The product of an attachment may also be the
ADDUCTof the two reactants, but not all adducts can
be represented as the products of an attachment.
(For example, the Diels-Alder CYCLOADDITIONresults
in an adduct of buta-1,3-diene and ethene, but the
reaction cannot be described as an attachment, since
bonds are formed between more than two centers.)
See alsoCOLLIGATION; ELECTRON ATTACHMENT.
aufbau (“building up”) principle Describes the
order in which electrons fill orbitals in atoms.
auranofin SeeGOLD DRUGS.
autacoid A biological substance secreted by various
cells whose physiological activity is restricted to the
vicinity of its release; it is often referred to as local
HORMONE.
autocatalytic reaction A CHEMICAL REACTION in
which a product (or a reaction INTERMEDIATE) also
functions as a CATALYST. In such a reaction, the
observed RATE OF REACTIONis often found to increase
with time from its initial value.
See alsoORDER OF REACTION.
autoionization Ionization reaction between identical
molecules.
automerization SeeDEGENERATE REARRANGEMENT.
autophytic The process whereby an organism uses
photosynthesis to make complex foods from inorganic
substances.
autoprotolysis A PROTON(hydron) TRANSFER REAC-
TIONbetween two identical molecules (usually a sol-
vent), one acting as a BRONSTED ACIDand the other as
a BRONSTED BASE. For example:
2 H 2 O →H 3 O++ OH–
See alsoHYDRON.autoprotolysis constant The product of the activities
(or, more approximately, concentrations) of the species
produced as the result of AUTOPROTOLYSIS. For solvents
in which no other ionization processes are significant,
the term is synonymous with ionic product.The auto-
protolysis constant for water, Kw, is equal to the product
of activities a(H 3 O+)a(OH–) = 1.0 × 10 –14at 25°C.autoreceptor An autoreceptor, present at a nerve
ending, is a RECEPTORthat regulates, via positive or
negative feedback processes, the synthesis or release of
its own physiological ligand.
See alsoHETERORECEPTOR.autotrophic organism An organism that is capable
of using carbon dioxide as the sole carbon source for
growth and product formation. Organisms that use
light as a source of energy are said to be PHOTOAU-
TOTROPHs; those that use the energy from chemical
reactions are CHEMOAUTOTROPHs.auxins A group of plant hormones that produce a
number of effects, including plant growth, phototropic
response through the stimulation of cell elongation
(photopropism), stimulation of secondary growth, api-
cal dominance, and the development of leaf traces and
fruit. An important plant auxin is indole-3-acetic acid.
(IAA and synthetic auxins such as 2,4-D and 2,4,5-T
are used as common weed killers.)auxotroph An organism that requires a particular
organic compound for growth.Avalue The conformational preference of an equa-
torial compared with an axial substituent in a mono-Avalue 21