ore is smelted in this way so that the
metal melts and, being denser than
the molten *slag, sinks below the
slag, enabling it to be removed from
the furnace separately.SMILESSimpliÜed molecular input
line entry system. A type of line nota-
tion for representing molecules and
reactions. It uses standard atom sym-
bols but hydrogen atoms are omitted.
A double bond is represented by an
equals sign and a triple bond by a
hash sign. So ethane is CC, ethene is
C=C, and ethyne is C#C.
Branched structures are given
using brackets. For example, triethyl-
amine ((C 2 H 5 ) 3 N) is represented by
CCN(CC)CC; ethanoic (acetic acid)
(CH3COOC) is CC(=O)O. Ring struc-
tures use numbers to show connec-
tions. For instance, cyclohexane is
C1CCCCC1, where the C1 atoms are
the ones joined to form the ring.
SMILES notation for aromatic mol-
ecules uses lower case letters. Ben-
zene is c1ccccc1; chlorobenzene
could be c1c(Cl)cccc1. Rules exist for
expressing isotope and stereochem-
istry information and there is a nota-
tion for reactions. Rules also exist for
choosing a unique SMILES, which is a
deÜnitive representation of the many
possible valid strings that may be
written for a given compound. An
extension of SMILES to describe mo-
lecular patterns (for substructure
searches) is called SMARTS.
A- A SMILES tutorial produced by Daylight
Chemical Information Systems Inc.
smokeAÜne suspension of solid
particles in a gas.
SN1 reaction See nucleophilic sub-
stitution.SN2 reaction See nucleophilic sub-
stitution.SNGSubstitute (or synthetic) nat-ural gas; a mixture of gaseous hydro-
carbons produced from coal, petro-
leum, etc., and suitable for use as a
fuel. Before the discovery of natural
gas *coal gas was widely used as a do-
mestic and industrial fuel. This gave
way to natural gas in the early part
of this century in the US and other
countries where natural gas was
plentiful. The replacement of coal
gas occurred somewhat later in the
UK and other parts of Europe. More
recently, interest has developed in
ways of manufacturing hydrocarbon
gas fuels. The main sources are coal
and the naphtha fraction of petro-
leum. In the case of coal three meth-
ods have been used: (1) pyrolysis – i.e.
more efÜcient forms of destructive
distillation, often with further hydro-
genation of the hydrocarbon prod-
ucts; (2) heating the coal with
hydrogen and catalysts to give hydro-
carbons – a process known as
hydroliquefaction (see also bergius
process); (3) producing carbon
monoxide and hydrogen and obtain-
ing hydrocarbons by the *Fischer–
Tropsch process. SNG from naptha is
made by steam *reforming. See crg
process.soapA substance made by boiling
animal fats with sodium hydroxide.
The reaction involves the hydrolysis
of *glyceride esters of fatty acids to
glycerol and sodium salts of the acids
present (mainly the stearate, oleate,
and palmitate), giving a soft semi-
solid with *detergent action. Potas-
sium hydroxide gives a more liquid
product (soft soap). By extension,
other metal salts of long-chain fatty
acids are also called soaps. See also
saponification.sodaAny of a number of sodium
compounds, such as caustic soda
(NaOH) or, especially, washing soda
(Na 2 CO 3 .10H 2 O).SMILES 486s