Stereo presentation.ppt

7/27/2019 Stereo presentation.ppt

1/24

BASIC CONCEPT OF STEREOCHEMISTRY


2/24

BASIC CONCEPTS

Isomers

Constitutional Isomers

Stereoisomers

Enantiomers

Diastereomers Chiral and Achiral

Meso Compounds

Optical Activity

R and S Nomenclature


3/24

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


4/24


5/24

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


6/24

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


7/24

Stereoisomers

Stereoisomersare molecules with identical connectivity

but different spatial arrangements of their constituent

atoms that cannot be interconverted by bond rotation.


8/24

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.


9/24

The two enantiomers are:


10/24

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


11/24

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.


12/24

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)


13/24


14/24

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".


15/24

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.


16/24

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.


17/24

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.


18/24

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.


19/24

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??


20/24

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.


21/24

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


22/24


23/24

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.


24/24

THANKS

Documents

Stereo presentation.ppt