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c h A P T e r 2 u nomenclature of Organic compounds
OH
CH
OH
CH
3
O
O
O
CH
2
CH
3
HC
3
HC
2
131
17. The iuPAc name of the compound
is
(a) 3-methylcyclobut-1-en-2-ol (b) 4-methylcyclobut-2-en-1-ol
(c) 4-methylcyclobut-1-en-3-ol (d) 2-methylcyclobut-3-en-1-ol
18. The iuPAc name of the compound
is
(a) 4-methylcylopent-1-en-2-ol (b) 2-methylcyclopent-4-en-1-ol
(c) 3-methylcyclopent-1-en-2-ol (d) 5-methylcyclopent-2-en-1-ol
19. The correct iuPAc name of is
(a) isopropylbenzene (b) cumene
(c) Phenyl isopropene (d) none of these
20. The correct order of priority for the –CONH2, –CN and –COOR is
(a) –CONH2, –COOR, –CN (b) –COOR, CONH2, –CN
(c) –CN, –COOR, –CONH2 (d) –CN, CONH2, –COOR
21. The iuPAc for the hydrocarbon represented by the swastic sign is
(a) neononane (b) Tetraethylcarbon
(c) 2-ethylpentane (d) 3,3-Diethylpentane
22. The iuPAc name for the compound
is
(a) Propionic anhydride (b) Dipropionic anhydride
(c) ethoxypropanoic acid (d) Propanoic anhydride
132
PhArmAceuTicAl OrgAnic chemisTry
CH
23. The iuPAc name for the compound
(a) ethyl acrylate (b) ethyl methylbutenoate
(c) ethyl acetoethenoate (d) ethyl-3-methylbut-3-enoate
24. The correct iuPAc name for the following compound is
3
O
COOC2H
is
5
H3C CH
CH
3
3
(a) sym-Trimethylbutanone (b) 1,3,5-Trimethylbenzophenone
(c) 1-Keto-2,4-6-Trimethylcyclohexane (d) 2,4,6-Trimethylcyclohexanone
ANSWERS
1. (d) 2. (b) 3. (c) 4. (c) 5. (c) 6. (c) 7. (a) 8. (b) 9. (d) 10. (c) 11. (c) 12. (b)
13. (a) 14. (d) 15. (d) 16. (c) 17. (b) 18. (d) 19. (a) 20. (b) 21. (d) 22. (d) 23. (d) 24. (d)
StereochemiStry
R R
Enzyme
Only one enantiomer matches
chiral enzyme
Stereochemistry is a very important facet of chemistry
3
3.1 Introduction, 3.2 Isomerism, 3.3 Structural Isomerism, 3.4 Stereoisomerism, 3.5 Optical Isomerism, 3.6 Concept of Chiraility, 3.7 Types of
Optical Isomers, 3.8 Planar Representation of Three-Dimensional Formulae: Fischer Projection Formulae, 3.9 Diastereomerism: Compounds Having
More Than One Chiral Carbon, 3.10 Meso Compounds, 3.11 Number of Possible Stereoisomers in Compounds Containing Different Number of
Chiral Centres, 3.12 Racemization, 3.13 Internal and External Compensation, 3.14 Resolution of Racemic Modifications, 3.15 Specification of
Configuration, 3.16 Specification of Configuration by
3.1 INTRODUCTION
Organic chemistry is the branch of chemistry devoted to the study of the compounds of carbon. What is
special about the compounds of carbon that they should be exclusively studied compared to other elements of
the periodic table? The simple answer is that there are very large number of compounds of carbon resulting
from molecules that are large and complex. Organic molecules containing thousands of atoms are known
and the arrangement of atoms in these molecules and even in small molecules can be very complicated. One
of the major problems in organic chemistry is to nd out how the atoms are arranged in molecules, i.e. to
determine the structure of organic compounds. Each different arrangement of atoms corresponds to a different
compound and each compound has its own characteristic set of chemical and physical properties.
There is denitely a relationship between the molecular structure, i.e. arrangement of atoms or groups of
atoms in molecule, and its properties.
Learning Plus
The prefix ‘stereo’ means ‘three-dimensional’, so
stereochemistry is also known as 3D chemistry.
and the study of stereochemical problems spans the
entire range of organic, inorganic, pharmaceutical,
biological, physical, and supramolecular chemistry.
