Title: Lesson 10 Optical Isomerism
Learning Objectives:
– Understand and identify optical isomerism
– Understand the chemical significance of optical isomerism
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The molecular formula, C3H4Cl2 represents several isomeric compounds.
Some isomers are cyclic and some are unsaturated.
a)
Draw the structures of two cyclic compounds that are structural
isomers and state the names of both isomers.
b)
Two of the non-cyclic compounds have geometrical isomers. Draw the
structures of these compounds and their geometrical isomers.
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Modelling
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Build a model of 2-methylbutan-2-ol
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Compare your model with your neighbours
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Have you all made the same thing?
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Stereoisomerism: true or false?
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Optical isomerism
Another form of stereoisomerism is optical isomerism, in
which a molecule can exist as two isomers that are nonsuperimposable, mirror images of each other, just like a left
hand and right hand.
optical isomers of the
amino acid alanine
Optical isomers have the same physical properties, but they
rotate polarized light in opposite directions.
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Optical Isomerism
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Optical isomerism occurs when you have at four different groups all bonded
to a central atom.
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For example: amino acids
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Build these for yourself to see that they are different…
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They are said to be Chiral.
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The four groups are arranged tetrahedrally around the carbon atom with
bond angles of 109.5o.
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They can be arranged in two different 3D configurations with mirror images
of each other…
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Chiral molecules have no plane of symmetry…
Mirror images are non-superimposable…
If we put them on top of each other they
do not line up…
Optical isomers are non-superimposable
forms known as enantiomers.
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Some terminology
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The central carbon is referred to as ‘chiral’
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You will see books refer to the ‘chiral carbon’ or ‘chiral centre’
You may see books talking about the ‘chirality’ of a molecule
A molecule might be described as ‘chiral’ if it has a ‘chiral centre’
A carbon must have at least 4 different groups to be chiral
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The two optical isomers are referred to as enantiomers
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A racemic mixture is a one with a 50:50 mix of the two enantiomers
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Enantiomers can be referred to as right or left handed
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If you see ‘R-’ or ‘L-’ in a name, this is what they are referring to
There is a standard way to work this out but we don’t need it!
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Molecules that have more than one chiral centre…
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Enantiomers have opposite configurations at each chiral centre… So the
whole molecule will be a mirror image.
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Diastereomers will have opposite configurations at some but not all chiral
centres… So they will not be a mirror image of each other
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Properties of optical isomers
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Enantiomers share virtually identical
chemical and physical properties
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However:
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Two enantiomers will rotate planepolarised light in opposite
directions…hence the term ‘optical
isomerism’
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More on the next slide
Chemical properties differ significantly in
chemical systems where 3D-shape is
important, particularly biochemistry
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The image above right shows an enzyme,
these are sensitive to the exact shape of a
molecule
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Rotation of plane-polarised light
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Light normally vibrates in many different
directions
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Plane-polarised light only vibrates in one
direction
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The polarizer blocks out light waves in all
other planes. Only plane polarised
light passes through.
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A pure solutions of enantiomers rotate
the plane-polarised light in opposite
directions
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To compare different solutions,
you need the same:
• Concentrations of the
solutions
• Wavelength of light
• Sample path
A racemic mixture will not rotate light as
the rotations from each enantiomer cancel
each other out… OPTICALLY
INACTIVE!!!
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This rotation can be detected using a
polarimeter.
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The analyser is a second polarizer that
can be detected until the rotated light
passes through. Extent and direction of
rotation can be deduced.
Richard Thornley - Distinguishing
Between Optical Isomers Video
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Reactivity with other chiral molecules
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Left rotation of plane polarized light is (-)
Right rotation of plane polarized light is (+)
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When a racemic mixture is reacted with a single isomer of another chiral
compound, the (-) and (+) components react to form two different products.
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These products have distinct chemical and physical properties.
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These can be separated relatively easily, so can be used to separate the
enantiomers from the racemic mixture, a process known as resolution.
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Solutions
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To do:
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Assess whether or not the following compounds display optical isomerism. If they do,
draw each of the two enantiomers in 3D and place a * next to the chiral carbon.
Build them from molecular models if it helps:
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2-chloropropane
2-chlorobutane
2-chloropropanal
chlorocyclopropane
1,2-dichlorocyclobutane
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Research the importance of optical isomerism in biochemistry. Give details of two
important examples.
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Draw a Venn diagram summarising geometrical and optical isomerism
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Key Points
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Optical isomers (enantiomers) rotate plane-polarised light in opposite
directions
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To display optical activity, a compound must have at least four different groups
attached to the same atom
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The central atom is referred to as chiral
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