Encyclopedia of Chemistry

awards, including being knighted in 1969. He died of
heart failure in Ireland on August 12, 1979.

chain reaction A reaction in which one or more
reactive reaction INTERMEDIATEs (frequently RADICALs)
are continuously regenerated, usually through a repeti-
tive cycle of elementary steps (the “propagation step”).
For example, in the chlorination of methane by a radi-
cal MECHANISM, Cl.is continuously regenerated in the
chain propagation steps:

Cl.+ CH 4 →HCl + H 3 C.

H 3 C.+ Cl 2 →CH 3 Cl + Cl.

In chain polymerization reactions, reactive intermedi-
ates of the same types, generated in successive steps or
cycles of steps, differ in relative molecular mass, as in

RCH 2 C.HPh + H 2 CCHPh →RCH 2 CHPhCH 2 C.HPh
See also CHAIN TRANSFER; INITIATION; TERMINA-
TION.

chain transfer The abstraction, by the RADICALend
of a growing chain polymer, of an atom from another
molecule. The growth of the polymer chain is thereby
terminated, but a new radical, capable of chain propa-
gation and polymerization, is simultaneously created.
For the example of alkene polymerization cited for a
CHAIN REACTION, the reaction

RCH 2 C.HPh + CCl 4 →RCH 2 CHClPh + Cl 3 C.

represents a chain transfer, with the radical Cl 3 C.

inducing further polymerization

H 2 CCHPh + Cl 3 C.→Cl 3 CCH 2 C.HPh

Cl 3 CCH 2 C.HPh + H 2 CCHPh →

Cl 3 CCH 2 CHPhCH 2 C.HPh

The phenomenon occurs also in other chain reac-

tions such as cationic polymerization.

See alsoTELOMERIZATION.

chalcogen One of the elements in the same column

of the periodic table as oxygen.

channels Transport proteins that act as gates to con-

trol the movement of sodium and potassium ions

across the plasma membrane of a nerve cell.

See alsoACTIVE TRANSPORT.

chaperonin Member of the set of molecular chaper-

ones, located in different organelles of the cell and

involved either in transport of proteins through BIOMEM-

BRANEs by unfolding and refolding the proteins or in

assembling newly formed polypeptides.

charge density SeeELECTRON DENSITY.

charge population The net electric charge on a spec-

ified atom in a MOLECULAR ENTITY, as determined by

some prescribed definition.

See alsoELECTRON DENSITY.

charge-transfer complex An aggregate of two or

more molecules in which charge is transferred from a

donor to an acceptor.

charge-transfer transition An electronic transition in

which a large fraction of an electronic charge is trans-

ferred from one region of a molecular entity, called the

electron donor, to another, called the electron acceptor

(intramolecular charge-transfer), or from one molecular

entity to another (intermolecular charge-transfer).

charge-transfer transition 45

Chain reaction. Any reaction where one reaction leads to another
that leads to another, and so forth