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Optical Isomerism

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Antonello Frau

on 4 January 2013

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Transcript of Optical Isomerism

Antonello Frau Optical Isomers and Biological Molecules Main Idea: Isomerism Isomerism Classes Constitutional isomers are molecules whose "pieces" have different connectivity—analogous to simple bracelets in which the order of red and green beads is different.

Stereoisomers are isomers whose connectivity is the same, but the pieces are oriented differently in space. 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. 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. 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. 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. Optical isomers rotate the plane of plane polarised light. OPTICALLY ACTIVE 2-chlorobutane NOT OPTICALLY ACTIVE propan-2-ol 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. 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. Left and right hands are an example of non-superimposable mirror images. For some molecules the mirror image is a different molecule (the mirror image is non-superimposable). R limonene (oranges) S limonene (lemons) R carvone (spearmint) S carvone (caraway seed) R thalidomide (dangerous drug) S thalidomide (effective drug)

The body racemises each enantiomer, so even pure S is dangerous as it converts to R in the body. butanone
2-methylbutanoic acid
butan-2-ol
1-chloro-3-methylpentane propan-2-ol
2-chlorobutane
1-chlorobutane
3-methylhexane TASK Which of the following molecules are optically active? All molecules have a mirror image – but in most of the cases image and object are the same molecule. OPTICAL ISOMERISM R limonene (oranges) S limonene (lemons) R carvone (spearmint) S carvone (caraway seed) R thalidomide (dangerous drug) S-thalidomide (effective drug)

The body racemises each enantiomer, so even pure S is dangerous as it converts to R in the body. 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!!!!! For example, most amino acids (and so proteins) are chiral, along with many other molecules.

POP QUESTION: what is the only
non-chiral amino acid? 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. Optical isomers rotate the plane of plane polarised light. 1-The wave vibrations are perpendicular to the direction of travel of the wave. Their effect on plane polarised light
Their reaction with other chiral molecules Enantiomers have identical chemical and physical properties, except: Molecule pairs that are optical isomers are called enantiomers. OPTICALLY ACTIVE 1-chloro-3-methylpentane OPTICALLY ACTIVE butan-2-ol OPTICALLY ACTIVE 2-methylbutanoic acid NOT OPTICALLY ACTIVE propan-2-ol NOT OPTICALLY ACTIVE butanone NOT OPTICALLY ACTIVE 1-chlorobutane OPTICALLY ACTIVE 2-chlorobutane NOT OPTICALLY ACTIVE propan-2-ol butanone
2-methylbutanoic acid
butan-2-ol
1-chloro-3-methylpentane propan-2-ol
2-chlorobutane
1-chlorobutane
3-methylhexane POP QUESTION(S):
Which of the following molecules are optically active? The optical isomers are called enantiomers.

These are distinguished by +/-, D/L or more correctly Rectus/Sinister (right/left...you gotta pick up a little Latin some time!!!!).

A 50/50 mixture of the two enantiomers is called a racemic mixture or a racemate. This usually happens when a molecule contains a C atom with four different groups attached (chiral/asymmetric carbon).
Such molecules are said to be chiral or optically active. Left and right hands are an example of
non-superimposable mirror images. For some molecules the mirror image is a different molecule
(the mirror image is non-superimposable). 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. OPTICALLY ACTIVE 3-methylhexane All molecules have a mirror image – but in most of the cases image and object are the same molecule. Molecules that have same brute formula but
differ in chemical and physical properties.
Greek: "isos" + "meros", or “equal parts.”


Colloquially, isomers are chemical compounds that
have the same "pieces" but are not the same compound. Constitutional (a.k.a. structural) Isomers Chiral molecules are optical isomers!!!! More examples... Almost all amino acids
have 4 different groups
round the carbon atom Optical Isomerism
in Amino acids
Full transcript