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BASIC CONCEPT OF STEREOCHEMISTRY
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BASIC CONCEPTS
Isomers
Constitutional Isomers
Stereoisomers
Enantiomers
Diastereomers Chiral and Achiral
Meso Compounds
Optical Activity
R and S Nomenclature
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ISOMERS Any of two or more substances that are
composed of the same elements in the same
proportions but differ in properties because of
differences in the arrangement of atoms. i.e. A
simple example ofisomerism is given formulaC3H8O .
http://en.wikipedia.org/wiki/Carbonhttp://en.wikipedia.org/wiki/Hydrogenhttp://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Hydrogenhttp://en.wikipedia.org/wiki/Carbon -
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Constitutional Isomers
Structural Isomers are molecules which have the same
molecular formula but have different connectivity's .
Three categories of constitutional isomers are skeletal,
positional, and functional isomers. In skeletal isomerism, orchain isomerism,
components of the (usually carbon) skeleton are
distinctly re-ordered to create different structures. For
example 3-methylpentane is a chain isomer of2-
methylpentane
http://en.wikipedia.org/wiki/3-Methylpentanehttp://en.wikipedia.org/wiki/2-methylpentanehttp://en.wikipedia.org/wiki/2-methylpentanehttp://en.wikipedia.org/wiki/2-methylpentanehttp://en.wikipedia.org/wiki/2-methylpentanehttp://en.wikipedia.org/wiki/2-methylpentanehttp://en.wikipedia.org/wiki/3-Methylpentanehttp://en.wikipedia.org/wiki/3-Methylpentanehttp://en.wikipedia.org/wiki/3-Methylpentane -
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In position isomerism a functional group changes position
on the chain. In the diagram, pentane-2-ol has become
pentan-3-ol
In functional group isomerism a functional group splits up
and becomes a different group. Here is an example of
functional group isomerism: take cyclohexane, C6H12 and
hex-1-ene, also C6H12. These two are considered functionalgroup isomers because cyclohexane is an alkane and hex-1-
ene is an alkene. Both must have the same molecular
formula.
http://en.wikipedia.org/wiki/Functional_grouphttp://en.wikipedia.org/wiki/Pentan-3-olhttp://en.wikipedia.org/wiki/Cyclohexanehttp://en.wikipedia.org/wiki/Hex-1-enehttp://en.wikipedia.org/wiki/Alkanehttp://en.wikipedia.org/wiki/Alkenehttp://en.wikipedia.org/wiki/Molecular_formulahttp://en.wikipedia.org/wiki/Molecular_formulahttp://en.wikipedia.org/wiki/Molecular_formulahttp://en.wikipedia.org/wiki/Molecular_formulahttp://en.wikipedia.org/wiki/Alkenehttp://en.wikipedia.org/wiki/Alkanehttp://en.wikipedia.org/wiki/Hex-1-enehttp://en.wikipedia.org/wiki/Hex-1-enehttp://en.wikipedia.org/wiki/Hex-1-enehttp://en.wikipedia.org/wiki/Hex-1-enehttp://en.wikipedia.org/wiki/Hex-1-enehttp://en.wikipedia.org/wiki/Cyclohexanehttp://en.wikipedia.org/wiki/Pentan-3-olhttp://en.wikipedia.org/wiki/Pentan-3-olhttp://en.wikipedia.org/wiki/Pentan-3-olhttp://en.wikipedia.org/wiki/Pentan-3-olhttp://en.wikipedia.org/wiki/Pentan-3-olhttp://en.wikipedia.org/wiki/Functional_group -
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Stereoisomers
Stereoisomersare molecules with identical connectivity
but different spatial arrangements of their constituent
atoms that cannot be interconverted by bond rotation.
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Enantiomers
Either of two stereoisomers that are mirror images ofone another but cannot be superimposed on one
another and that rotate the plane of polarized light in
opposite directions. Enantiomers usually behave the
same chemically but differ in optical behavior and
sometimes in how quickly they react with other
enantiomers. Also called optical isomer.
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The two enantiomers are:
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Diastereomers
Diastereomers are stereoisomers that are not
enantiomers; that is, they are distinct molecules with the
same structural arrangement of atoms that are non-
superimposible, non-mirror images of each other.
