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4.1 OBJECTIVES
By the end of this unit you will be able to
• Describe isomers and explain the structural formulae for a variety of isomeric organic
compounds
• Explain various kinds of structural and stereo isomerism along with their
representation.
• Differentiate geometrical and optical isomers
• Represent three dimensional organic molecules in two dimensions
• Learn chirality, enantiomers, diastereomers and their relative/absolute configurations
• Learn the nomenclature (cis-trans, E/Z, D/L, d/l, erythro/threo and R/S) of different
stereoisomers
4.2 INTRODUCTION
Stereochemistry deals with three dimensional representation of molecule in space.
This has sweeping implications in biological systems. For example, most drugs are often
composed of a single stereoisomer of a compound. Among stereoisomers one may have
positive effects on the body and another stereoisomer may not or could even be toxic. An
example of this is the drug thalidomide which was used during the 1950s to suppress the
morning sickness. The drug unfortunately, was prescribed as a mixture of stereoisomers, and
while one stereoisomer actively worked on controlling morning sickness, the other
stereoisomer caused serious birth defects.
The study of stereochemistry focuses on stereoisomers and spans the entire spectrum
of organic, inorganic, biological, physical and especially supramolecular chemistry.
Stereochemistry includes method for determining and describing these relationships; the
effect on the physical or biological properties.
4.3 CONCEPT OF ISOMERISATION
The word isomerism originated from Greek word isomer (iso= equal; mers = part). When
two or more compounds having the same molecular formula but exhibit difference in their
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chemical and/or physical properties are called isomers and the phenomenon is known as
isomerism.
4.4 TYPES OF ISOMERISM
Generally isomerism can be divided in to two categories;
a. Structural (constitutional) Isomerism
b. Stereo (configurational) Isomerism
a. Structural (constitutional) Isomerism
Structural isomerism is also known as ‘constitutional isomerism’. Structural isomerism arises
when a molecule can be represented in to two or more than two different structures. The
difference in structure is due to the difference in the arrangement of atoms within the
molecules, irrespective of their position in space. In other words, structural isomers are
compounds those have identical molecular formulae but different structural formulae; and the
phenomenon is called structural isomerism.
Examples 1: Structural isomer of Butane (C4H10) and Bromobutane (C4H9Br)
CH3CH2CH2CH3 CH3CH2CH2CH2Br
n-Butane 1-Bromobutane
C4H10 C4H9Br
Butane CH3CHCH3 Bromobutane CH3CHCH2CH3
CH3 Br
Isobutane 2-Bromobutane
Structural isomerism can also be subdivided in to five types
1) Chain Isomerism
2) Functional Isomerism
3) Position Isomerism
4) Metamerism
5) Tautomerism
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1) Chain Isomerism: Chain isomers are those isomers having difference in the order in
which the carbon atoms are bonded to each other. In other words chain isomers have variable
amounts of branching along the hydrocarbon chain.
If you observe two or more than two molecules having same molecular formulae, but
difference in their hydrocarbon chain length, you should understand that these are
chain isomers of each other.
Example 2: Chain isomers of Butane (A) and Pentane (B)
A) C4H10 B) C5H12
Butane Pantane CH3
CH3CH2CH2CH3 CH3CHCH3 CH3CH2CH2CH2CH3 CH3CHCH2CH3 CH3CCH3
CH3 CH3 CH3
n-Butane Isobutane n-Pantane Isopetane Neopetane
2) Functional Isomerism: Two or more than two molecules those having the same
molecular formulae but have different functional groups are called functional isomers and the
phenomenon is termed as functional isomerism.
If you observe two or more than two molecules having same molecular formulae, but
difference in their functional groups, you should understand that these are functional
isomers of each other.
