Isomerism
Teacher: Jelena Marinković
School: Gimnazija, Banja Luka
Students age: 18
This lesson plans should be read in conjunction with the power point in Additional Materials
Introduction:
 Isomers are different compounds that have the same molecular formula. Different
compounds means that they have different physical properties (melting point, boiling
point etc.). They may also have very different chemical properties depending on the type
of isomerism present. Isomers may be divided into
 structural isomers, in which the atoms are joined in a different order, so that they have
different structural formulae
 stereoisomerism, in which the order in which the atoms are joined is the same, but the
molecules have a different spatial arrangement of the atoms and hence different three
dimensional shapes.
 Students should realize that different structural formulas can be made from the same
molecular formula.
 The aim is developing the logical way of thinking, in the propose that the knowledge
could be used in everyday life.
 The students should be able to gain this knowledge in the further field of biochemistry,
e.g. proteins, nucleid acids etc.
First class: Isomerism-revision
Strategies:
 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
Introduction:
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.
The relationship of different types of isomers
Isomers
Structural Isomers
Different bonding
Positional
Isomers
Hydrocarbon
Chain Isomers
Stereoisomers
Different shapes
Functional
Group Isomers
Geometric
Enantiomers
Isomers
Discussing about isomerism:
Students should explain different types of isomerism, and how that influence to the physical and
chemical properties of compounds.
Working in the groups:
Students are divided in four groups.
First group has a task to make different structural 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 them.
Second class: Detailed explanation of optical isomerism
Strategies:
 Collaborative work amongst students and between students and teacher.
 Diversity of teaching methods, resources and aids
See power point files for group work
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. The asymmetric carbon atom has a * on it:
NH2
NH2
NH2
Mirror images
Rotate by 180o
*C
*C
COOH
CH3
H
*C
HOOC
H
H
CH3
CH3
COOH
Note that all four groups must be different for this to occur so that glycine (H2N-CH2-COOH), in which the
methyl group of alanine has been replaced by a second hydrogen atom, does not exist in enantiomeric
forms. All other 2-amino acids exist as two enantiomeric forms. 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 (see below). 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.
Double bonds comprise a σ-bond and a π-bond (see section 4.6). Because the π-bond involves two regions
of high electron density on opposite sides of the bond, atoms joined by a double bond are not free to rotate
because rotation would involve breaking this π-bond. This means that if the form, produced by rotating one
end of the bond by 180o relative to the other in a molecule containing double bond, is not identical to the
original, then there can be two separate forms of the molecule. This is illustrated below using
1,2-dichloroethene as an example. The form which has two similar groups on the same side of the double
bond is known as the cis- isomer, the other the trans- isomer:
Cl
cis-1,2-dichloroethene
│
Cl
Cl
│
│
│
H
trans-1,2-dichloroethene
C
C
C
C
│
│
│
│
H
H
H
Cl
Frequently, as in this case, the cis-isomer will have a dipole, whilst the more symmetrical trans-isomer will
not. This affects physical properties such as the boiling points, the cis-isomer boiling at 333 K the transisomer at 321 K. Usually the chemical properties are similar, but in some cases the proximity of the
functional groups allows interaction in the cis-isomer more easily than in the trans-isomer. For example cisbutenedioic acid dehydrates at under 470 K to form the anhydride. The trans-isomer sublimes unchanged
at ~470 K. Dehydration of the trans isomer requires a much higher temperature, so as to achieve the
activation energy required for rotation about the double bond, and so gives the same product as the cisisomer.
H - C - CO - OH
H - C - CO

║
H - C - CO - OH
║
O + H2O
H - C - CO
Cycloalkanes also display geometrical isomerism because being part of a ring prevents rotation of carboncarbon single bonds, hence 1,2-dichlorocyclopropane can exist as both cis-and trans- isomers:
Cl
Cl
C
C
H
Cl
H
│
│
C
C
C
C
│
│
│
│
│
H
H
H
│
H
H
H
Cl
As explained above, 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.
Illustrating plane polarised light and
the effect of optically active compounds
Optically inactive
Output when second filter
Vertical
Incident
Vertical
normal polarising
light
filter
Second Light
polarising
filter
maximum
passing
through
none
Horizontal
Optically active
Output when second filter
Maximum light at an angle
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.
Third class: Monosaccharides and disaccharides
Strategies:
 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
See 2nd power point file for homework using Internet sites
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. Structures for the D and L isomer of the simplest aldose, glyceraldehyde, are
shown below.
