ISOMERISM
v Isomers
are different compounds with the same molecular formula.
v These
compounds are grouped into two broad classes:
structural
isomers and stereoisomers.
v Structural
isomers differ in their bonding sequence; their atoms are connected
differently.
v Stereoisomers
have the same bonding sequence but they differ in the orientation of their
atoms in space.
v Stereoisomers
often have remarkably different physical, chemical and biological
properties.
Structural isomerism(constitutional isomerism)
Ø Structural isomerism is the type of isomerism
where the molecules are same molecular formula and different arrangement of
atoms or groups.
Ø Types
of structural isomerism
- chain
isomerism, or skeletal isomerism
- Position
isomerism
- Functional
group isomerism
- Metamerism
- Tautomerism
- chain
isomerism, or skeletal isomerism
Ø These
isomers arise because of the possibility of branching in carbon chains.
Ø The
skeleton are distinctly re-ordered to create different structures. For
Example
2.Position isomerism
Ø In
position isomerism, the basic carbon skeleton remains unchanged, but important
groups are moved around on that skeleton.
3.Functional group isomerism
Ø The
isomers contain same molecular formula but, different functional groups
Example
CH3-CH2-OH CH3-O-CH3
Ethyl alcohol Di methyl ether
4. Metamerism
Ø This
form of isomerism is rare and is limited to molecules having a divalent atom
like O or S and alkyl groups around it.
The main examples come from ethers and thioethers.
5.Tautomerism
Ø This
isomerism is due to spontaneous inter conversion of two isomeric forms with
different functional groups.
Ø The
prerequisites for this is the presence of the C=O, C==N or N=O in the usual
cases and an alpha H atom.
Ø Example
Types of Stereoisomers
There are two categories of stereoisomers:
- Configurational
Isomers:
v Configurational
isomers differ in their arrangement in space and they can not be inter
converted without breaking a bond.
v They
are of two types:
- Optical
isomers ii. Geometric
isomers
B. Conformational Isomers:
They also differ in spatial arrangement of atoms/groups but
they can be inter converted easily by rotation around a single bond.
v Stereoisomers are two types such as
Enantiomers & Diastereomers.
v Enantiomers, also known as optical
isomers, the two stereoisomers that are mirror images of each other
that are non-superimposable.
v
Ex-Lactic acid
v Diastereomers are stereoisomers not mirror
images of each other. These include meso compounds, cis–trans (E-Z)
isomers, and non-enantiomeric optical isomers. Ex-Tartaric acid
Meso form or meso isomer - A meso form is a
stereoisomer of a compound with two or more chiral centers that is
superimposable on its own mirror image.
v
Racemic mixture, racemic modification or racemate –
v
A
mixture consisting of equal amounts of enantiomers.
v
A
racemic mixture exhibits no optical activity because the activities of the
individual enantiomers are equal and opposite in value, therby canceling each
other out.
Optical isomers (+)/(-)
v
Compounds
with chiral carbons mainly produce optical isomerism.A molecule with n
chiral atoms may have up to 2n stereoisomers.
v
A carbon atom attached to four different atoms
or groups is known as asymmetric or chiral carbon.
v
Identifies
rotation under plane-polarized light
v
The substance which rotate the plane of
polarized light to the right (or clockwise direction) are called dextrorotatory(+), from the Greek
word dexios, meaning “toward the right” while those which rotate
to the left (or in the anticlockwise direction) are called laevorotatory(-)
Geometrical isomerism
v
Stereoisomerism
about double bonds arises because rotation about the double bond is restricted.
v
The
geometrical isomers often show different physical and chemical properties.
v
There
are two types
Cis isomer – identical groups on same side
Trans
isomer – identical groups on
opposite sides.
v
Usually
the dipole moment of cis-isomers is greater than that of trans isomers. Hence
the cis isomers usually have more solubility in polar solvents.
v
In
general, the trans isomers are more stable than cis isomers.
E-Z NOTATION
v
The
simple convention of denoting the geometrical isomers by cis/trans descriptors
is not sufficient when there are more than two different substituents on a
double bond.
v
To
differentiate the stereochemistry in them, a new system of nomenclature known
as the E-Z notation method is to be adopted.
v
According
to this method, if the groups with higher priorities are present on the
opposite sides of the double bond, that isomer is denoted by E.
v
Where
E = Entgegen ( the German word for 'opposite')
v
However,
if the groups with higher priorities are on the same side of the double bond,
that isomer is denoted by Z.
v
Where
Z = Zusammen (the German word for 'together')
v
The
letters E and Z are represented within parentheses and are separated from the
rest of the name with a hyphen.
Conformational Isomers:
Conformational isomerism is a form of isomerism
that describes the the same structural formula but with different shapes due to
rotations about one or more bonds.
R/S
NOMENCLATURE SYSTEM (Cahn–Ingold–Prelog convention)
v
Assign priorities to the atoms directly
attached to the chirality center. The highest priority goes to the atom with
the highest atomic number.
v
In
case there are isotopes, use the mass number instead, since they have the same
atomic number.
v
If
two or more of the atoms directly attached to the chiral center are of the same
type, look at the next atom to break the tie. Do not do this unless there is a
tie. Repeat this process until the tie is broken.
v
For
examples
v
When
assigning absolute configuration to a chiral carbon the lowest priority group
that’s attached to chiral carbon must be pointing away.
v
In
the 3-D formula, the groups above the plane is represent as thick line and
groups below the plane is represent as dotted line.
To determine whether R or S, find the direction
of the priority groups numbered 1-3.
1. (R)- priority #1-3 are clockwise
2. (S)- priority #1-3 are anti- clockwise
v
The
3-D structures are converted in to Fischer formulas by draw the cross with the four substituents
attached to the chiral carbon, making sure the lowest priority group is lying
on a horizontal line.
v
Now
let’s consider the case of chiral molecules that contain two or more
stereo centers.
v
Such molecules can have enantiomers because
they are not the same as their mirror images.
v
Meso
forms can also be open chain, as illustrated below.
v
Different
conformations can have different energies, can usually interconvert, and are
very rarely isolatable.
v
For
example, cyclohexane can exist in a variety of different
conformations including a chair conformation and a boat conformation.
