ISOMERISM

 The phenomenon of existence of two or more compounds with same

molecular formula but different properties ( physical, chemical or both) is
known as isomerism and the compound exhibiting this phenomenon are
called isomers
 The term was used by Berzelius
 Isomerism are of two types

STRUCTURAL ISOMERISM:
 It is due to the difference in arrangement of atoms or groups within the
molecule, without any reference to space.
 Structural isomers are compound having same molecular formula but
different structural formula.
 These are of following types
(a) CHAIN ISOMERISM
When the isomers have similar molecular formula but differ
in nature of carbon chain are called chain isomers and phenomenon is known
as chain isomerism.
Example:
C4H10 (Butane) exists in two forms
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Pentane (C5H12) exists in three form

C4H10O

C5H8

(b) FUNCTIONAL ISOMERISM

Compounds having same molecular formula but different functional group are
known as functional isomers and the phenomenon is functional isomerism.
Examples
 Alcohol and ether ( CnH2n+2O)
C3H8O
CH3 – CH2 – CH2-OH : n-propyl alcohol
C2H5 – O – CH3 : ethyl methyl ether
 Aldehydes, ketones, ethers etc. (CnH2nO)
C3H6O
CH3 – CH2 – CHO : propanal
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 Amines ( Primary, secondary, tertiary)
C3H9N

 Alcohol, phenol and ethers

C7H8O

(c) POSITION ISOMERISM

It is due to the difference in the positions occupied by the particular atom or
group ( substituent) in the same carbon chain or due to different positions of
double or triple bonds in alkenes and alkynes.
Example:
 C4H6
CH3 – CH2 – C ≡ CH : But – 1 – yne
CH3 – C ≡ C – CH3 : But – 2 – yne
 C3H8O
CH3 – CH2 – CH2 – OH : propan – 1 – ol
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 C4H8
CH3 – CH2 – CH = CH2 : But-1-ene
CH3 – CH = CH – CH3 : But – 2 – ene
 C6H4Cl2

Aldehydes, carboxylic acid and their derivatives do not exhibit position isomerism

(d) Metamerism
Metamers are the isomers which have same molecular formula but differ in
nature of alkyl group, groups attached to the either side of the same
functional group. This isomerism is shown by ethers, ketones, esters,
secondary amines.
Examples
 C4H10O
C2H5 – O – C2H5 : Diethyl ether
C3H7 – O – CH3 : Methyl propyl ether
 C5H10O

 C4H10S

 C4H11N
C2H5 – NH – C2H5 : Diethyl amine
C3H7 – NH – CH3 : Methyl propyl amine
(e) TAUTOMERISM
( Greek word : tauto = same ; meros = parts)
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It was used by Laar in 1885
Tautomerism may be defined as the phenomenon in which a single
compounds exists in two readily inter convertible structures that differ
markedly in the relative position of at least one atomic nucleus generally
hydrogen. The two different structures are known as tautomers of each other.
There are two types of tautomerism
(i) Dyad system
If two hydrogen atom oscillates between two polyvalent atoms, linked
together, then system is called dyad system.

(ii) Triad system

 If the hydrogen atom travels from first to third in a chain, the system is triad
 The most important type of triad system is keto-enol tautomerism keto-enol
system

The keto form is more stable

Mechanism of tautomerism

(i) Base catalysed tautomerism

(ii) Acid catalysed tautomerism

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Examples of keto-enol system

 Acetaldehyde

 Acetone

 Acetyl acetone

 Benzoyl acetophenone

Enolisation

 The conversion of keto form into enol form is known as enolisation.

 The percentage of enol from has been found to increase in the order:
Simple aldehydes and ketone < β keto ester < β diketones having phenyl
group < phenols
 Enolisation is in order
CH3COCH3 < CH3COCOOC2H5 < C6H5COCH2COOC2H5 < CH3COCH2CHO <
CH3COCH2CHO < CH3COCH2COCH3 < C6H5COCH2COCH3 < phenoxide ion <
C6H5COCH2COC6H5
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Composition of tautomeric mixture

The relative amount of keto and enol form of tautomeric mixture depends upon their
relative stabilities. In simple monocarbonyl compounds like acetaldehyde, acetone
etc, the amount of enolic form is negligibly small because of comparatively lower
stability.

However if enolic form is stabilized by intermolecular bonding, the amount of enolic

form becomes higher. In 1,3-dicarbonyl compounds also called β-dicarbonyl
compounds can be attributed to the following reasons.

i) Stability gained through reasonance stabilization of conjugated double bond

eg. Acetylacetone

ii) H –Bonding in enol form results in the formation of cyclic structure. Eg. Acetyl

iii) Stabilisation of enolic form increases if double bond of enol form is in

conjugation with electron cloud of benzene ring. Eg. Benzoylacetophenone.