–Emil Fischer
Chapter Outline
R
and S Notations, 3.17 Geometrical Isomerism, 3.18 Conformation of Alkanes
Compounds that have the same molecular formula, but
different structural formula are called structural isomers and the
compounds having different conguration, i.e. different relative
position of atoms or groups in space, but the same molecular and
structural formulae are called structural stereoisomers. In this
chapter, we shall deal with different types of structural as well as
stereoisomers of organic compounds.
3.2 ISOMERISM
In organic chemistry, we nd numerous organic compounds having same molecular formula, differing in
their physical and chemical properties. Such compounds are called isomers (in Greek iso means ‘the same’
and meros means ‘parts’), and the property exhibited by such compounds is known as isomerism.
ISOMERISM
Structural isomerism
(Structural isomers have
different structure formulae,
i.e. different arrangement of atoms
within the molecule.)
Stereo or space isomerism
(Stereomers have the same structure
formula but different configurations,
i.e. different arrangement of atoms
or groups in space.)
Chain or
nuclear
isomerism
Position
isomerism
Functional
isomerism
Metamerism Tautomerism
Geometrical
isomerism
Optical
isomerism
Conformational
isomerism
134
PharMaCEuTICal OrGanIC ChEMISTry
As different compounds possess the same molecular formula, the difference in their physical and
chemical properties is attributed to varying internal structures of the molecules. There are two main types
of isomerism, as given below:
1. Structural isomerism
2. Stereoisomerism
1. Structural isomerism: It is due to different arrangement of atoms within the molecule. Structural
isomerism is further divided into the following categories:
a. Chain isomerism
b. Position isomerism
c. Functional isomerism
d. Metamerism
e. Tautomerism
2. Stereoisomerism: It is due to different arrangement of atoms or groups around the carbon atom in
space. Stereoisomers have the same molecular as well as structural formulae but possess different
spatial arrangement of atoms in space. It is also known as space isomerism. It is classied as the
following:
a. Geometrical or cis–trans isomerism
b. Optical isomerism
c. Conformational isomerism
Various types of isomerism have been summarized below.
n-Butane
Isobutane
CH
3
CH
3
CH
CH
3
CH2CH
2
CH
3
CH
3
n-Butyl chloride
Isobutyl chloride
ClCH
3CH2CH2CH2
CH2ClCHCH
3
CH
3
α-Butylene (Butene-2)
Isobutylene (2-methyl propene)
CH
2
CH3CH2CH
C
CH
3
CH
3
CH
2
Butanol-1
Butanol-2
CH
2
OHCH
2
CH
3
CH
2
OH
CHCH
2
CH
3
CH
3
C h a P T E r 3 Stereochemistry
3.3 STRUCTURAL ISOMERISM
1. chain isomerism: This is due to the difference in the nature of carbon chain.
For example,
a. Butane (C4h10) exists in two isomeric forms, i.e. n-butane and isobutane:
b. Butyl chloride (C4h9Cl) exists in chain isomers as n-butyl chloride and isobutyl chloride:
c. Butene (C4h8) has the following two chain isomers:
135
2. Position isomerism: This is due to the difference in the position occupied by an atom or a functional
a. Butanol (C4h9OH) exists as two position isomers as follows:
b. Butene exists in the form of two position isomers as follows:
CH
Butene-1 Butene-2
c. 1,1-Dichloro ethane and 1,2-dichloro ethane are position isomers:
CH
1,1-Dichloroethane 1,2-Dichloroethane
group in a carbon chain.
For example,
CH2 CH CH
3
CHCl
CH
3
2
CH
2
CH CH CH
3
ClCH2Cl
2
3
The position of the alcoholic group, carbon–carbon double bond and the chlorine atom changes
from rst carbon atom to the second in all the above examples.
Cl
Cl
Cl
Cl
Cl
Cl
o-Dichlorobenzene
1,2-Dichlorobenzene
m-Dichlorobenzene
1,3-Dichlorobenzene
p-Dichlorobenzene
1,4-Dichlorobenzene
Dimethylamine
CH
3CH2NH2
Ethylamine
H
CH
3
CH
3
N
C2H
5
CH3CH2CH
2
C2H
5
CH
3
NH NH
136
PharMaCEuTICal OrGanIC ChEMISTry
d. o-, m- and p-Dichlorobenzenes also show position isomerism:
3. Functional isomerism: This is shown by compounds having same molecular formula but different
functional groups.
