CO 2 are fixed into sugar (glucose) and mediated by the
enzyme rubisco (ribulose-1-5-biphosphate carboxy-
lase). It occurs in the stroma of chloroplasts. The
Calvin cycle is also known as the dark reaction, as
opposed to the first-stage light reactions.
canonical form SeeCONTRIBUTING STRUCTURE.
capillary A tube with a very small inside diameter.
capillary action The rising of a liquid up the inside
of a small-bore tube or vertical space when adhesive
forces (the attractive forces between the capillary mate-
rial and the liquid) exceed cohesive forces (the attrac-
tive forces between the molecules of the liquid itself).
captodative effect Effect on the stability of a car-
bon-centered RADICAL determined by the combined
action of a captor (electron withdrawing) and a dative
(electron releasing) substituent, both attached to the
radical center. The term is also used for certain unsatu-
rated compounds.
carbanion Generic name for anions containing an
even number of electrons and having an unshared pair
of electrons on a tervalent carbon atom (e.g., Cl 3 C–or
HCC–) or—if the ion is mesomeric (see MESO-
MERISM)—having at least one significant CONTRIBUT-
ING STRUCTUREwith an unshared pair of electrons on a
tervalent carbon atom, for example,
CH 3 C(–O–)CH–C(O)CH 3 ↔
CH 3 C(O)–CH––C(O)CH 3
See alsoRADICAL ION.carbene Generic name for the species H 2 C: and sub-
stitution derivatives thereof, containing an electrically
neutral bivalent carbon atom with two nonbonding
electrons. The nonbonding electrons may have antipar-
allel spins (singlet state) or parallel spins (triplet state).
Use of the alternative name methyleneas a generic term
is not recommended.
See alsoBIRADICAL.
carbenium center The three-coordinate carbon atom
in a CARBENIUM ION, of which the excess positive
charge of the ion (other than that located on het-
eroatoms) can be formally considered to be largely
attributed, i.e., which has one vacant PORBITAL. (It is
not always possible to uniquely identify such an atom.)
This formal attribution of charge often does not express
the real charge distribution.carbenium ion A generic name for CARBOCATION,
real or hypothetical, that has at least one important
CONTRIBUTING STRUCTUREcontaining a tervalent car-
bon atom with a vacant PORBITAL. (The name implies a
protonated carbene or a substitution derivative thereof.)
The term was proposed (and rejected) as a replace-
ment for the traditional usage of the name CARBONIUM
ION.
To avoid ambiguity, the name should not be used
as the root for the systematic nomenclature of carboca-
tions. The corresponding difficulty confused carbonium
ion nomenclature for many years. For example, the
term ethylcarbonium ionhas at times been used to refer
either to CH 3 CH 2 + (ethyl cation) or (correctly) to
CH 3 CH 2 CH 2 +(propyl cation).carbenoid A CARBENE-like CHEMICAL SPECIES but
with properties and REACTIVITYdiffering from the free
carbene itself, e.g., R^1 R^2 C(Cl)M (M = metal).carbocation A cation containing an even number of
electrons with a significant portion of the excess posi-
tive charge located on one or more carbon atoms. This
is a general term embracing all types of CARBENIUM
IONs, vinyl cations, etc. Carbocations can be named by
adding the word cationto the name of the correspond-
ing RADICAL. Such names do not imply structure (e.g.,
whether three-coordinated or five-coordinated carbon
atoms are present).
See alsoBRIDGED CARBOCATION; RADICAL ION.carbohydrate A large class of compounds that con-
tain carbon, hydrogen, and oxygen in a general formula
of Cn(H 2 O)n. Classified from simple to complex, they
form mono-, di-, tri-, poly-, and heterosaccharides.carbohydrate 39