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CHE 220: STEREOCHEMISTRY
LECTURE 2
Chirality & Enantiomers
D.M. Shadrack
(Chem Dept-SJUT)
Wednesday, February 01,2012
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Chirality and Enantiomers
Chirality, what does it mean?
Chiral mean handedness.
The left and right hands are mirror image and nonsuperimposable
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Chirality and Enantiomers, cont..,
Chirality:Is ability of molecule to exist intwo non superimposable mirror-imageforms.
Note, The effects of chirality give rise to
Enantiomerism
Enantiomer: molecules that are non-superposable mirror images
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Chiral molecule
A B
What is a chiral molecule?
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Chiral molecule
Definition
A molecule that is non superposable
mirror image.Note,
Non-superimposability on the mirror
image is a necessary and sufficientcondition for chirality; no exception hasever been found
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Chiral molecule, cont..,
A molecule is said to be chiral if itlacks symmetry and its mirrorimages are not superimposable.
To be chiral a molecule must lack
symmetry, that is, a chiral moleculecan not have any type or symmetry.
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Chiral center Fig A
Also known as stereogenic center ,
stereocenter or chiral carbon.
The asterisk (*) indicate the stereocentercarbon.
Look at fig A & B, then,
What is a chiral center? OH
*
* *
Fig BWednesday, February 01,2012
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Chiral center, cont..,
Is a centre of asymmetry in a molecule, that is,
the carbon atom that has FOUR different
atoms attached to it.
Other examples
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Achiral molecules
Are stereoisomers that are
superimposable on its mirror imagedue to plane of symmetry though
may possess chiral centers. (e.gmost of the socks).
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Generally,
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Practice problem
Which of the following compounds would
form enantiomers because the molecule is
chiral?
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Practice problem - answer
The second carbon atom in 2-bromo-2-
methylbutane contains two identical
CH3 substituents. As a result, this compound
is achiraland does not form enantiomers.
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The second carbon atom in 1-bromo-2-
methylbutane carries four different
substituents: H, Br, CH3, and CH2CH3. As a
result, this molecule is chiral and it forms
enantiomers.
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Test for chirality
Plane of symmetry (or Mirror plane):Is an imaginary line that bisects the
molecule such that the two halvesare mirror image of each other.
A molecule will not be chiral if itpossesses a plane of symmetry
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Test for chirality, cont..,
All molecules with plane ofsymmetry are
achiral.
For example.
H
ClH3C
H3C
H
Cl
C2H6
H3C
2-Chloropentane has a plane of symmetry 2-Chlorobutane does not posses a plane of symmetry
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Test for chirality, cont..,
C
HBr
C
H
H3C
Br
Mirror
Br HHBr
CH3 H3C
**
*
*
line of symmetry
CH3
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Below are the examples of superposable
and nonsuperposable molecules
C
CH3
HCH3CH2CH2CH3CH2
CH3
HCH2CH2CH3
CH2CH
3
=
3-Methylhexane Mirror
3-Methylhexane and its mirror image
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C
=
3-Methylpentane
CH3
HCH3CH2
CH3CH2
CH3
HCH2CH3
CH2CH3
Mirror
3-Methylpentane and its mirror image
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Now,
For the case of 3-Methylpentane
(above), rotating a molecule at 180o
the molecule will be superimposed(= identical).
Unlike 3-Methylhexane no
superimposition (= chiral molecule)
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Note:1. A molecule with one chiral C
atom has two enantiomers, two
molecules that are each other's mirrorimage. There are many molecules in
nature with more than one chiral C atom,
glucose, cholesterol, prostaglandins toname a few.
Each chiral C atom adds another two
enantiomers to the total number of
possible compounds.
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In other words, the answer to the question,
how many different compounds exist when
there are n chiral centers combined in one
molecule, is 2n.
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NOTE 2,
Presence of one chiral center in compound
is a sufficient condition for chirality , but
the presence of more than one chiral center
in compound is NOT a sufficient condition
to cause chirality (compound be a chiral).
Some chiral molecules do not possess chiral
centers, hence chiral center is not a
necessary condition for chirality.
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Physical properties of Enantiomers
Have identical physical properties (e.g. melting
point, boiling point, solubility etc.) except for
optical activity.
They are also identical in chemical reactivity
(except with chiral reagents or chiral
catalysts).
Pair of enantiomers can be distinguished by
their optical activity.
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Optical Activity
The rotation by a molecule of the
plane-polarized light
Plane polarized light: is a light whichtravel only in one plane.
Ordinary light is in a wave motionoscilating in all planes.
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Optical Activity, cont..,
Example
light
end view of plane
of oscilation
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Polarimeter
Is a device tomeasure opticalactivity
Or
Is an instrumentthat determinethe plane ofpolarization.
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Principal parts of polarimeter
Source of light (lamp)
A polarizer: Is the lense made up of cystaline
material like Nickel prism. i.e special crytaline
with CaCO3/calcite. It acts as a filter.
Sample tube
Analyzer (lense no. 2)
Scale (in degree)
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Specific rotation
The observed rotations () of enantiomers
are opposite in direction. One enantiomer
will rotate polarized light in a clockwise
direction, termed dextrorotatory or (+),
and its mirror-image partner in a counter-
clockwise manner, termed levorotatory or().
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The prefixes dextro and levo come from the
Latin dexter, meaning right, and laevus, for
left, and are abbreviated dand lrespectively.
If equal quantities of each enantiomer are
examined, using the same sample cell, then
the magnitude of the rotations will be the
same, with one being positive and the other
negative.
Hence, Dextrorotatory (+) or (d) and
Levorotatory (-) or (l)
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Angle of rotation, o
Angle of rotation, o depends on;
(i) wave length of light
(ii) Temperature(iii) concentration
(iv) length of sample tube
(v) solvent e.g CH2Cl2
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Angle of rotation, o
In order to place measured rotation on a
standard basis chemists calculate a
quantity called the specific rotation *+.
That is, Specific rotation,
l.c
t
D
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Where,
*+ = Specific rotation
= the observed rotation
l = length of sample tube in decimeters (1 dm =
10 cm)
c = the concentration of the the solution in g/ml
of solution or density in g/mL for neat
liquids
t = temperature
D = wave length of sodium lamp ( = 599.6 nm).
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Practice problems
Problem 1: enantiomers and chirality
Problem 2: chiral and achiral molecules
Problem 3: specific rotation
Problem 4: diastereomers
The Origin of optical Activity (GO AND READ)
Wednesday February 01 34D M Shadrack (Chem Dept-SJUT)
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