stereoregular 504s
stereoregular Describing a *poly-
mer that has a regular pattern of side
groups along its chain.stereospeciÜcDescribing chemi-
cal reactions that give products with
a particular arrangement of atoms in
space. An example of a stereospeciÜc
reaction is the *Ziegler process for
making polyethene.steric effect An effect in which
the rate or path of a chemical reac-
tion depends on the size or arrange-
ment of groups in a molecule.
steric hindranceAn effect in
which a chemical reaction is slowed
down or prevented because large
groups on a reactant molecule hinder
the approach of another reactant
molecule.Stern–Gerlach experiment An
experiment demonstrating the space
quantizationof rotating bodies,Ürst
conducted by the German scientists
Otto Stern (1888–1969) and Walther
Gerlach (1899–1979) in 1921. Their
experiment involved passing a beam
of silver atoms through an inhomo-
geneous magneticÜeld. The purpose
of the experiment was to investigate
the interaction between a charged ro-
tating body and theÜeld. A charged
rotating body acts like a magnet. In
classical mechanics the orientation of
angular momentum can have any
value so that the magnet associated
with it can take any orientation.
However, in quantum mechanics an-gular momentum is quantized,
which means that the associated
magnet can only lie in certain dis-
crete orientations. As expected, sharp
bands of atoms are observed, pro-
vided that the beam has low inten-
sity. The low intensity reduces the
blurring effects caused by collisions
between atoms.steroid Any of a group of lipids de-
rived from saturated compound
called cyclopentanoperhydrophenan-
threne, which has a nucleus of four
rings (see formula). Some of the most
important steroid derivatives are the
steroid alcohols, or sterols. Other
steroids include the bile acids, which
aid digestion of fats in the intestine;
the sex hormones (androgens and oe-
strogens); and the corticosteroid hor-
mones, produced by the adrenal
cortex. *Vitamin D is also based on
the steroid structure.
A- Information about IUPAC nomenclature
sterol Any of a group of *steroid-
based alcohols having a hydrocarbon
side-chain of 8–10 carbon atoms.
Sterols exist either as free sterols or
as esters of fatty acids. Animal sterols
(zoosterols) include cholesterol and
lanosterol. The major plant sterol
(phytosterol) is beta-sitosterol, while
fungal sterols (mycosterols) include
ergosterol.
stimulated emissionSee induced
emission; laser.
Steroid