For example,
a. CH
CH2 OH CH3 O CH
3
3
Ethanol Dimethyl ether
b. CH
CH2 CHO CH3 CO CH
3
3
Propionaldehyde Acetone
c. CH
CH2 COOH CH3 COOCH
3
3
Propionic acid Methyl acetate
d. CH
C
N CH3 N
3
→
C
Methyl cyanide Methyl isocyanide
e.
4. metamerism: This isomerism is shown by the compounds having the same molecular formula and
even the same functional group, but the two alkyl groups attached to the same functional group are
different. These isomers belong to same homologous series.
For example,
a. C
O C2H
2H5
CH
5
O C3H
3
7
Diethyl ether Methyl propyl ether
b. C
CO C2H
2H5
CH
5
CO C3H
3
7
Diethyl ketone Methyl propyl ketone
c.
Diethylamine Methyl n-propylamine
O
C(
(
OH
CC
(
(
H
CH3CO
HH
HHCCO
(Keto form) (Enolic form)
O
COOC
2H5
CH
2
CCH
3
OH
COOC2H
5
CHCCH
3
HC
H
H
N
O
O
HCHN
O
OH
(Normal form) (Aci form)
C h a P T E r 3 Stereochemistry
5. tautomerism: This is a special case of functional isomerism and arises due to 1,3 migration
of a proton from one carbon atom to another atom (oxygen or nitrogen) with the rearrangement
of carbon–carbon single and double bond. This type of isomers are called tautomers and the
phenomenon is called tautomerism.
Further, the two tautomeric forms also exist in equilibrium with each other, the stable isomer
being in large amount and less stable isomer being in small amount. For example:
a. acetaldehyde and vinyl alcohol are tautomers of each other. acetaldehyde being more
stable isomer is in large amount as compared to vinyl alcohol. Since one of the isomer, i.e.
137
acetaldehyde contains a keto group
group
, this type of tautomerism is also called keto–enol tautomerism.
and the other, i.e. vinyl alcohol contains an enolic
b. Similarly, acetoacetic ester also shows keto–enol tautomerism, keto form being major product
and enolic form being minor product.
c. normal and aci forms of nitromethane are tautomers.
The compounds showing tautomerism take part in reactions of the functional groups present in each
tautomer, i.e. reactions of both the functional groups. The two tautomers can be separated by special methods
and can be kept in separate forms only under special conditions. under ordinary conditions tautomers exist
in equilibrium with each other.
138
MEMORY FOCUS
Isomers are different compounds with the same molecular formula.1.
There are two major classes of isomers: 2. structural or constitutional and stereoisomers.
Structural isomers have
3.
a. different IUPAC names;
b. the same or different functional groups;
c. different physical properties, so they are separable by physical techniques such as distillation;
d. different chemical properties. They behave differently or give different products in chemical
reactions.
Stereoisomers differ only in the way atoms are oriented in space.
4.
Stereoisomers have identical IUPAC names (except for a prefix like 5. cis or trans).
Because they differ only in the three-dimensional arrangement of atoms, stereoisomers always have
6.
the same functional group(s).
A particular three-dimensional arrangement is called a
7. configuration.
Stereoisomers differ in configuration, while structural isomers differ in structure.
8.
3.4 STEREOISOMERISM
PharMaCEuTICal OrGanIC ChEMISTry
The isomerism that arises due to difference in the arrangement of atoms in space is called stereoisomerism. The isomers that have the same molecular structure but different relative arrangements of atoms or groups in
space, i.e. different conguration, are called stereoisomers and the phenomenon is called stereoisomerism.
There are three types of stereoisomerism:
1. Optical isomerism
2. Geometrical isomerism
3. Conformational isomerism
It may be emphasized that the different structures of conformational isomers of a substance are
nonsuperimposable on one another but are easily interconverted into each other by rotation around single
bonds. In contrast to this, the different structures of optical or geometrical isomers are nonsuperimposable
over one another and at the same time they cannot be interconverted without breaking and making of
bonds. Hence, optical and geometrical isomers are also collectively known as congurational isomers.
While conformational isomers of a substance exist only as a mixture of different conformational forms,
the congurational isomers exist in individual substances. We shall now discuss these different types of
stereoisomerism one by one.