Example 3: Ethyl alcohol and Dimethyl ether
CH3CH2OH CH3OCH3
Ethyl alcohol Dimethyl ether
Example 4: n-Butyl alcohol and Diethyl ether
CH3CH2CH2CH2OH CH3CH2OCH2CH3
n-Butayl alcohol Diethyl ether
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3) Position Isomerism: Two or more than two molecules those having same molecular
formulae but having difference in the position of functional group on the carbon chain are
called position isomers and the phenomenon is called as position isomerism.
If you observe two or more than two molecules having same molecular formulae, but
difference in their functional groups, you should understand that these are functional
isomers of each other.
Example 5: 1-Butene and 2-Butene
CH3CH2CH CH2 CH3CH CHCH3
1-Butene 2-Butene
Example 6: 1-Butyl alcohol, 2-Butyl alcohol and t-Butyl alcohol
CH3
CH3CH2CH2CH2OH CH3CHCH2CH3 CH3C OH
OH CH3
1-Butyl alcohol 2-Butyl alcohol t-Butyl alcohol
4) Metamerism: Two or more than two molecules those having same molecular
formulae and functional group but having difference in the distribution of carbon atoms on
either side of functional group are called metamers and the phenomenon is called the
metamerism.
When you see two or more than two molecule with identical molecular formulae but
while structural representation you observe there is a difference in the alkyl group
attached to same functional group you should understand these molecules are
metamers of each other.
Example 7: Diethyl ether, Methyl propyl ether and isopropyl methyl ether
CH3
CH3CH2OCH2CH3 CH3CH2CH2OCH3 CH3CHOCH3
Diethyl ether Methyl propyl ether Isopropyl methylether
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Example 8: Diethyl amine, Methyl propyl amine and isopropyl methyl amine
CH3
CH3CH2NHCH2CH 3 CH3CH2CH2NHCH3 CH3CHNHCH3
Diethyl amine Methyl propyl amine Isopropyl methylamine
5) Tautomerism: This is a special kind of isomerism where both the isomers are
interconvertible and always exist in a dynamic equilibrium to each other. Due to their
interconversion change in functional group takes place that gives two different isomers of an
organic compound. This phenomenon is called Tautomerism.
When you observe two different isomeric forms of an organic compound are rapidly
interconvertible to each other you should recognize them as tautomer of each other.
i. Remember: Tautomers are not the resonance structure of same compound
Example 9: Acetone exists in rapid equilibrium with Prop-1-en-2-ol
O OH
CH3CCH3 CH3C CH2
Acetone Prop-1-ene-2-ol
(keto form) (enol form)
<99% >1%
Example 10: Tautomeric forms of Ethyl acetoacetate under rapid equilibrium
O O OH O
CH3CCH2COC2H5 CH3C CHCOC2H5
(keto form) (enol form)
93% 7%
b. Stereo (configurational) Isomerism
Stereoisomerism is arises due to the difference in arrangement (configuration) of atoms or
groups in space. When two or more than two isomers have the same structural formulae but
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having difference in the arrangement (configuration) of atoms in space are called stereo
isomer and the phenomenon is called stereo isomerism.
Stereo isomerism can be further classified as
i. Geometrical or cis-trans isomerism
ii. Optical isomerism
Geometrical isomerism is generally observed in alkenes and cyclic compounds due to
their restricted rotation around carbon- carbon bond. For example cis- and trans 2-
butene have same connection of bond and molecular formulae.
If you observe two similar groups are on the same side of C=C bond this is
called cis- isomer; whereas, if two similar groups are on opposite side of
C=C bond this is known as trans- isomer.
Example 11: cis- and trans- isomerism in 2-butene
H 3C CH3 H 3C H
H H H CH3
cis-2-butene trans-2-butene
You can understand that due to the presence of one σ (sigma) and one π (pi)
bond in carbon–carbon double bond, rotation around C=C bond is not
possible. The restricted rotation around C=C bond is responsible for
geometrical isomerism in alkenes.
You can easily observe that rotation around C-C bond is also not possible in cyclic
compounds as the rotation would break the bonds and break the ring. Thus geometrical
isomerism is also possible in cyclic compounds.
