matic compounds. The method is to
diazotize an aromatic amide at low
temperature and add an equimolar
solution of the halogen acid and cop-
per(I) halide. A complex of the diazo-
nium salt and copper halide forms,
which decomposes when the temper-
ature is raised. The copper halide acts
as a catalyst in the reaction of the
halide ions from the acid, for exam-
ple
C 6 H 5 N 2 +(aq) + Cl–(aq) + CuCl(aq) →
C 6 H 5 Cl(l) + N 2 (g) + CuCl(aq)
The reaction was discovered in 1884
by the German chemist Traugott
Sandmeyer (1854–1922). See also gat-
termann reaction.sandwich compoundA transi-
tion-metal complex in which a metal
atom or ion is ‘sandwiched’ between
two rings of atoms. *Ferrocene was
theÜrst such compound to be pre-
pared, having two parallel cyclopen-
tadienyl rings with an iron ion
between them. In such compounds
(also known as metallocenes) the
metal coordinates to the pi electron
system of the ring, rather than to in-
dividual atoms. A wide variety of
these compounds are known, having
Üve-, six-, seven-, or eight-membered
rings and involving such metals as
Cr, Mn, Co, Ni, and Fe. Other similar
compounds are known. A multi-
decker sandwich has three or more
parallel rings with metal atoms be-
tween them. In a bent sandwich, the
rings are not parallel. A half sand-
wich(or piano stool) has one ring,
with single ligands on the other side
of the metal.Sanger’s reagent 2,4-dinitro-
Ûuorobenzene, C 6 H 3 F(NO 2 ) 2 , used to
identify the end *amino acid in a
protein chain. It is named after Fred-
erick Sanger (1918– ).saponiÜcation The reaction of es-ters with alkalis to give alcohols and
salts of carboxylic acids:
RCOOR′+ OH–→RCOO–+ R′OH
See esterification; soap.saponinA type of toxic *glycoside
that forms a frothy colloidal solution
on shaking with water. Saponins
occur in many plants (such as horse
chestnut). They break down red
blood cells and have been used for
poisoningÜsh. On hydrolysis they
yield a variety of sugars.
sapphire Any of the gem varieties
of *corundum except ruby, especially
the blue variety, but other colours of
sapphire include yellow, brown,
green, pink, orange, and purple. Sap-
phires are obtained from igneous and
metamorphic rocks and from alluvial
deposits. The chief sources are Sri
Lanka, Kashmir, Burma, Thailand,
East Africa, the USA, and Australia.
Sapphires are used as gemstones and
in record-player styluses and some
types of laser. They are synthesized
by the VerneuilÛame-fusion process.sarinA highly toxic colourless liq-
uid, C 4 H 10 FO 2 P; r.d. 1.09; m.p. –56°C;
b.p. 158°C. It is an organophosphorus
compound, O-isopropyl methylphos-
phonoÛuoridate. Sarin was discov-
ered in 1938 and belongs to the
G-series of *nerve agents (GB). It was
used by Iraq in the Iran–Iraq war
(1980–88). In 1988, sarin was also
used by Iraq in a poison gas attack on
the Kurd city of Halabja in the north
of Iraq. About 5000 people died. In
1994, a Japanese religious sect re-
leased sarin in Matsumoto and in
1995 released it in the Tokyo subway.saturable laserSee dye laser.
saturated1.(of a compound) Con-
sisting of molecules that have only
single bonds (i.e. no double or triple
bonds). Saturated compounds can un-
dergo substitution reactions but notsandwich compound 472s