3.5.1 Optical Isomerism
It has been observed that certain compounds resemble one another in their chemical properties as well as
in most of their physical properties but they differ in their behaviour towards the action of plane polarized
light. Such compounds are called optically active and this phenomenon is called optical activity. Compounds
that rotate the plane polarized light towards the right are said to be dextrorotatory and are represented as
d or 1 form, whereas those that rotate it to the left are called laevorotatory represented as l or (–) form.
Ordinary
light
Nicol
prism
Plane
polarized
light
Solution of
an optically
active
substance
Plane
rotated
towards
right
Plane
rotated
towards
left
or
First
Nicol prism
(polarizer)
Second
Nicol prism
(analyser)
Light
source
α
Eye
α
C h a P T E r 3 Stereochemistry
These isomers are said to show optical isomerism. Before proceeding further, it is important to understand
the terms polarized light and optical activity.
Polarized light
an ordinary ray of light consists of electromagnetic waves and its vibrations taking place in all planes
perpendicular to the direction in which it travels. Monochromatic light, on the other hand, consists of waves
of one wavelength. Even such a ray has its vibrations in all the planes.
However, if a ray of monochromatic light is passed through a Nicol prism, the wave motion of emergent
light is restricted to only one plane. Such a beam of light which has vibration only in one plane is called plane
polarized light or unidirectional light. The nicol prism used to obtain plane polarized light is called a polarizer
(Fig. 3.1).
Optical activity
When the plane polarized light is allowed to fall on another Nicol prism with its axis parallel to the rst
Nicol prism, the plane polarized light passes through it without undergoing any deviation. The rst Nicol
prism is known as polarizer and the second nicol prism is known as analyser. If the axis of analyser is kept
at right angle to the polarizer, complete darkness appears due to total internal reection (Fig. 3.2).
The arrangement of polarizer and analyser with their axis perpendicular to each other is used to study the
behaviour of solutions of organic compound towards plane polarized light. For this purpose, a polarimeter
tube containing solution of organic compound is placed between two Nicol prisms. If some light appears, it
indicates that organic compound has rotated plane polarized light through a certain angle. Such compounds
139
Figure 3.1 Polarization of light.
Figure 3.2 Representation of polarimeter (dotted lines indicate the angle of rotation).
α
θ
[]
=
×
×
D
C
100
140
PharMaCEuTICal OrGanIC ChEMISTry
that can rotate the plane polarized light through a certain angle are known as optically active compounds.
The property by virtue of which the organic compounds can rotate the plane polarized light is known as
optical activity.
Specific rotation
The angle through which plane polarized light is rotated by an optically active compound is known as angle
of rotation. It can be represented by alpha (a) and can be determined with the help of an instrument called
polarimeter.
The angle of rotation depends upon the following factors:
1. nature of the compound
2. nature of solvent
3. Concentration of solution
4. length of the solution column through which the light passes
5. Temperature
6. Wavelength of light used
The rotatory power of a given solution is usually expressed as specic rotation—which can be dened as
the angle of rotation (a) produced by one decimetre length of solution having one gram of the substance per
millilitre. The measurement of rotation is carried out at temperature T using sodium light (the D lines)
Specic rotation =
100 × Observed angle of rotation
length in decimetres × grams of substances present in 100 ml of solution
q
or
T
The specic rotation of a compound is written as [a]
temperature. For example, [a]
20°
= - 20° means that a specic rotation of –20° is obtained at 20°C of
D
°
= X° where X° is the rotation in degree at T°
D
temperature and using with wavelength of sodium line (5893a°). Molecular rotation is the product of
specic rotation and molecular mass.
3.6 CONCEPT OF CHIRALITY
3.6.1 Chiral Structures
a structure or object is said to be chiral (or asymmetric) if it
has no plane of symmetry and is not superimposable on its
mirror image. The most perfect example of a chiral object
is human hand. It does not have any plane of symmetry and
cannot be superimposed on its mirror image. If you hold your
left hand up to a mirror, the image looks like a right hand but
if you try to superimpose one hand over the other (both palms
down), you cannot do it (Fig. 3.3). Similarly a right-handed
Naming a carbon atom with four different
groups is a topic that currently has no firm
agreement among organic chemists. The IUPAC
recommends the term chirality centre, but the
term has not gained wide acceptance among
organic chemists since it was first suggested
in 1996. Other terms in common use are chiral
centre, chiral carbon, asymmetric carbon and
stereogenic centre. The term used in this text is
chiral carbon or asymmetric carbon.
REMEMBER
glove and a left-handed glove are chiral objects.