ISOMERS.PPT organic chemistry slides by V.U

n= Number of carbon atoms present in fatty acid
Number of acetyl CoA produced = n/2
Number of cycles for fatty acids = (n/2 -1)
Number of reduced coenzyme = (n/2-1) (FADH2 + NADH)
For example if 16C (palmitic acid) undergoes beta oxidation
No. of acetyl CoA produced = 8 {1 Acetyl CoA = 12 ATP in TCA}
8×12 = 96 ATPs
No. of cycles for palmitic acid = 7
No. of reduced coenzymes produced= 7 (FADH2 + NADH)
7(2+3) = 35 ATPs
Total no. of ATPs produced= 96+35= 131 ATPs
No. of ATPs utilized during activation= 2 ATPs
Net gain = 131 - 2= 129 (ATPs According to old energetics concept)

1. © E.V. Blackburn, 2005
Isomers
•Isomers are molecules with the same molecular formula (i.e. the
same number of atoms of each element), but different structural or
spatial arrangements of the atoms within the molecule.
•Therefore, isomers are the different compounds with the same
molecular formula.

2. © E.V. Blackburn, 2005
Classification of
Isomers

3. © E.V. Blackburn, 2005
Constitutional or Structural Isomers
•Constitutional or structural isomers are compounds with the same
molecular formula but different structural formulae.
•In isomerism, constitutional isomers are molecules with different
connectivity.
•Therefore, constitutional isomers differ in the way the atoms are
connected to each other.

4. © E.V. Blackburn, 2005
Properties of Constitutional isomers
•Different IUPAC names.
•Different bonding connectivity.
•The same or different functional groups.
•Different physical properties, so they are separable by physical
techniques such as distillation.
•Different chemical properties.

5. © E.V. Blackburn, 2005
Constitutional or Structural Isomers
Structural isomers can be split again into three main subgroups:
•Chain isomers.
•Position isomers.
•Functional group isomers.
Chain isomers
•Are molecules with the same molecular formula, but different
arrangements of the carbon ‘skeleton’.
•Organic molecules can be arranged differently either as one
continuous chain, or as a chain with multiple side groups of carbons
branching off.

6. © E.V. Blackburn, 2005
Constitutional or Structural Isomers
•Positional isomers are constitutional isomers that
have the same carbon skeleton and the same
functional groups but differ from each other in the
location of the functional groups on or in the
carbon chain.
Functional isomerism
•Occurs when substances have the same molecular
formula but different functional groups.
•This means that functional isomers belong to
different homologous series.
•E.g alcohols and ethers; aldehydes and ketones;
carboxylic acids and esters, etc.

7. © E.V. Blackburn, 2005
Stereoisomers
•Stereoisomerism, or spatial isomerism, here molecules have the same
molecular formula and same sequence of bonded atoms (bonding
connectivity), but differ in the three-dimensional orientations of their atoms
in space.
•This contrasts with structural isomers, which share the same molecular
formula, but the bond connections differs.
•Stereoisomers have identical IUPAC names (except for a prefix like cis or
trans).
•Because they differ only in the three-dimensional arrangement of atoms,
stereoisomers always have the same functional group(s).
There are two main types of stereoisomerism-
•Geometric isomerism and Optical isomerism.

8. © E.V. Blackburn, 2005
Geometrical Isomers
•These isomers have different directional arrangements of specifically
located groups in the molecule, and considered to be caused by
prevention of free rotation in parts of the molecule by the double bond
or a ring).
•This type of isomerism is involved in compounds containing carbon-
carbon double bonds with suitable substituents.
•Rotation of these bonds is restricted, compared to single bonds which
can rotate freely.
•This means that, if there are two different atoms, or groups of atoms,
attached to each carbon of the carbon-carbon double bond, they can
be arranged in different ways to give different molecules

9. © E.V. Blackburn, 2005
Optical Isomers
•Optical isomers are two compounds which contain the same number and
kinds of atoms and bonds (i.e., the connectivity between atoms is the
same), but different spatial arrangements of the atoms, and have non-
superimposable mirror images.
• Each non-superimposable mirror image structure is called Enantiomer.
•Optical isomers are named due to their effect on plane-polarised light,
and come in pairs.
•They usually (although not always) contain a Chiral centre – this is a
carbon atom, with four different atoms (or groups of atoms) attached to it.
•These atoms or groups can be arranged differently around the central
carbon, so that the molecule can’t be rotated to make the two
arrangements align.

10. © E.V. Blackburn, 2005
Enantiomers-Optical Isomers
•Enantiomers are optical stereoisomers that are
non-superimposable on their mirror images.
A AB B
C C
D D
mirror
CO
2H
H
3C
NH
2
H
CH
3
CO
2H
H
2N
H

11. © E.V. Blackburn, 2005
Diastereomers-Optical Isomers
Diastereomers are optical stereoisomers that are not mirror
images of each other, and they arenot enantiomers.
CC
H
CH
3
H
H
3C
CC
CH
3
H
H
H
3C

12. © E.V. Blackburn, 2005
Chirality
•Chirality means non-superimposable mirror-images of
molecules, and a molecule is chiral if its mirror image is
not the same as itself(non-superimposable) thus chiral
centers.
•Chirality is a necessary and sufficient condition for the
existence of enantiomers.
•Chiral compounds actively rotate plane polarized light.
•Molecules that are superimposable on their mirror images
are said to be achiral and optically inactive.
Molecules that can exist as enantiomers are said to be
chiral, and they are non-superimposable on their mirror
images.

