Extended materials
Jelena Marinković
The existence of chemical compounds that have the same
molecular formulae but different molecular structures or
different arrangements of atoms in space is called
isomerism. In structural isomerism the molecules have
different types of compound or they make simply differ in
the position of the functional group in the molecule.
Structural isomerism generally have different physical and
chemical properties. In stereoisomerism , the isomers have
the same formula and functional groups, but differ in the
arrangement of groups in space. Optical isomerism is one
form of this. Another type is cis-trans isomerism, in which
the isomers have different positions of groups with respect
to a double bond or ring or central atom.
Isomers
Structural Isomers
Different bonding
Stereoisomers
Different shapes
Students should explain different types of isomerism,
and how that influence to the physical and chemical
properties of compounds.
 Students are divided in four groups.
 First group has a task to make different structural
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isomers of a molecular formula C8H18.
Second group has a task to make different cis-trans
isomers of octen.
Third group has a task to make different isomers of
dimetil cyclo pentane.
Fourth group has a task to make different optical
isomers of buthanol, penthanol and hexanol.
At the end of the class all the groups have to present
their results and discuss about the the results.
 Collaborative work amongst students and between
students and teacher.
 A participatory approach to class, with students taking
responsibility for contributing.
 Diversity of teaching methods, resources and aids.
 If a carbon atom has four different groups attached to
it then there are two different ways in which these
groups can be arranged around this carbon atom,
which is known as a chiral or asymmetric carbon atom.
The two forms of the molecule, which are known as
enantiomers, are mirror images of each other that
cannot be superimposed (like a pair of gloves),. This is
illustrated below using 2-aminopropanoic acid
(alanine) as an example.
 Because these molecules are so similar, there is very
little difference in their physical and chemical
properties. In fact the only difference is that they have
differing effects on polarised light, one isomer rotating
the plane of polarisation clockwise, the other
anticlockwise . Biological systems are much more
sensitive to the shape of the molecule and so the
different enantiomers usually have different biological
effects, for example, one isomer of aspartame tastes
sweet, but the other enantiomer tastes bitter.
 If a molecule has four different groups attached to a single
carbon atom, then the compound can exist as a pair of
enantiomers. The only difference in the properties of these
compounds is in their interaction with plane polarised light.
Plane polarised light can be considered to be light in which the
oscillation of the wave is restricted to one plane, say the vertical.
This can be achieved by passing the light through a polarising
filter. If the light is now passed through a second polarising
filter orientated in the same direction (e.g. vertical) then there is
virtually 100% transmission . If the second polarising filter has
its axis at right angles to the first (e.g. horizontal) then no light
will pass. A pure enantiomer placed between the two filters will
rotate the plane of polarisation in one direction (say clockwise)
so that maximum transmission is no longer when the second
filter is aligned with the first one. The second enantiomer will
rotate the plane of polarisation by exactly the same amount but
in the opposite direction (anticlockwise). Substances that affect
polarised light in this way are said to be optically active. An
instrument containing two polarising filters that can be rotated
relative to each other, so as to allow the angle between their
orientations to be measured is called a polarimeter.
 The other enantiomer would rotate it an equal amount but
anticlockwise. Apart from this the properties of enantiomers are
identical. Chemically the behaviour of the enantiomers is
identical unless the reaction also involves a pure enantiomer.
 Chemical reactions that produce an asymmetric carbon atom in
a molecule give rise to a mixture containing exactly equal
amounts of the two enantiomers. Such a mixture is known as a
racemic mixture. The effects of the two enantiomers in a
racemic mixture cancel each other out and so it is not optically
active. In contrast almost all natural products, produced by
enzyme catalysed biochemical processes, result in just one pure
enantiomer and hence produce optically active material. Pure
turpentine (produced from pine tree resin) can, for example, be
differentiated from white spirit (a substitute produced by the
chemical industry) because turpentine is optically active and will
rotate the plane of polarised light, whereas white spirit will not.
 Collaborative work amongst students and between
students and teacher.
 Diversity of teaching methods, resources and aids.
 The term carbohydrate was originally used to describe compounds that
were literally "hydrates of carbon" because they had the empirical
formula CH2O. In recent years, carbohydrates have been classified on
the basis of their structures, not their formulas. They are now defined
as polyhydroxy aldehydes and ketones. Among the compounds that
belong to this family are cellulose, starch, glycogen, and most sugars.
 There are three classes of carbohydrates: monosaccharides,
disaccharides, and polysaccharides. The monosaccharides are white,
crystalline solids that contain a single aldehyde or ketone functional
group. They are subdivided into two classes aldoses and ketoses on the
basis of whether they are aldehydes or ketones. They are also classified
as a triose, tetrose, pentose, hexose, or heptose on the basis of whether
they contain three, four, five, six, or seven carbon atoms.
 With only one exception, the monosaccharides are optically active
compounds. Although both D and L isomers are possible, most of the
monosaccharides found in nature are in the D configuration.
 Students should find different sources of information
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about sugars and isomerism
Results:
http://en.wikipedia.org/wiki/Monosaccharide
http://users.rcn.com/jkimball.ma.ultranet/BiologyPag
es/C/Carbohydrates.html
http://chemistry2.csudh.edu/rpendarvis/monosacch.
html
http://www.chemguide.co.uk/basicorg/isomerism/opt
ical.html
http://en.wikipedia.org/wiki/Isomer
 Collaborative work amongst students and between
students and teacher.
 Diversity of teaching methods, resources and aids.
 Subject material with “real life application” and
connections to the world outside the classroom.
 Students are in four groups. Each group gets cards with
structural formulae of monosaccharides and their
names at separated cards. They have to link structures
with names, and to determine α and β stereoisomers.
After that, they have to make disaccharides making
different bonds between those monosaccharides .
 At the end of the class groups have to exchange their
papers and make corrections if they are necessary.
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 Collaborative work amongst students and between
students and teacher.
 Diversity of teaching methods, resources and aids.
 Practical work in tandem with theory.
 Student were very interested in this topic. They prefer
working within a group. They are looking forward
going to the Science University, to do laboratory
investigations.
 I expect that test results are going to be better than
usually, because I notice that they are a little bit more
involved in learning.
 The most important is that students realized how
small number of monosaccharides, making different
isomers and different bond, produce a large number of
di- and polisaccharides.