Che 220 Lecture 2

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CHE 220: STEREOCHEMISTRY

LECTURE 2

Chirality & Enantiomers

D.M. Shadrack

(Chem Dept-SJUT)

Wednesday, February 01,2012

1D.M. Shadrack (Chem Dept-SJUT)

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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)
http://enatiomers%20and%20chirality%20problems.docx/http://practise%20problems%20chiral%20%26achiral.pdf/http://specific%20rotation%20examples%20and%20problems.docx/http://diastereomers.docx/http://che%20220%20lecture%20notes.doc/http://che%20220%20lecture%20notes.doc/http://che%20220%20lecture%20notes.doc/http://diastereomers.docx/http://diastereomers.docx/http://diastereomers.docx/http://specific%20rotation%20examples%20and%20problems.docx/http://practise%20problems%20chiral%20%26achiral.pdf/http://practise%20problems%20chiral%20%26achiral.pdf/http://practise%20problems%20chiral%20%26achiral.pdf/http://practise%20problems%20chiral%20%26achiral.pdf/http://practise%20problems%20chiral%20%26achiral.pdf/http://practise%20problems%20chiral%20%26achiral.pdf/http://practise%20problems%20chiral%20%26achiral.pdf/http://enatiomers%20and%20chirality%20problems.docx/http://enatiomers%20and%20chirality%20problems.docx/http://enatiomers%20and%20chirality%20problems.docx/http://enatiomers%20and%20chirality%20problems.docx/

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