D-Glyceraldehyde L-Glyceraldehyde
The structures of many monosaccharides were first determined by Emil Fischer in the 1880s and
1890s and are still written according to a convention he developed. The Fischer projection
represents what the molecule would look like if its three-dimensional structure were projected
onto a piece of paper. By convention, Fischer projections are written vertically, with the
aldehyde or ketone at the top. The -OH group on the second-to-last carbon atom is written on the
right side of the skeleton structure for the D isomer and on the left for the L isomer. Fischer
projections for the two isomers of glyceraldehyde are shown below.
D-Glyceraldehyde L-Glyceraldehyde
These Fischer projections can be obtained from the skeleton structures shown above by imaging
what would happen if you placed a model of each isomer on an overhead projector so that the
CHO and CH2OH groups rested on the glass and then looked at the images of these models that
would be projected on a screen.
Fischer projections for some of the more common monosaccharides are given in the figure
below.
ALDOSES
D-ribose
D-xylose
D-arabinose
D-glucose
KETOSES
D-ribulose
D-fructose
D-galactose
D-mannose
If the carbon chain is long enough, the alcohol at one end of a monosaccharide can attack the
carbonyl group at the other end to form a cyclic compound. When a six-membered ring is
formed, the product of this reaction is called a pyranose, shown in the figure below.
When a five-membered ring is formed, it is called a furanose, shown in the figure below.
+
D-ribose
a-D-ribofuransoe
b-D-ribofuranose
+
D-fructose
a-D-fructofuranose
b-D-fructofuranose
There are two possible structures for the pyranose and furanose forms of a monosaccharide,
which are called the a- and b-anomers.
The reactions that lead to the formation of a pyranose or a furanose are reversible. Thus, it
doesn't matter whether we start with a pure sample of a-D-glucopyranose or b-D-glucopyranose.
Within minutes, these anomers are interconverted to give an equilibrium mixture that is 63.6% of
the b-anomer and 36.4% of the a-anomer. In the b-anomer, all of the bulky -OH or -CH2OH
substituents lie more or less within the plane of the six-membered ring. In the a-anomer, one of
the -OH groups is perpendicular to the plane of the six-membered ring, in a region where it feels
strong repulsive forces from the hydrogen atoms that lie in similar positions around the ring. As
a result, the b-anomer is slightly more stable than the a-anomer.
Carbohydrates: The Disaccharides and Poly-Saccharides
Disaccharides are formed by condensing a pair of monosaccharides. The structures of three
important disaccharides with the formula C12H22O11 are shown in the figure below.
Maltose, or malt sugar, which forms when starch breaks down, is an important component of the
barley malt used to brew beer. Lactose, or milk sugar, is a disaccharide found in milk. Very
young children have a special enzyme known as lactase that helps digest lactose. As they grow
older, many people lose the ability to digest lactose and cannot tolerate milk or milk products.
Because human milk has twice as much lactose as milk from cows, young children who develop
lactose intolerance while they are being breast-fed are switched to cows' milk or a synthetic
formula based on sucrose.
The substance most people refer to as "sugar" is the disaccharide sucrose, which is extracted
from either sugar cane or beets. Sucrose is the sweetest of the disaccharides. It is roughly three
times as sweet as maltose and six times as sweet as lactose. In recent years, sucrose has been
replaced in many commercial products by corn syrup, which is obtained when the
polysaccharides in cornstarch are broken down. Corn syrup is primarily glucose, which is only
about 70% as sweet as sucrose. Fructose, however, is about two and a half times as sweet as
glucose. A commercial process has therefore been developed that uses an isomerase enzyme to
convert about half of the glucose in corn syrup into fructose. This high-fructose corn sweetener is
just as sweet as sucrose and has found extensive use in soft drinks.
The monosaccharides and disaccharides represent only a small fraction of the total amount of
carbohydrates in the natural world. The great bulk of the carbohydrates in nature are present as
polysaccharides, which have relatively large molecular weights. The polysaccharides serve two
principal functions. They are used by both plants and animals to store glucose as a source of
future food energy and they provide some of the mechanical structure of cells.
Very few forms of life receive a constant supply of energy from their environment. In order to
survive, plant and animal cells have had to develop a way of storing energy during times of
plenty in order to survive the times of shortage that follow. Plants store food energy as
polysaccharides known as starch. There are two basic kinds of starch: amylose and amylopectin.