Example 12: cis- and trans- isomers of 1,2-dimethylcyclopropane
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CH3 CH3
H H
CH3 H
H
cis trans CH3
4.5 OPTICAL ISOMERISM
Optical isomerism is another class of stereoisomerism. The organic compounds that
exhibit optical isomerism must have a unique ability to rotate the plane polarized light either
towards left or towards right hand directions. This unique ability is generally known as
optical activity. Optical activity of any compound is measured by analyzing the sample in an
instrument called Polarimeter. A solution of known concentration of optically active
compound is when exposed to the beam of plane polarized light, the beam of plane polarized
light is rotated through a certain number of degrees, either to the clockwise (right) direction
or anti-clockwise (left) direction. The compound which rotates the plane polarized light
towards clockwise direction is called to be dextrorotatory (represented by +); whereas, the
compound which rotates the plane polarized light towards anti-clockwise direction is called
to be levorotatory (represented by -). Figure 1 shows the schematic representation of
polarimeter.
Figure 1. Schematic representation of simple polarimeter
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The degree of rotation depends upon the number of the molecules of the
compounds falls in the path of beam. To compare the rotating power of different
optically active compounds, the specific rotation of each compound is calculated
and then comparison should be made.
Specific rotation is defined as the degree of rotation offered for the given
wavelength of plane polarized light at given temperature by a solution of 1g/mL
concentration is filled in a 10 cm length sample cell. Specific rotation is
represented by and can be calculated as
Where α is observed angle of rotation; t is the temperature of during experiment;
λ is the wavelength of light used; l is the length of the tube in decimeter; and c is
the concentration of the compounds per 100 mL of solution.
ii. Remember:
Optically active compounds always exist in two isomeric forms which rotates the plane
polarized light by equal degrees in opposite directions. The optical isomer which
rotates the plane polarized light towards right (clockwise direction) is known as
Dextrorotatory Isomer or (+)-isomer, whereas, the optical isomer which rotates the
plane polarized light towards left (anticlockwise direction) is known as Levorotatory
Isomer or (-)-isomer.
4.5.1 Elements of symmetry:
All optically active molecules/object are chiral and they exhibit enantiomerism (Figure 2). A
chiral molecule is that which cannot be superimposed on its mirror image; however, both the
non-superimposable isomers are called enantiomers. We will learn more about chirality and
enantiomerism in separate section of this unit.
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Figure 2. (a) Non superimposable mirror image relationship of right and left hands. (b)
Ball and stick model of tetravalent chiral carbon atom.
Elements of symmetry are a simple tool to identify whether a molecule is chiral or not. The
necessary condition for optically active molecule to be chiral is that, the molecule should not
possess any kind of symmetry elements. The elements of symmetry are generally categorized
as follows:
(i) Simple axis of symmetry (Cn)
(ii) Plane of symmetry (σ)
(iii) Centre of symmetry (Ci)
(iv) Alternating axis of symmetry(Sn)
(i) Simple axis of symmetry (Cn):
When a rotation of 360°/n (where n is any integer like 1,2,3…etc.) around the axis of a
molecule or object is applied, and the rotated form thus obtained is non-differentiable
from the original, then the molecule/object is known to have a simple axis of symmetry. It
is represented by Cn.
Example 13: Water molecule has C2 (two fold axis of symmetry) whereas chloroform has C3
axis of symmetry.
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H
O
C
Cl
H H Cl
Cl
C2 C3
water Chloroform
From above example you can
easily understand that if you rotate the water molecule by 180° (i.e. 360°/2=180°)
along its molecular axis you will get the identical (non-differentiable) form of
water molecule, hence water molecule has two fold of symmetry. Similarly, if you
rotate the chloroform molecule by 120° (i.e. 360°/3=120°) along its molecular
axis you will get the identical (non-differentiable) form of chloroform molecule,
hence chloroform molecule has three fold of symmetry.
