Alder reaction is a (4+2) cycloaddition; (b) the initial
reaction of ozone with an alkene is a (3+2) cycloaddi-
tion; and (c) the reaction shown below is a (2+2+2)
cycloaddition. (Parentheses (...) are used in the descrip-
tion based on numbers of atoms.)
(2) The symbolism [i+j+...] for a cycloaddition
identifies the numbers i, j,... of electrons in the inter-
acting units that participate in the transformation of
reactants to products. In this notation, the reactions (a)
and (b) of the preceding paragraph would both be
described as [2+4] cycloadditions, and (c) as a [2+2+2]
cycloaddition. The symbol aor s(a= ANTARAFACIAL, s
= suprafacial) is often added (usually as a subscript
after the number to designate the stereochemistry of
addition to each fragment). A subscript specifying the
orbitals, viz., σ, π(sigma, pi), with their usual signifi-
cance, or n(for an orbital associated with a single atom
only) may be added as a subscript before the number.
Thus the normal Diels-Alder reaction is a [4s+2s] or
[π 4 s+ π 2 s] cycloaddition, while the reaction
would be a [14a+2s] or [π 14 a+ π 2 s] cycloaddition.
(Square brackets [...] are used in the descriptions based
on numbers of electrons.)
Cycloadditions may be PERICYCLIC REACTIONs or
(nonconcerted) STEPWISE REACTIONs. The term dipolar
cycloadditionis used for cycloadditions of 1,3-dipolar
compounds.
See also CHELETROPIC REACTIONs; CONCERTED
PROCESS.cycloalkane An alkane that contains a ring of car-
bon atoms.cycloelimination The reverse of CYCLOADDITION.
The term is preferred to the synonyms cycloreversion,
retroaddition,and retrocycloaddition.cycloreversion SeeCYCLOELIMINATION.cyclotron A machine for accelerating charged parti-
cles along a spiral path to high energies. The first
cyclotron was built by Ernest Orlando Lawrence and
his graduate student, M. Stanley Livingston, at the Uni-
versity of California, Berkeley, in the early 1930s.cytochrome A HEMEprotein that transfers electrons
and exhibits intense absorption bands (the α-band and
β-bands, with the α-band having the longer wave-
length) between 510 and 615 nm in the reduced form.
Cytochromes are designated types a, b, c,or d,depend-
ing on the position of the α-band, which depends on
the type of heme. The iron undergoes oxidation-reduc-
tion between oxidation states Fe(II) and Fe(III). Most
cytochromes are hemochromes, in which the fifth and
sixth COORDINATIONsites in the iron are occupied by
strong field LIGANDs, regardless of the oxidation state
of iron. Cytochromes may be distinguished by the
wavelength of the α-band, such as cytochrome c-550.
Certain specific cytochromes with particular functions
are designated with suffixes, such as cytochrome a 1 ,b 2 ,
etc., but this practice is discouraged.cytochrome-c oxidase An ENZYME, ferrocytochrome-
c: dioxygen OXIDOREDUCTASE, CYTOCHROMEaa 3. The
major respiratory protein of animal and plant MITO-
CHONDRIA, it catalyzes the oxidation of Fe(II)-
cytochrome c and the reduction of dioxygen to water.
Contains two HEMEs and three copper atoms, arranged
in three centers. Heme a 3 and copper-B form a center thatcytochrome-c oxidase 67