Charles’ law At constant pressure, the volume occu-
pied by a definite mass of gas is directly proportional
to its absolute temperature.
chelation The formation or presence of BONDs (or
other attractive interactions) between two or more sepa-
rate BINDING SITEs within the same LIGANDand a single
central atom. A MOLECULAR ENTITYin which there is
chelation (and the corresponding CHEMICAL SPECIES) is
called a “chelate.” The terms bidentate(or didentate), tri-
dentate, tetradentate,..., multidentateare used to indi-
cate the number of potential binding sites of the ligand,
at least two of which must be used by the ligand in form-
ing a “chelate.” For example, the bidentate ethylenedi-
amine forms a chelate with CuI in which both nitrogen
atoms of ethylenediamine are bonded to copper. (The use
of the term is often restricted to metallic central atoms.)
The phrase “separate binding sites” is intended to
exclude cases such as [PtCl 3 (CH 2 CH 2 )]–, ferrocene,
and (benzene)tricarbonylchromium, in which ethene,
the cyclopentadienyl group, and benzene, respectively,
are considered to present single binding sites to the
respective metal atom, and which are not normally
thought of as chelates (see HAPTO).
See alsoCRYPTAND.
chelation therapy The judicious use of chelating
(metal binding) agents for the removal of toxic
amounts of metal ions from living organisms. The
metal ions are sequestered by the chelating agents and
are rendered harmless or excreted. Chelating agents
such as 2,3-dimercaptopropan-1-ol, ethylenediaminete-
traacetic acid, DESFERRIOXAMINE, and D-penicillamine
have been used effectively in chelation therapy for
arsenic, lead, iron, and copper, respectively.
See alsoCHELATION.cheletropic reaction A form of CYCLOADDITION
across the terminal atoms of a fully CONJUGATED SYS-
TEM, with formation of two new SIGMA BONDs to a sin-
gle atom of the (“monocentric”) reagent. There is
formal loss of one PI BONDin the substrate and an
increase in COORDINATION NUMBER of the relevant
atom of the reagent. An example is the ADDITIONof
sulfur dioxide to butadiene:The reverse of this type of reaction is designated
“cheletropic elimination.”
See alsoCHELOTROPIC REACTION.chelotropic reaction Alternative (and etymologically
more correct) name for CHELETROPIC REACTION.chemical bond The attractive force that binds atoms
together in elements or compounds.chemical change Occurs when atoms in a substance
are rearranged so that a new substance with a new
chemical identity is formed.chemical equation A graphic representation of a
chemical reaction.chemical equilibrium The condition when the for-
ward and reverse reaction rates are equal and the con-46 Charles’ law
Charles’ law. The volume (V)of a fixed mass of gas at constant
pressure (P)is dependent on its temperature (T).