(2S,3S)-tartaric acid (2R,3R)-tartaric acid (2R,3S)-tartaric acid
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In tartaric acid, both of the central carbon atoms are
stereogenic centers. Thus, they both must be assigned R or
S configurations according to the Cahn
Ingold
Prelogsystem. The first two molecules shown above, (2S,3S)-
tartaric acid and (2R,3R)-tartaric acid, are clearly
enantiomers of each other since they are mirror images. But
what about the third molecule, (2R,3S)-tartaric acid? It is
clearly distinct from the first two, yet the same atoms arebonded to each other. It is also very clearly not a mirror
image of either of the first two. Thus, as a non-
superimposible, non-mirror image with the same
arrangement of atoms, (2R,3S)-two molecules.
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Chiral and Achiral
Chiral objects have nonsuperimposable mirror images
and form a pair of enantiomers. Achiral objects have
mirror images that are identical (superimposable). The
following objects were tested: snail shell (chiral), pencil(achiral), glove (chiral), hand (chiral), a carbon atom with
four different substituents (chiral), a carbon atom with
three different substituents (achiral)
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Optical ActivityYour left hand is the mirror image ofyour right.
The two hands are "non-superimposable mirror images".
Molecules that are "non-superimposable mirror images"are optically active. We say the molecules are "chiral".We say the pair of molecules is an "enantiomeric pair".
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Meso Compounds
A compound with two chirality centers where there is the
same set of four groups at each chirality center, the
combination where the four groups are arranged such
that the centers are mirror images of each other (i.e.where the molecule has an internal mirror plane) is a
meso compound.
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Example of Meso compound
For example consider 2,3-dichlorobutane. There are twochirality centers (C2 and C3), each with -H, -Cl, -CH3 andthe other -CHClCH3 groups attached. Since there are twochirality centers, then, at least in principle, there are 4possible permutations (R,R), (R,S), (S,R) and (S,S)
If we look at the (R,S) and (S,R) as drawn above, these two
structures are actually the same thing because they aresuperimposable. This structure is the meso isomer.
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Optical Activity Optical activity is the ability of a chiral molecule to rotate
the plane of plane-polairzed light. It is measured using apolarimeter, which consists of a light source, polarizinglens, sample tube and analyzing lens.
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Simple substances which show optical isomerism existas two isomers known as enant iomers.
A solution of one enantiomer rotates the plane ofpolarisation in a clockwise direction. This enantiomer isknown as the (+) form.
For example, one of the optical isomers (enantiomers) ofthe amino acid alanine is known as (+)alanine.
A solution of the other enantiomer rotates the plane of
polarisation in an anti-clockwise direction. Thisenantiomer is known as the (-) form. So the otherenantiomer of alanine is known as or (-)alanine.
If the solutions are equally concentrated the amount ofrotation caused by the two isomers is exactly the same -but in opposite directions.
When optically active substances are made in the lab,they often occur as a 50/50 mixture of the twoenantiomers. This is known as a racemic m ixtureorracemate. It has no effect on plane polarised light.
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R and S Nomenclature
Since two enantiomers are different compounds, we willneed to have nomenclature which distinguishes themfrom each other. The convention which is used is calledthe (R,S) system because one enantiomer is assingedas the R enantiomer and the other as the S enantiomer.
What are the rules which govern which is which??
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Priorities are assigned to each of the four differentgroups attached to a given stereogenic center (one
through four, one being the group of highest priority). (Itshould be understood that each stereogenic center hasto be treated separately.)
Orient the molecule so that the group of priority four(lowest priority) points away from the observer.
Draw a circular arrow from the group of first priority tothe group of second priority.
If this circular motion is clockwise, the enantiomer is theR enantiomer. If it is counterclockwise, it is the Senantiomer.
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The Sequence Rule for Assignment of Configurationsto Chiral Centers
Assign sequence priorities to the four substituents by
looking at the atoms attached directly to the chiral center. 1. The higher the atomic number of the immediate
substituent atom, the higher the priority.For example, H < C < N < O < Cl. (Differentisotopes of the same element are assigned a priorityaccording to their atomic mass.)2. If two substituents have the same immediate substituentatom,evaluate atoms progressively further away from the chiralcenter until a difference is found.For example, CH3 < C2H5 < ClCH2 < BrCH2
< CH3O.3. If double or triple bonded groups are encountered assubstituents, they are treated as an equivalent set of single-bonded atoms.For example, C2H5 < CH2=CH < HCC
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The Viewing Rule
Once the relative priorities of the four substituents
have been determined, the chiral center must beviewed from the side opposite the lowest priority
group. If we number the substituent groups from 1
to 4, with 1 being the highest and 4 the lowest in
priority sequence, the two enantiomericconfigurations are shown in the following diagram
along with a viewers eye on the side opposite
substituent #4.
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