Essential conditions for tautomerism.

In order to exhibit keto-anol tautomerism, an aldehyde or ketone or ester must

possess at least, one α- hydrogen atom. Which can show 1,3 – migration. Example
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Difference between tautomerism and resonance

i) Tautomers are definite compounds and can be separated and characterized by

suitable methods, but resonating structure cannot be separated as they are
imaginary structures of same compounds
ii) Two tautomers have different functional groups, but the various resonating
structures have the same functional group.
iii) Tautomerism has no effect on bond length, while resonance is accompanied by
an increase in bond length of double bond and decrease of a single bond length.
iv) Tautomerism has no contribution in stabilizing the molecule but resonance give
rise to extra stability to molecule.
v) Tautomerism may occur in planar or non-planar molecules while resonance
occurs only in planar molecules.
(f) RING-CHAIN ISOMERISM
In this type of isomerism compounds are having same molecular formula but differ in
modes of linking of carbon atoms, i.e. it may either be open chain or closed chain
structures.
Eg. 1 C3H6
CH3 – CH = CH2 : Propene

Eg2 C3H4
CH3 – C ≡ CH : Propyne

Eg 3 C4H8
CH3 – CH2 – CH = CH2 : But -1 – ene
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Eg 4
C6H12

CH3 – CH2 – CH2 – CH2 –CH =CH2 : hex-1-ene

DOUBLE BOND EQUIVALENT ( D.B.E )
Number of structural isomers can be predicted using double bond equivalents.
Double bond equivalent gives the number of double bonds ( π – bonds ) or rings in
compounds
𝑛 (𝑉 − 2)
𝐷. 𝐵. 𝐸 = ∑ +1
2
N = number of different kinds of atoms present in molecules
V= valency of each atom.
Eg 1 C4H6
4(4 − 2 ) + 6(1 − 2)
𝐷. 𝐵. 𝐸 = +1
2

𝐷. 𝐵. 𝐸 = 2
Thus the compound may contain
i) Two double bond or a triple bond
ii) One ring and one double bond
iii) Two rings

For the compounds of general formula CaHbNcOd

(𝑏 − 𝑐 )
𝐷. 𝐵. 𝐸 = 𝑎 + 1 −
2
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Univalent atoms such as halogen atom may be replaced by one hydrogen atom and
bivalent atom such as oxygen may be ignored example

Eg, Benzene ( C6H6 )

6
𝐷. 𝐵. 𝐸 = 6 + 1 − =4
2
4 D.B.E. in benzene corresponds to 3 double bond and one ring

C3H6O

6
𝐷. 𝐵. 𝐸 = 3 + 1 − =1
2
i.e. Molecule may contain double bond ( C=C) or ( C = O ) or a ring

It’s possible isomers are

CH2 = CH – CH2 – OH ( prop-2-en-1-ol) CH2 = CH – O – CH3 ( Methoxy ethane)

STEREOISOMERISM

Compounds have same molecular and structural formulae but different spatial
arrangement of atoms or groups

There are two types of stereoisomerism:

a) Conformational isomerism.
b) Configurational isomerism.
a) CONFORMATIONAL ISOMERISM
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The different arrangements of atoms in space that result from the free rotation of
groups about C – C bond axis are called conformation or conformational isomers
or rotational isomers and the phenomenon as conformational isomerism
This type of isomerism is found in alkanes, cycloalkanes and their derivatives.
Representation of conformers
(i) Sawhorse formula
In this representation, molecule is viewed slightly from above and from right side
of one carbon atom
Carbon – carbon bond is drawn diagonally and slightly elongated and remaining
six bonds attached to each carbon atom are represented as straight line

(ii) Newman projection formula

In this representation, the molecule is viewed along the carbon – carbon single
bond
The front carbon atom is represented by a point and groups attached to it are
represented by equally spaced radii. Whereas rear carbon atom is represented by
circle and groups attached to it are represented by three equally spaced radial
extensions

Conformations of ethane ( CH3 – CH3 )

Two extreme conformation are important, staggered and eclipsed. There can be
number of arrangements between staggered and eclipsed forms and these
arrangements are called skew forms.