(ii) Plane of symmetry (σ):
It is defined as ‘when a plane that devised a molecule or object in to two equal halves
which are related to object and mirror image is known as plane of symmetry. It is
represented by σ.
Example 14: Plane of symmetry in Tartaric acid
Palne that devides
d molecule in two equal halves COOH
a C b H C OH
a C b H C OH
d COOH
Sybmbolic representation 2,3-dihydroxysuccinic acid
(Tartaric acid)
From above example you can
easily understand that if we put a mirror plane/reflection plane exactly at the
centre axis of the molecule/object; you will found that the mirror image thus
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obtained is the complementary of the original and both will give us the
appearance of complete molecule/object.
COOH
Real object
H C OH
H C OH
Mirror palne that devides Mirror image of
molecule in two equal halves COOH real object
2,3-dihydroxysuccinic acid
(Tartaric acid)
(iii) Centre of symmetry (Ci): A molecule has a centre of symmetry when, for any
atom in the molecule, an identical atom exists diametrically (diagonally) opposite
to this centre and at equal distance from it.
Example 15: An isomer of 1,3-dichloro-2,4-dibromocyclobutane has a centre of symmetry
H Cl
Br H
H Br
Cl H
Center of Symmetry
(Ci)
From above example you may
understand that all the identical atoms are situated diagonally and at equal
distance from the centre. This is called centre of symmetry.
(iv) Alternating axis of symmetry (Sn): An alternate axis of symmetry is defined
as, when a molecule is rotated by 360°/n degrees about its axis and then a
reflection plane is placed exactly at perpendicular to the axis, and the reflection of
the molecule thus obtained is identical to the original. It is represented by Sn.
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Example 16. An isomer of 1,3-dichloro-2,4-dibromocyclobutane has a 2 fold alternate axis
of symmetry
H Cl Br H
Br H H Cl
180o roation
H Br Cl H
about axis
Cl H H Br Mirror Plane perpendicular to
axis of rotation
H Cl
Same
Br H
H Br
Cl H
4.6 MOLECULAR CHIRALITY, ENANTIOMERS
The necessary condition for a molecule to have optical isomerism is that molecule should not
have any kind of symmetry elements present in it, in other words the molecule should be
dissymmetric. Such molecules are called ‘Chiral’ and the property is called ‘molecular
chirality’. Optically active chiral molecules which are non-superimposable on their mirror
images are called ‘enantiomers’ and the phenomenon is known as ‘enantiomerism’. To
exhibit optical isomerism an organic compound must have at least one asymmetric carbon
atom. An asymmetric carbon atom is that which is bonded to four different atoms or groups.
We can easily understand the
chirality by comparing our hands (left hand and right hand). Our left hand and
right hand are the best example of non-superimposable mirror image of each
other. Each hand is therefore considered as chiral.
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iii. Remember: Our left hand and
right hand are non-superimposable mirror image of each other each one of them
is chiral.
iv. Remember: Chirality is the
necessary and sufficient condition for the existence of enantiomers.
Example 17. Tartaric acid has two asymmetric carbon and it exists in four forms, out of them
two form are optically active and two are optically inactive.
Two asymmetric
COOH carbon atoms of
chiral Tartaric acid
H C* OH
*
H C OH
COOH
Mirror plane divides molecule in two equal
halves
COOH COOH COOH COOH
HO C H H C OH H C OH HO C H
H C OH HO C H H C OH HO C H
COOH COOH COOH COOH
Non-superimposable mirror images Non-superimposable mirror images
(Enantiomers) with each one having plane of symmetry
optically inactive
4.6.1 Stereogenic Centre:
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As we discussed in previous section that if a molecule contains one carbon atom which is
directly bonded with four different groups or atoms, and the molecule do not have any kind
of symmetry element present in it, such molecule is called asymmetric or chiral.
