 2 OH, C 6 H 5 , C 2 H 5 , CH 3 , H. The sys-
tem was jointly developed by the
British chemists Robert Sidney Cahn
(1899–1981) and Sir Christopher Kelk
Ingold (1893–1970) and the Bosnian–
Swiss chemist Vladimir Prelog (1906–
98).
circular birefringence A phenom-
enon in which there is a difference
between the refractive indices of the
molecules of a substance for right-
and left-circularly polarized light. Cir-
cular birefringence depends on the
way in which the electromagnetic
Üeld interacts with the molecule, and
is affected by the handedness of the
molecule, and hence its polarizabil-
ity. If a molecule has the shape of a
helix, the polarizability is dependent
on whether or not the electricÜeld
of the electromagneticÜeld rotates in
the same sense as the helix, thus giv-
ing rise to circular birefringence.
circular dichroism (CD)The pro-
duction of an elliptically polarized
wave when a linearly polarized light
wave passes through a substance that
has differences in the extinction
coefÜcients for left- and right-handed
polarized light. The size of this effect
is given by φ= π/λ(ηl– ηr), where φis
the ellipticity of the beam that
emerges (in radians), λis the wave-
length of the light, and ηland ηrare
the absorption indices of the left- and
right-handed circularly polarized
light, respectively. Circular dichro-
ism is a property of optically active
molecules and is used to obtain infor-
mation about proteins. See also cd
spectrum.
circular polarization See polariza-
tion of light.
cis See isomerism; torsion angle.
127 Clapeyron–Clausius equation
c
A
- Information about IUPAC nomenclature
cisplatinA platinum complex, cis-
[PtCl 2 (NH 3 ) 2 ], used in cancer treat-
ment to inhibit the growth of
tumours. It acts by binding between
strands of DNA.
citrateA salt or ester of citric acid.citric acidA white crystalline
hydroxycarboxylic acid,
HOOCCH 2 C(OH)(COOH)CH 2 COOH;
r.d. 1.54; m.p. 153°C. It is present in
citrus fruits and is an intermediate in
the *Krebs cycle in plant and animal
cells.citric-acid cycleSee krebs cycle.CL20See hniw.Claisen condensationA reaction
of esters in which two molecules of
the ester react to give a keto ester,
e.g.
2CH 3 COOR →CH 3 COCH 2 COOR
+ ROH
The reaction is catalysed by sodium
ethoxide, the mechanism being simi-
lar to that of the *aldol reaction. It is
named after Ludwig Claisen
(1851–1930).Clapeyron–Clausius equation A
differential equation that describes
the relationship between variables
when there is a change in the state
of a system. In a system that has two
*phases of the same substance, for
example solid and liquid, heat is
added or taken away very slowly so
that one phase can change reversibly
into the other while the system re-
mains at equilibrium. If the two
phases are denoted A and B, the
Clapeyron–Clausius equation is:
dp/dT = L/T(VB–VA),
where p is the pressure, T is the ther-
modynamic temperature, L is the
heat absorbed per mole in the