Dictionary of Chemistry [6th Ed.]

reaction of alcohols with sodium or
potassium metal. Alkoxides are salt-
like compounds containing the ion
R–O–.

alkyd resin A type of *polyester
resin used in paints and other sur-
face coating. The original alkyd
resins were made by copolymerizing
phthalic anhydride with glycerol, to
give a brittle cross-linked polymer.
The properties of such resins can be
modiÜed by adding monobasic acids
or alcohols during the polymeriza-
tion.

alkylation A chemical reaction
that introduces an alkyl group into
an organic molecule. The Friedel–
Crafts reaction results in alkylation
of aromatic compounds.

alkylbenzenes Organic com-
pounds that have an alkyl group
bound to a benzene ring. The sim-
plest example is methylbenzene
(toluene), CH 3 C 6 H 5. Alkyl benzenes
can be made by the *Friedel–Crafts
reaction.

alkyl group A group obtained by
removing a hydrogen atom from an
alkane, e.g. methyl group, CH 3 –, de-
rived from methane.

alkyl halidesSee haloalkanes.

alkynes(acetylenes)Unsaturated
hydrocarbons that contain one or
more triple carbon–carbon bonds in
their molecules. In systematic chemi-
cal nomenclature alkyne names end
in the sufÜx -yne. Alkynes that have
only one triple bond form a *ho-
mologous series: ethyne (acetylene),
CH≡CH, propyne, CH 3 CH≡CH, etc.
They can be made by the action of
potassium hydroxide in alcohol solu-
tion on haloalkanes containing halo-
gen atoms on adjacent carbon atoms;
for example:

RCHClCH 2 Cl + 2KOH →2KCl +

2H 2 O + RCH≡CH

Like *alkenes, alkynes undergo addi-

tion reactions.

A

• Information about IUPAC nomenclature
  allenesCompounds that contain
  the group >C=C=C<, in which three
  carbon atoms are linked by two adja-
  cent double bonds. The outer carbon
  atoms are each linked to two other
  atoms or groups by single bonds. The
  simplest example is 1,2-propadiene,
  CH 2 CCH 2. Allenes are *dienes with
  typical reactions of alkenes. Under
  basic conditions, they often convert
  to alkynes. In an allene, the two
  double bonds lie in planes that are
  perpendicular to each other. Con-
  sequently, in an allene of the type
  R 1 R 2 C:C:CR 3 R 4 , the groups R 1 and
  R 2 lie in a plane perpendicular to
  the plane containing R 3 and R 4.
  Under these circumstances, the mol-
  ecule is chiral and can show optical
  activity.

allosteric enzymeAn enzyme

that has two structurally distinct

forms, one of which is active and the

other inactive. In the active form, the

quaternary structure (see protein) of

the enzyme is such that a substrate

can interact with the enzyme at the

active site (see enzyme–substrate

complex). The conformation of the

substrate-binding site becomes al-

tered in the inactive form and inter-

action with the substrate is not

possible. Allosteric enzymes tend to

catalyse the initial step in a pathway

leading to the synthesis of molecules.

The end product of this synthesis can

act as a feedback inhibitor (see inhibi-

tion) and the enzyme is converted to

the inactive form, thereby control-

ling the amount of product synthe-

sized.

allosteric siteA binding site on

the surface of an enzyme other than

the *active site. In noncompetitive

21 allosteric site

a