(i) Eclipsed conformation

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In this conformation rotation about C – C single bond is such that hydrogen atoms
of front carbon atom completely cover or eclipse the hydrogen atom of rear
carbon atom.
In this conformation, hydrogen atoms of two carbon atoms are at minimum
distance which makes conformation unstable
(ii) Staggered conformation
In this confirmation rotation about C – C bond is by an angle of 60O so that
hydrogen atoms of two carbon atoms are at maximum distance from each other
making it stable.
In staggered conformation, all the six hydrogen atoms are visible
(iii) Skew conformation
In this conformations, hydrogen atoms are closer than in staggered but way than
in eclipsed conformation

The relative stabilities of the various conformation of ethane are in the following
order
Staggered > Skew > eclipsed
Ethane is mostly in staggered form.

Conformations of propane

Since it has two C – C single bonds, rotation about any of C-C bond give rise to two
extreme conformation like that of ethane
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Propane molecules exist mostly in the more stable staggered conformation.

Conformation of Butane

n-Butane may be considered as derivative of ethane whose one hydrogen of each

carbon atom is replaced by a methyl group

1) Eclipsed conformations of n-butane

There are three eclipsed conformations of n-butane. In the fully eclipsed form, a
methyl group is eclipsed by another methyl group, while in partially group is
eclipsed by hydrogen.
Fully eclipsed feel more repulsive force than partially eclipsed. Thus fully eclipsed
is less stable than partially eclipsed
2) Staggered conformations of n-Butane
Three staggered conformations are possible; anti and two gauche.
In anti –conformation, the methyl groups are 180O apart and hence confirmations
is most stable. In gauche conformations, the two methyl group are only 60 O apart
and hence less stable than anti-form
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The order of stability of n-Butane conformations is

Anti > gauche = gauche > partially eclipsed = partially eclipsed > fully eclipsed

Factors affecting stability of conformations

(i) Torsional strain

Torsional strain arise due to repulsive interaction between bonds on
adjacent atoms
As the repulsive interaction between electronic cloud increases, torsional
strain increases and thus stability decreases.
(ii) Steric strain
Steric strain arises due to crowding around central atom more the bulky
groups present around the central more will be steric strain and thus less
will be stability.
For example, gauche conformation of n-Butane is less stable than anti-
conformation
(iii) Dipole – dipole interactions
Molecule in which polar bonds are attached in central atom, stability of
greatly affected by dipole-dipole interactions.
Stronger the dipole – dipole interaction lesser will be the stability
(iv) Angle strain
Any deviation from the band angle suggested by the state of hybridization
bring angle strain in the molecule. It mainly influences stability of
cycloalkane.
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Baeyer’s strain theory
In year 1885, Baeyer proposed a theory of angle stain for cycloalkanes and
the main postulates of this theory are
(i) Baeyer assumed the planer structure for all cycloalkanes. Thus the
deviation from the tetrahedral bond angle varies with the size of the
ring.
(ii) Deviation from regular tetrahedral angle introduces strain in the
ring which brings unstability. Larger the deviation, greater will be
the strain and thus lesser will be its stability.
Amount of deviation (d) = ( 109O28’ – Bond angle of the ring)

In cyclopropane = 109O 28’ – 60O = 49.5O

In cyclobutane = 109O 28’ – 90o = 19o 28’

In cyclopentane = 109O 28’ – 180O = 1O28’

Thus relative order of their stability is

Cyclopentane > cyclobutane > cyclopropane

In cyclohexane = 109O 28’ – 120O = 10.5O

Cyclohexane is free from angle strain and hence is quite stable and unreactive.

Therefore, cyclohexane adopts a non-planar structure.

(v) Intramolecular hydrogen bonding also influences the relative stability of

conformations of a molecule

For example in ethylene glycol gauche conformations are more stable than anti form
due to intramolecular hydrogen bonding.

Conformations of cyclohexane
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1. Chair conformations
It is most stable conformation of cyclohexane as it is free angle and torsional
strain as all groups are staggered and bond angles are tetrahedral.

Axial and equatorial bond in cyclohexane

Hydrogen atoms are perpendicular to the ring are called axial hydrogen atoms
and hydrogen atom lying in the plane of ring are called equatorial hydrogen
atom.

2. Boat conformation
If left end of the chair conformation is flipped, keeping rest of the molecule
fixed, we get boat conformation of cyclohexane
It is highly unstable conformation
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3. Twist boat confirmation

If in the boat conformation of cyclohexane bond along C2 – C3 and C5 – C6 is
twisted in such a way that flagpole hydrogens move away.
Torsional strain in this conformation is less, making it more stable than boat
conformation

4. Half chair conformation

It transition state conformation chair and twist boat conformation

Order of relative stabilities of various conformations of cyclohexane is :

chair > twist boat > boat > half chair

CONFIGURATIONAL ISOMERISM

These are the stereoisomers which differ in spatial arrangement of atoms and thus
show different properties.

