• All molecules have a mirror image – but in most of the cases image and object are the same molecule. H H C C H H H H F F fluoromethane
• For some molecules the mirror image is a different molecule (the mirror image is non-superimposable). OH OH C C COOH HOOC H H3C H CH3 (-) lactic acid (+) lactic acid in sour milk in muscles
• Left and right hands are an example of non-superimposable mirror images.
• This usually happens when a molecule contains a C atom with four different groups attached (chiral / asymmetric C).• Such molecules are said to be chiral or optically active. a a C C d d b c c b
• The optical isomers are called enantiomers.• These are distinguished by +/-, D/L or more correctly Rectus-right/Sinister-left.• A 50/50 mixture of the two enantiomers is called a racemic mixture or a racemate.
TASK Which of the following molecules are optically active?1) propan-2-ol • butanone2) 2-chlorobutane • 2-methylbutanoic acid3) 1-chlorobutane • butan-2-ol4) 3-methylhexane • 1-chloro-3-methylpentane
propan-2-ol CH3 CH CH3 OH NOT OPTICALLY ACTIVE
2-methylbutanoic acid CH3 O CH3 CH2 CH C OH CH2CH3 CH2CH3 C C H HCH3 CH3 COOH HOOC OPTICALLY ACTIVE
butan-2-ol OH CH3 CH2 CH CH3 CH2CH3 CH2CH3 C C H HCH3 CH3 OH HO OPTICALLY ACTIVE
1-chloro-3-methylpentane CH3 Cl CH3 CH2 CH CH2 CH2 CH2CH3 CH2CH3 C C H HCH3 CH3 CH2CH2Cl CH2ClCH2 OPTICALLY ACTIVE
• Molecules that are optical isomers are called enantiomers.• Enantiomers have identical chemical and physical properties, except: • Their effect on plane polarised light; • Their reaction with other chiral molecules
1-The wave vibrations are perpendicular to the direction of travel of the wave. normal light plane-polarised light plane-polarised light after(waves vibrate in all directions) (vibrates in only one direction) clockwise rotation
• Optical isomers rotate the plane of plane polarised light. (-)-enantiomer (+)-enantiomer (±)-racemate(anticlockwise rotation) (clockwise rotation) (no overall effect)
2- Chiral molecules often react differently with other chiral molecules.• This is like the idea that a right hand does not fit a left handed glove – the molecule must be the correct shape to fit the molecule it is reacting with.• Many natural molecules are chiral and most natural reactions are affected by optical isomerism.
• For example, most amino acids (and so proteins) are chiral, along with many other molecules.• The human body can consume amino acids that rotate polarised light to the left.
• Many drugs are optically active, with one enantiomer only having the beneficial effect.• In the case of some drugs, the other enantiomer can even be harmful, e.g. thalidomide.
• In the 1960’s thalidomide was given to pregnant women to reduce the effects of morning sickness.• This led to many disabilities in babies and early deaths in many cases.
O NH O O NH O O O H2C C C H2C CH2 N N CH2 H H O OS thalidomide (effective drug) R thalidomide (dangerous drug) The body racemises each enantiomer, so even pure S isdangerous as it converts to R in the body.
CH3 CH3 O O H C CH2 H2C C H H3C CH3S carvone (caraway seed) R carvone (spearmint)
CH3 CH3CH2 C H H C CH2 CH3 H3C S limonene (lemons) R limonene (oranges)
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