steriochemstry, isomerism ,elements of symmetry with important terms.

1. STEREOCHEMISTRY
PRESENTED BY:
SALMAN KHAN

2. “The branch of chemistry concerned with the
three dimensional arrangement of atoms and
molecules and the effect of this on chemical
reactions.”
“Stereo-” means “three dimensional”
For this reason it is also known as -3D chemistry

3. isomerism
structural Stereo
geometrical optical
“Configuration”- arrangement of atoms of a molecules in
space
Stereoisomerism- It is a phenomenon by which two or more
molecules have same structure but different configurations
i.e, different orientations of atoms and groups in space.

4. OPTICAL ISOMERISM
“Compounds which possess similar chemical
and physical properties and differ only in their
optical activity are known as optical isomers and
the phenomenon as optical isomerism.”
Optical activity: The phenomenon of rotating the
plane polarized light is known as optical activity.
It could be either towards right or left.
right- dextrorotatory(d)(+) left-levorotatory(l)(-)
The substances exhibiting this property are
called optically active compounds.

5. • Plane polarized light:
A light vibrating in one plane only is known as plane polarized light.
NOTE: Optically activity of a compound can both be detected and
measured by an instrument called polarimeter.

6. SPECIFIC ROTATION
• The rotatory power of an optically active
substance is measured by a term, called specific
rotaion.
• It is defined as “the number of degrees of
rotation observed when plane polarized light is
passed through one decimeter length of its
solution having concentration 1 g per milliletre.
• Specific rotation value is generally expressed
with(+) or (-) sign.

7. CHIRAL ATOM
• Optical activity are shown by molecules having
an asymmetric carbon atom or chiral atom.
• It is a carbon atom to which four different atoms
or groups are attached.

8. • These models are asymmetric because they do
not possess a plane of symmetry , molecules that
are non-superimposable on their mirror images
are chiral.

9. • The number of optical isomers is equal to 2n,
where n is the number of asymmetric carbon
atoms. Thus lactic acid having one asymmetric
carbon atom, has two optical isomers called
dextro and laevo.
• STEREOGENIC CENTER: just any location in a
molecule where the interchange of any two
groups gives a new stereoisomer. A chiral center
is specifically a stereogenic center.

10. Elements Of Symmetry
• One determines whether the molecule possesses
any form of symmetry via
1) PLANE OF SYMMETRY
2) CENTRE OF SYMMETRY
3) AN ALTERNATING AXIS OF SYMMETRY
If the molecule contains at least one of these
elements of symmetry ,the molecule is
symmetric and optically inactive.

11. PLANE OF SYMMERTY
• It is a plane that divides the molecules into two
identical halves in such a way, one half of the
molecules is the mirror image of the other half.

12. Meso-tartaric acid although has two asymmetric
carbon atoms is a symmetrical molecule because
it can be divided into two identical halves by a
plane of symmetry.

13. CENTRE OF SYMMETRY
• It is a point from which lines, when drawn on
one side and produced an equal distance on the
other side, will meet identical points in the
molecule e.g, 1,3-dicarboxylic acid

14. ALTERNATING AXIS OF SYMMETRY
• A molecule possesses an n-fold alternating axis
of symmetry if when rotated through an angle of
360˚ /n about this axis and then reflected in a
plane perpendicular to this axis, the molecule is
indistinguishable from the original
one.e.g,1,2,3,4-tetrametyl cyclobutane.

15. Criteria for optical isomerism
• The compound must contain an asymmetric
carbon atom.
• Its three-dimensional structure should not have
a plane of symmetry.
• The two possible formulas should be non super
imposable mirror images

16. ENANTIOMERS
• “The optical isomers which are non
superimposable mirror images of each other are
known as enantiomers or optical antipodes”

17. ▫ They rotate the plane polarized light by equal
amounts but in opposite directions, i.e., their
value of specific rotation is equal but opposite in
sign.
▫ Their physical and chemical properties are same,
but differ in their rates of reaction with isomers of
other optically active compounds.

18. • 3-phenyl-2-butanol has two asymmetric carbon
atoms and has four optical isomers.
• Pairs of isomers I-II and III-IV are enantiomers.
• Enantiomers have identical chemical properties
except towards optical active reagents.

19. • For example,(+)-glucose is readily metabolized
by animals or fermented by yeast, whereas (-)-
glucose is neither metabolized by animals nor
fermented by yeast.
• This is because the enzymes which catalyze these
biological processes are optically active.