13. © E.V. Blackburn, 2005
Chiral Molecules and Chiral Samples
•When a molecule is chiral, it must be either “right-handed” or “left-
handed”. Meaning it can either rotate light clockwise or anti-
clockwise.
•A racemic mixture is a 50:50 mixture of two enantiomers.
Because they are mirror images, each enantiomer rotates
plane-polarized light in an equal but opposite direction and
is optically inactive.
•Any chiral macroscopic sample may be racemic or non-racemic.
•Chiral and non-racemic sample: The sample is made up of molecules
that all have the same sense of chirality (homochiral molecules).
•Chiral but racemic sample: The sample is made up of equal (or very
nearly equal) numbers of molecules of opposite sense of chirality
(heterochiral molecules).

14. © E.V. Blackburn, 2005
Chiral Molecules and Chiral samples
•Chiral, racemic means that the sample is made up of equal numbers
of molecules of opposite sense of chirality.

•But in a “chiral, non-racemic” sample, some molecules are of a
sense of chirality opposite to that of the majority, thus the sample
may not be enantiomerically pure (or enantiopure).
•All compounds have mirror images.

•What’s important in chemistry is whether a molecule is identical to
or different from its mirror image.
•Some molecules are like hands, the left and right hands are mirror
images of each other, but they are non-superimposable and not
identical.

15. © E.V. Blackburn, 2005
Chiral and Achiral Molecules
•To superimpose an object on its mirror image means to align all
parts of the object with its mirror image. With molecules, this
means aligning all atoms and all bonds.
•A molecule (or object) that is not superimposable on its mirror
image is said to be chiral.

16. © E.V. Blackburn, 2005
Tetrahedral stereogenic centres
A carbon atom bonded to four different groups is called a
tetrahedral stereogenic centre, asymmetric centre, or chirality
centre.
O
H
(+)-carvone
(+)-Carvone is responsible for the odour of caraway seed oil.
*

17. © E.V. Blackburn, 2005
•The two are non-superimposable, mirror images.
• Such isomers are called enantiomers.
DB
A
C
BD
A
C
mirror

18. © E.V. Blackburn, 2005
Configurations
•Configurations are not the same as conformations.
•Conformations are interconvertible by rotation about
single bond(s) whereas bonds must be broken to
change one configuration into another.
•The particular arrangement of atoms in space that is
characteristic of a given molecule is called its
configuration.

19. © E.V. Blackburn, 2005
Properties of enantiomers
b) Chemical: They have identical chemical properties
except for their reaction with reagents which are,
themselves, optically active. In this case, reaction rates
differ and depend on which enantiomer of the reagent is
used.
E.g (+)-Glucose is central to the fermentation process
whereas (-)-glucose doesn’t react
a) Physical: Enantiomers have identical physical
properties with the exception that they rotate the plane of
polarized light in opposite directions.

20. © E.V. Blackburn, 2005
Ordinary Light and Plane Polarised Light
•An ordinary light beam consists of a group of electromagnetic
waves of a range of different wavelengths that vibrate in many
different planes at right angles to the direction of propagation of the
light ray. It vibrates in all directions.
•When such a beam strikes a polarising film or a Nicol prism, only
those waves vibrating in a specific plane with respect to the axis of
the film or prism may pass through and all others are blocked out.
•Light beam is plane polarized once passed through the prism in one
direction.
•If light waves vibrate in the same light polarization plane, then light
of this kind is said to be plane polarised.

21. © E.V. Blackburn, 2005
Ordinary Light and Plane Polarised Light
•Monochromatic light: Monochromatic light, such as emitted by a
sodium lamp (λ = 589 nm), is of discrete wavelength but still
vibrates in an infinite number of planes.

22. © E.V. Blackburn, 2005
Plane-polarized light
•Ordinary light is a moving wave whose vibrations take place in
all directions perpendicular to the direction in which the light is
travelling.
•One of these components can be eliminated by passing
ordinary light through a polarizer(Polaroid filter).

•The resulting light is said to be polarized, and all its
vibrations are parallel to a single plane.

23. © E.V. Blackburn, 2005
Polarimeter
source
5893 Å
polarizer
sample
tube
analyzer


24. © E.V. Blackburn, 2005
•If from the point of the observer the rotation is in the clockwise
direction, the sample is said to be dextrorotatory.
The angle of rotation  is considered to be positive (+).
•If the rotation is in the counterclockwise direction, the sample
is said to be levorotatory and the angle  is then negative (-).
•R chiral isomers are dextrorotatory and S chiral isomers
are levorotatory
•Thus (S)-2-chlorobutane is the levorotatory enantiomer.
Optical activity
•An optically active compound is one which rotates the
plane of light polarization.

25. © E.V. Blackburn, 2005
Racemic mixtures
•Is an equimolar mixture of two enantiomers.
•Prefix the name +-
•Contain enantiomers which are mirror images of
each other and rotate plane polarized light in equal
but opposite directions, thus optically inactive.
•If enantiomers are separated in solution, then the
mixture is said to have been resolved.

26. © E.V. Blackburn, 2005
Meso compounds
•A meso compound is an achiral compound with chiral centers
but with superimposable mirror images.
• Meso compounds are achiral and optically inactive due to
their internal plane of symmetry that makes them
superimposable on their mirror images.
ClH
CH
3
ClH
CH
3
HCl
CH
3
HCl
CH
3

27. © E.V. Blackburn, 2005
Optically active natural products
CO
2H
HO
2C H
H
OH
HO
(+)-tartaric acid
H
3CO
H
3CO
OO
H
HH
H
NH
(-)-brucine

28. © E.V. Blackburn, 2005
•END