11.3) ISOMERISM - Part 3

Part 3

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11.3 ISOMERISM (Part 3)

At the end of the lesson, student should
be able to:

e) determine the optical activity of a
compound

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Enantiomer compounds have the ability to rotate
the plane-polarized light through/ in a polarimeter

and said to be optically active.

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• an instrument that allows polarized light to travel
through a sample tube containing an organic
compound.

• permits the measurement of the degree to which
an organic compound rotates plane-polarized
light.

• the light that has an electric vector that oscillates
in a single plane.

• arises from passing ordinary light through a
polarizer.

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Ordinary light
Plane polarised light

• Enantiomer molecules (chiral compounds)
have the ability to rotate the plane of
plane–polarized light



A Sample tube
contains chiral

compounds

• However, separate enantiomers rotate the
plane of plane–polarized light equal amount
but in opposite direction

• Separate enantiomers are said to optically
active

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

A pair of enantiomers rotate the plane polarised
light in opposite direction.



EXAMPLE:

PROPERTIES OF 2–BUTANOL ENANTIOMERS

PROPERTIES d-(+)-butanol l-(-)-butanol

Boiling point (oC) 99.5 99.5

Density (g/mL at 20oC) 0.808 0.808

Index of refraction (20oC) 1.397 1.397

Specific rotation []D25 +13.52o –13.52o

CH3 CH3

C CH2CH3 C H
H CH3CH2
HO
OH

EXAMPLE: Enantiomers of limonene
HH

(+)-limonene (–)-limonene
odor of oranges odor of lemons

EXAMPLE: dihydroxyphenylalanine (DOPA)

COOH COOH

HO H NH2 HO NH2 H
OH OH
D–dopa
L–dopa

L–dopa is used to treat
Parkinson’s disease, but
D–dopa is toxic to human!

➢ The horrible thalidomide disaster in 1961 focused
scientist's attention on stereochemistry

The drug thalidomide used to alleviate the symptoms
of morning sickness in pregnant women





ISOMERS

different compounds with
the same molecular formula

constitutional stereoisomers
isomers
difference in 3–D
difference in arrangement only
connectivity of atoms

enantiomers diastereomers

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