S

Sabatier–Senderens process A
method of organic synthesis employ-
ing hydrogenation and a heated
nickel catalyst. It is employed com-
mercially for hydrogenating unsatu-
rated vegetable oils to make
margarine. It is named after Paul
Sabatier (1854–1941) and Jean-Bap-
tiste Senderens (1856–1937).

saccharideSee sugar.

A

• Information about IUPAC nomenclature

saccharinA white crystalline solid,
C 7 H 5 NO 3 S, m.p. 224°C. It is made
from a compound of toluene, derived
from petroleum or coal tar. It is a
well-known artiÜcial sweetener,
being some 500 times as sweet as
sugar (sucrose), and is usually mar-
keted as its sodium salt. Because of
an association with cancer in labora-
tory animals, its use is restricted in
some countries.

SO 2

NH

O

Saccharin

saccharose See sucrose.

Sachse reaction A reaction of
methane at high temperature to pro-
duce ethyne:

2CH 4 →C 2 H 2 + 3H 2

The reaction occurs at about 1500°C,
the high temperature being obtained
by burning part of the methane in
air.

Sackur–Tetrode equation An

equation for the entropy of a perfect

monatomic gas. The entropy S is

given by:

S = nRln(e5/2V/nNAΛ^3 ),

where Λ= h/(2πmkT)½, wheren is the

amount of the gas, R is the gas con-

stant, e is the base of natural loga-

rithms, V is the volume of the

system, NAis the Avogadro constant,

h is the Planck constant, m is the

mass of each atom, k is the Boltz-

mann constant, and T is the thermo-

dynamic temperature. To calculate

the molar entropy of the gas both

sides are divided by n. The Sackur–

Tetrode equation can be used to

show that the entropy change ∆S,

when a perfect gas expands isother-

mally from Vito Vf, is given by:

∆S = nRln(aVf) – nRln(aVi) =

nRln(Vf/Vi),

where aV is the quantity inside the

logarithm bracket in the Sackur–

Tetrode equation.

sacriÜcial protection (cathodic

protection) The protection of iron or

steel against corrosion (see rusting)

by using a more reactive metal. A

common form is galvanizing (see gal-

vanized iron), in which the iron sur-

face is coated with a layer of zinc.

Even if the zinc layer is scratched,

the iron does not rust because zinc

ions are formed in solution in prefer-

ence to iron ions. Pieces of magne-

sium alloy are similarly used in

protecting pipelines, etc.

sal ammoniacSee ammonium

chloride.

SALC(symmetry-adapted linear