Topic 17 - Organic Chemistry 2
Optical Isomerism
How does optical isomerism arise?
optical isomerism arises as a result of chirality in molecules
What makes a chiral carbon atom?
a chiral carbon atom has 4 different groups attached to it
it is possible to arrange the groups in two different ways - these are called
enantiomers or optical isomers
Explain the term chiral
a chiral molecule is a non-superimposable on its mirror image
What makes an enantiomer?
enantiomers are mirror images of each other
no matter which way to turn them they cannot be superimposed
Explain how optical isomers rotate plane polarised light
monochromatic, plane polarised ligth has a single wavelength and only
vibrates in one direction
optical isomers are optically active - they rotate the plane of polarisation
one enantiomer rotates it clockwise and the other anticlockwise
Topic 17 - Organic Chemistry 2 1
, Define what is meant by a racemic mixture and describe what they do
a racemic mixture contains equal quantaties of each enantiomer of a chiral
compound
racemic mixtures don't rotate plane polarised light - the two enantiomers cancel
each others light rotating effect
two achiral reactants can react to make racemic mixtures of chiral products
What is the difference between an optically active chiral molecule and a non
optically active one
optically active - single enantiomer
non optically active - racemic mixture
Describe the optical activity in an Sn1 mechanism and draw out the steps
you start with a single enantiomer reactant
the product will be a racemic mixture of the two optical isomers of each other,
so wont rotate plane-polarised light
Describe the optical activity in an Sn2 mechanism and draw out the steps
a single enantiomer reactant produces a single enantiomer product
Topic 17 - Organic Chemistry 2 2
, When an optically active isomer reacts to form an optically active product, what
does this suggest about the mechanism?
only one enantiomer formed
Sn2 mechanism
nucleophile attacks from the opposite side to the leaving group
Aldehydes and ketones
Describe the carbonyl functional group and where they're found
c=o
esters, aldehydes, ketones
not carboxylic acids
Describe why aldehyes and ketones have lower boiling points than alcohols
and carboxylic acids
because alcohols and carboxylic acids form hydrogen bonds
aldehydes and ketones dont have a polar O-H bond so they can't form
hydrogen bonds with other aldehydes and ketones
but aldehydes and ketones from only weak intermolecular bonds such as
permanent dipole-dipole and LDF's
Describe and draw the formation of hydrogen bonds between aldehydes and
ketones with water
lone pair electrons on the O atom
so small aldehydes and ketones dissolve in water
Topic 17 - Organic Chemistry 2 3
Optical Isomerism
How does optical isomerism arise?
optical isomerism arises as a result of chirality in molecules
What makes a chiral carbon atom?
a chiral carbon atom has 4 different groups attached to it
it is possible to arrange the groups in two different ways - these are called
enantiomers or optical isomers
Explain the term chiral
a chiral molecule is a non-superimposable on its mirror image
What makes an enantiomer?
enantiomers are mirror images of each other
no matter which way to turn them they cannot be superimposed
Explain how optical isomers rotate plane polarised light
monochromatic, plane polarised ligth has a single wavelength and only
vibrates in one direction
optical isomers are optically active - they rotate the plane of polarisation
one enantiomer rotates it clockwise and the other anticlockwise
Topic 17 - Organic Chemistry 2 1
, Define what is meant by a racemic mixture and describe what they do
a racemic mixture contains equal quantaties of each enantiomer of a chiral
compound
racemic mixtures don't rotate plane polarised light - the two enantiomers cancel
each others light rotating effect
two achiral reactants can react to make racemic mixtures of chiral products
What is the difference between an optically active chiral molecule and a non
optically active one
optically active - single enantiomer
non optically active - racemic mixture
Describe the optical activity in an Sn1 mechanism and draw out the steps
you start with a single enantiomer reactant
the product will be a racemic mixture of the two optical isomers of each other,
so wont rotate plane-polarised light
Describe the optical activity in an Sn2 mechanism and draw out the steps
a single enantiomer reactant produces a single enantiomer product
Topic 17 - Organic Chemistry 2 2
, When an optically active isomer reacts to form an optically active product, what
does this suggest about the mechanism?
only one enantiomer formed
Sn2 mechanism
nucleophile attacks from the opposite side to the leaving group
Aldehydes and ketones
Describe the carbonyl functional group and where they're found
c=o
esters, aldehydes, ketones
not carboxylic acids
Describe why aldehyes and ketones have lower boiling points than alcohols
and carboxylic acids
because alcohols and carboxylic acids form hydrogen bonds
aldehydes and ketones dont have a polar O-H bond so they can't form
hydrogen bonds with other aldehydes and ketones
but aldehydes and ketones from only weak intermolecular bonds such as
permanent dipole-dipole and LDF's
Describe and draw the formation of hydrogen bonds between aldehydes and
ketones with water
lone pair electrons on the O atom
so small aldehydes and ketones dissolve in water
Topic 17 - Organic Chemistry 2 3