The isomers cannot be obtained by free rotation around C-C single bond

1. Geometrical isomerism
Geometrical isomers are the stereoisomers which have different arrangement
of groups or atoms around rigid framework of double bonds
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Geometrical isomerism is generally seen in alkenes ( >C=C<) and oximes ( >C=NOH)

Geometrical isomerism in alkene

Isomer in which similar groups or atoms lie on the same side of double bond
are called cis-isomer where as isomers in which similar groups lie on the
opposite side of double bond are called trans-isomer.

Necessary conditions for geometrical isomerism

i) The molecule must have a C - C double bond.
ii) Two atoms or groups attached to doubly bonded carbon atom must be
different.

Distinction between cis and trans isomer

1. Dipole moment :
Cis –isomer
In cis isomer, dipole moment of polar groups have additive effect thus have
higher dipole moment than corresponding trans isomer

Trans-isomer
In trans isomer, dipole moment of polar groups have opposing effect, thus
tends to cancel each other
2. Melting point
Cis isomer
Cis-isomer has lower melting point because the structure is not symmetrical

Trans isomer
Due to symmetry, trans isomer fits better in crystal lattice, thus has higher
lattice energy and hence higher melting point.
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3. Solubility
Cis isomer
Cis isomer have higher solubility because these are weakly held in lattice
Trans isomer
Trans – isomers have lower solubility because these are tightly held in the
lattice
4. Density
Cis isomers
Cis – isomers have lower density
Trans isomers
Trans isomer due to higher lattice energy has higher density
5. Boiling point
Cis isomer
Cis-isomer have higher boiling point due to dipole-dipole interaction
Trans isomer
Trans isomers have comparatively low boiling point

Cis – trans isomer around single bond

Geometrical isomerism in oximes and azo compounds

In syn-isomers H atom of doubly bonded carbon and –OH group of doubly bonded
nitrogen lie on the same side of double bond

In anti-isomers H atom of doubly bonded carbon and –OH group of doubly bonded
nitrogen lie on opposite side of double bond

E – Z NOTATION OF GEOMETRICAL ISOMERISM

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This new system of nomenclature of geometrical isomerism was developed by Ingold
and Prelog

In this system we assign a priority to the groups attached to double bonded carbon
atom. If groups of similar priority lie on the similar side of double bond, the isomer is
designated as Z ( Zusammen, means together). If groups of similar priority lie on the
opposite side of double bond, the isomer is designated as E ( Entgegen, means
opposite)

Sequence rules: The following rules are followed for deciding the precedence order
of the atoms or groups:

(i) Higher atomic number atoms get higher priority.

(ii) Among the isotopes of same element, isotope of higher mass is given
higher priority.
(iii) In the groups, the order of precedence is also decided on the basis of
atomic number of first atom of the group.
For example
The order of precedence

When the order of precedence of the groups cannot be settled on the first
atom, the second atom or the subsequent atoms in the groups are
considered.
For example
The order of precedence

(iv) A double or triple bonded atom is considered equivalent to two or three

such atoms

For example, the group >C=O is equal to and the group

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(v) If one of the position is occupied by lone pair, it is given higher priority
over the bonded group.
Examples of E-Z isomerism

Number of geometrical isomers

If a compound has more than one double bond, the number of geometrical isomers
is 2n, where n is number of double bond

This formula applies only to the molecules in which ends are different. For example,

CHa = CH – CH = CHb occurs in four geometrical isomers

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When the ends of polyene are same

𝑛
i) When n is even number of geometrical isomers = 2(𝑛−1) + 2( 2 −1)
𝑛−1
ii) When n is geometrical isomers = 2(𝑛−1) + 2( 2
)

OPTICAL ISOMERISM

Compounds having similar physical and chemical properties but differing only in the
behavior towards polarized light are called optical isomers and this phenomenon is
known as optical isomerism

Plane polarized light and optical activity

The beam of light which vibrate only in one plane is called plane polarized light. It can
be obtained by passing ordinary light through a nicol prism which cuts vibrations in
all planes except in one.