When the interchange of the position of two directly bonded groups or atoms of a centre
carbon atom results a new stereoisomer, such chiral centre is called stereo centre or
stereogenic centre.
If the new stereoisomer is a non-superimposable mirror image of the original molecule such
carbon centre is called chiral carbon centre.
Remember: All the chiral centres are stereogenic centres but all stereogenic
centres are not chiral centre.
Example 18: Bromochlorofluoroidomethane exhibits chiral carbon centre
interchange
F F and Cl Cl
I C Cl I C F
Br Br
Interchange of F and Cl results
non-superimposable stereoisomers
4.7 OPTICAL ACTIVITY
It is already known to you (from section 4.5) that the optical activity is an ability of a chiral
molecule to rotate the plane of plane-polarized light either towards left or right direction. The
rotation is measured by an instrument called Polarimeter. When a beam of plane polarized
light passes through a sample that can rotate the plane polarized light, the light appears to
dim because it no longer passes straight through the polarizing filters. The amount of
rotation is quantified as the number of degrees that the analyzing lens must be rotated to
observe the no dimming of light appears. Optical rotation can be measured by using the
following formulae
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Where α is observed angle of rotation; t is the temperature of during experiment; λ is the
wavelength of light used; l is the length of the tube in decimeter; and c is the concentration of
the compounds per 100 mL of solution.
Optically active chiral compounds that are non-superimposable mirror image of each other
are called enantiomers.
4.7.1 Properties of enantiomers:
The main properties of enantiomers are given as follow
Enantiomers always exist in pair
Enantiomers are non-superimposable mirror image to each other
Enantiomers have same physical properties (like boiling point, melting point,
solubility, density, viscosity, refractive index etc.) and chemical properties in achiral
environment
Each enantiomer have opposite behavior with respect to plane polarized light, if one
of them will rotate the plane polarized light towards right hand direction then
definitely the other will rotate the plane polarized light towards left hand direction.
Each enantiomer shows the same chemical reactivity with achiral reagent; however
they have different reactivity with chiral reagent.
Example 19: Glyceraldehyde molecule is a chiral molecule. It has a pair of enantiomer with
same physical properties except their behavior towards plane polarized light
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Mirror CHO
CHO
HO C H H C OH
CH2OH CH2OH Enantiomeric pair of
Glycerldehyde
Molecular formula : C3H6O3 Molecular formula : C3H6O3
Mol. Wt.: 90.08 Mol. Wt.: 90.08
Boiling point : 228oC Boiling point : 228oC
Melting point : 145 °C Melting point : 145 °C
Density : 1.455 g/cm3 Density : 1.455 g/cm3
You can see that the glyceraldehyde molecule can exists in two enantiomeric
forms which differ only in the arrangement of bonded atoms around the centre
chiral carbon. The physical properties (like molecular formula, molecular
weight, melting point, boiling point and density etc.) of both the isomers are
same. But if one isomer will rotate the plane polarized light towards right hand
direction (dextrorotatory) then the other one will rotate the plane polarized
light towards left hand direction (levorotatory).
4.7 CHIRAL AND ACHIRAL MOLECULES WITH TWO
STEREOGENIC CENTRES
As we have discussed earlier in this unit (sec. 4.6) that chiral molecules are those in which
the centre carbon atom is bonded directly through four different atoms/groups and do not
have any kind of symmetry element present in it and the molecule has non-superimposable
mirror image. However, those molecule in which centre carbon atom is directly bonded
through four different atoms of groups and it satisfied any kind of symmetry elements are
called achiral molecule. Achiral molecules have superimposable mirror images.
Let us consider the stereoisomers of Tartaric acid which has two stereo centres with identical
atoms/groups attached to both the stereo centres. The tartaric acid have two stereo centres
and can have four stereoisomers out of which two stereoisomers are non-superimposable
mirror image of each other called enantiomers and chiral; and rest two are identical to each
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