Amylose is found in algae and other lower forms of plants. It is a linear polymer of
approximately 600 glucose residues whose structure can be predicted by adding a-Dglucopyranose rings to the structure of maltose. Amylopectin is the dominant form of starch in
the higher plants. It is a branched polymer of about 6000 glucose residues with branches on 1 in
every 24 glucose rings. A small portion of the structure of amylopectin is shown in the figure
below.
Amylose
n = 1000 - 6000
The polysaccharide that animals use for the short-term storage of food energy is known as
glycogen. Glycogen has almost the same structure as amylopectin, with two minor differences.
The glycogen molecule is roughly twice as large as amylopectin, and it has roughly twice as
many branches.
There is an advantage to branched polysaccharides such as amylopectin and glycogen. During
times of shortage, enzymes attack one end of the polymer chain and cut off glucose molecules,
one at a time. The more branches, the more points at which the enzyme attacks the
polysaccharide. Thus, a highly branched polysaccharide is better suited for the rapid release of
glucose than a linear polymer.
Polysaccharides are also used to form the walls of plant and bacterial cells. Cells that do not have
a cell wall often break open in solutions whose salt concentrations are either too low (hypotonic)
or too high (hypertonic). If the ionic strength of the solution is much smaller than the cell,
osmotic pressure forces water into the cell to bring the system into balance, which causes the cell
to burst. If the ionic strength of the solution is too high, osmotic pressure forces water out of the
cell, and the cell breaks open as it shrinks. The cell wall provides the mechanical strength that
helps protect plant cells that live in fresh-water ponds (too little salt) or seawater (too much salt)
from osmotic shock. The cell wall also provides the mechanical strength that allows plant cells to
support the weight of other cells.
The most abundant structural polysaccharide is cellulose. There is so much cellulose in the cell
walls of plants that it is the most abundant of all biological molecules. Cellulose is a linear
polymer of glucose residues, with a structure that resembles amylose more closely than
amylopectin, as shown in the figure below. The difference between cellulose and amylose can be
seen by comparing the figures of amylose and cellulose. Cellulose is formed by linking bglucopyranose rings, instead of the a-glucopyranose rings in starch and glycogen.
Cellulose
n = 5000 - 10,000
The -OH substituent that serves as the primary link between -glucopyranose rings in starch and
glycogen is perpendicular to the plane of the six-membered ring. As a result, the glucopyranose
rings in these carbohydrates form a structure that resembles the stairs of a staircase. The -OH
substituent that links the b-glucopyranose rings in cellulose lies in the plane of the six-membered
ring. This molecule therefore stretches out in a linear fashion. This makes it easier for strong
hydrogen bonds to form between the -OH groups of adjacent molecules. This, in turn gives
cellulose the rigidity required for it to serve as a source of the mechanical structure of plant cells.
Cellulose and starch provide an excellent example of the link between the structure and function
of biomolecules. At the turn of the century, Emil Fischer suggested that the structure of an
enzyme is matched to the substance on which it acts, in much the same way that a lock and key
are matched. Thus, the amylase enzymes in saliva that break down the a-linkages between
glucose molecules in starch cannot act on the b-linkages in cellulose.
Most animals cannot digest cellulose because they don't have an enzyme that can cleave blinkages between glucose molecules. Cellulose in their diet therefore serves only as fiber, or
roughage. The digestive tracts of some animals, such as cows, horses, sheep, and goats contain
bacteria that have enzymes that cleave these b-linkages, so these animals can digest cellulose.
For many years, biochemists considered carbohydrates to be dull, inert compounds that filled the
space between the exciting molecules in the cell
the proteins. Carbohydrates were impurities
to be removed when "purifying" a protein. Biochemists now recognize that most proteins are
actually glycoproteins, in which carbohydrates are covalently linked to the protein chain.
Glycoproteins play a particularly important role in the formation of the rigid cell walls that
surround bacterial cells.
Fourth class: Determining the monosaccharides and stereoisomers
Strategies:
 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

Practical work in tandem with theory.
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.
Exemplar:
Conclusion:
 Students 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.
References:
 Julijana Petrović, Smiljana Velimirović; “Hemija za 4. Razred gimnazije prirodnomatematičkog I opšteg smjera” , Zavod za udžbenike I nastavna sredstva, Srpsko
Sarajevo, 2002.
 http://en.wikipedia.org/wiki/Monosaccharide
 http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/Carbohydrates.html
 http://chemistry2.csudh.edu/rpendarvis/monosacch.html
 http://www.chemguide.co.uk/basicorg/isomerism/optical.html
 http://en.wikipedia.org/wiki/Isomer