Sometimes on passing a plane polarized light through solution of certain substances,

a change in plane polarized light. Such substances which rotate the plane of plane
polarized light are called optically active substances

On the basis of study of optical activity, the various organic compounds were divided
into three types.

i) The optical isomer which rotates the plane polarized light to the right (
clockwise) is known as dextro-rotatory isomer or d-form or indicated by
+ve sign
ii) The optical isomer which rotates the plane polarized light to the left (
anticlockwise) is known as laevo-rotatory isomer or l- form or indicated by
–ve sign
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iii) The optical powers of the above two isomers are equal in magnitude but
opposite in sign. An equimolar mixture of the two forms, therefore, will be
optically inactive. This mixture is termed as racemic mixture or fl- form or
(±) mixture

Enantiomers

An optically active substances may exist in two or more isomeric forms which have
same chemical and physical properties but differ in terms of direction of rotation of
plane polarized light. Such optical isomers which rotate the plane of polarized light
with equal angle but in opposite directions are known as enantiomers and
phenomenon is known as enantiomerism.

In order to exhibit optical activity an object must be chiral. A carbon atom whose
tetra valency is satisfied by four altogether different substituents is called chiral
carbon atom or asymmetric carbon atom. A molecule possessing chiral carbon atom
and non-superimposable to its own mirror image is said to be chiral and the property
is called chirality.

Molecule which is superimposable on its own mirror image is said to achiral

Prochiral carbon

A carbon atom is said to be prochiral if replacement of one of its group or atom by

other substituent makes it chiral centre. For example propanoic acid is prochiral
molecule.
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Representation of enantiomers

i) Newman projection formulae

ii) Wedge and dash formulae

iii) Fischer projection

Specification of configuration

i) Relative configuration ( D, L – Nomenclature)

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Earlier than 1951, the absolute configuration of a compound was not known.
Therefore glyceraldehydes was chosen as standard compound and all compounds
were studied which respect is

Now the configuration with –OH group at right side was given D-configuration
where as the configuration with –OH group at left side was given L-configuration.

Example

In polyhdroxy compounds ( sugars ) having more than one chiral centre, the
configuration of stereocentre farthest from carbonyl group is compared with
glyceraldehydes. Example

In case of α- amino acids, the configuration is assigned by comparing –NH2 group of

α- amino acid with –OH group of glyceraldehydes.
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The sign ( + ) or ( - ) added after D- and L- letters indicates the direction of optical
rotation

(iii) Absoulte configuration ( R, S – nomenclature)

Limitations of D and L notations were overcome by R and S notations developed by
R.S. Cahn and C.K. Ingold and V. Prelog

The centre is then viewed with the substituent with lowest priority pointing away
from the viewer.

If the path for remaining three substituents going from highest priority to lowest
priority is clockwise, the configuration is R ( R stands for Rectus i.e. right) and if the
path is anticlockwise the configuration is assigned as S ( S stands for sinister i.e. left)

Sequence rules for assigning the priority order :

Rule 1: The atom with highest atomic number has highest priority. For example I > Br
> Cl > F > C > H

Rule 2 : If the atom attached to the asymmetric carbon atom are the same. We
determine the priority by considering the next atom from the asymmetric carbon
atom. For example

Ethyl has a higher priority than methyl because the ethyl group has ( C, H, H)
attached to first carbon, where as the methyl carbon has only hydrogen ( H, H, H)

Rule3: If the atoms attached to two group are of the same atomic number then the
priority is given to the group which have more substituent CHCl2 > CH2Cl
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Rule 4 : If the groups attached to the asymmetric carbon atom has double bond or
triple bond then the priority is given to the group which has the maximum bonds.

Example Glyceraldehyde

Priority order : OH > CHO > CH2OH > H

The priority sequence for most common groups

-I , -Br , -Cl, -SH. _F, -OCOR , -OR , -OH, -NO2 , -COCl, -COOR, -COH, -CONH2 , -COR, -
CHO, -CN, CH2OH, -C6H5 , -CR3 , -CHR2 , -CH2R

Golden rule

There is an easy way of assigning R and S configuration to optical isomers

represented by Fischer projection formula. First assign priorities to the group, atom
attached to chiral centre.

If the lowest priority group occupies vertical position in the original Fisher projection
then the configuration obtained above gives the actual configuration in the molecule

If the lowest priority group occupies horizontal position in the Fischer projection
formula, then change the configuration obtained above from (R) to (S) or (S) to (R)
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Examples

Example II

MESO COMPOUNDS:

A compound with two or more asymmetric carbon atoms but also having a plane of
symmetry (a mirror plane) is called meso compounds. The figure shows two meso
compounds. These molecules have plane of symmetry dividing them midway
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between the two asymmetric carbon in each. Notice that one half of the molecule is
the mirror image of the other. Both molecules are optically inactive; even though
each has two asymmetric centres. Neither will rotate the plane polarized light.