1
Enantiomeric excess
• Optical purity - an outdated measurement of the enantiomeric excess (amount of
two enantiomers) in a solution / mixture
• If a solution contains only one enantiomer, the maximum rotation is observed...
H NH2
CO2H
measure rotation
derive [α]D = +14 100% (+) enantiomer
100% of maximum
observed rotation
H2N H
CO2H
measure rotation
derive [α]D = -14 100% (–) enantiomer
100% of maximum
observed rotation
• The observed rotation is proportional to the amount of each enantiomer present...
Advanced organic
,2
Enantiomeric excess II
= +
90% (+) enantiomer 10% anti- 90% clockwise 10% of major 80% of maximum
10% (–) enantiomer clockwise enantiomer is rotation observed
‘cancelled out’
= +
60% (+) enantiomer 40% anti- 60% clockwise 40% of major 20% of maximum
40% (–) enantiomer clockwise enantiomer is rotation observed
‘cancelled out’
= +
50% (+) enantiomer 50% anti- 50% clockwise 50% of major 0% of maximum
50% (–) enantiomer clockwise enantiomer is rotation observed
‘cancelled out’
Advanced organic
, 3
Enantiomeric excess III
• Previous slide indicates that a polarimeter measures difference in the amount of
each enantiomer
• Racemate (racemic mixture) - 1 to 1 mixture of enantiomers (50% of each)
• Racemisation - converting 1 enantiomer to a 1:1 mixture of enantiomers
• Optical rotation very unreliable so use new methods to measure amounts and use
the value enantiomeric excess
Enantiomeric excess (% ee) = [R] – [S] = %R – %S
[R] + [S]
• How do we measure enantiomeric excess?
• Problem - all the physical properties of enantiomers are identical (in an achiral
environment) except rotation of plane polarised light
• Solution - the interaction of a chiral molecule with other chiral compounds is
different depending on the enantiomer used...
• Imagine you have a mixture of left and right-handed gloves and you are asked to
separate them...suddenly there is a power cut, and you are left in a darkened room.
How would you do it? Use just one hand and try the gloves on...
Advanced organic
Enantiomeric excess
• Optical purity - an outdated measurement of the enantiomeric excess (amount of
two enantiomers) in a solution / mixture
• If a solution contains only one enantiomer, the maximum rotation is observed...
H NH2
CO2H
measure rotation
derive [α]D = +14 100% (+) enantiomer
100% of maximum
observed rotation
H2N H
CO2H
measure rotation
derive [α]D = -14 100% (–) enantiomer
100% of maximum
observed rotation
• The observed rotation is proportional to the amount of each enantiomer present...
Advanced organic
,2
Enantiomeric excess II
= +
90% (+) enantiomer 10% anti- 90% clockwise 10% of major 80% of maximum
10% (–) enantiomer clockwise enantiomer is rotation observed
‘cancelled out’
= +
60% (+) enantiomer 40% anti- 60% clockwise 40% of major 20% of maximum
40% (–) enantiomer clockwise enantiomer is rotation observed
‘cancelled out’
= +
50% (+) enantiomer 50% anti- 50% clockwise 50% of major 0% of maximum
50% (–) enantiomer clockwise enantiomer is rotation observed
‘cancelled out’
Advanced organic
, 3
Enantiomeric excess III
• Previous slide indicates that a polarimeter measures difference in the amount of
each enantiomer
• Racemate (racemic mixture) - 1 to 1 mixture of enantiomers (50% of each)
• Racemisation - converting 1 enantiomer to a 1:1 mixture of enantiomers
• Optical rotation very unreliable so use new methods to measure amounts and use
the value enantiomeric excess
Enantiomeric excess (% ee) = [R] – [S] = %R – %S
[R] + [S]
• How do we measure enantiomeric excess?
• Problem - all the physical properties of enantiomers are identical (in an achiral
environment) except rotation of plane polarised light
• Solution - the interaction of a chiral molecule with other chiral compounds is
different depending on the enantiomer used...
• Imagine you have a mixture of left and right-handed gloves and you are asked to
separate them...suddenly there is a power cut, and you are left in a darkened room.
How would you do it? Use just one hand and try the gloves on...
Advanced organic
