Stereochemistry

Structual Isomers: 

Have the same molecular formula and a different structual formula

can be devided in to 3 groups:

Carbon Chain:

Have different arrangements of carbon-chain, some are straight and others are  branched eg.  butane, 2-methyl-propane

CH3-CH2-CH2-CH3         CH3-CH2-CH3

                                                 CH3    

Positional

Have the same functional group but on different positions of the carbon chain eg. 1-chloro-propane, 2-chloro-propane

CH3-CH2-CH2CL       CH3-CH2-CHCL-CH3 

Functional groups

Have the same atoms arranged in to different functional groups

eg. propanone (ketone), propanal (aldehyde)

Stereoisomers  Have the same molecular formula but their atoms occupy Different arrangements in space

There are 2 types

Geometric Isomers

Optical Isomers (enantiomers)

Geometric Isomers

Geometric isomers occur due to the lack of free rotation about the C=C bond 

Isomers can be

CIS/Z

TRANS/E

to show geometrical isomerism a molecule must have

1. A C=C bond

2. 2 different groups attached to each carbon of the C=C bond

Look at the atomic numbers of the groups attached to each carbon of the C=C bond, 

the groups of HIGHER ATOMIC MASS are given HIGHER PRIORITY

If the 2 groups of Higher priority are on the same side of the C=C bond the molecule is CIS/Z

If they are on different sides they are called a TRANS/E isomer

(remeber E=enemy, enemies are on opposite sides)

A chiral carbon atom is one with 4 DIFFERENT GROUPS ATTACHED

This creates an ENANTIOMER

A MOLECULE WITH A NON SUPERIMPOSIBLE MIRROR IMAGE

Molecules containing chiral carbon atoms will display optical isomerism and exist as a pair of enantiomers

Enantiomes have:

Identical chemical properties

Identical physical properties (melting bpt and boiling bpt)

They DIFFER IN:

BIOCHEMICAL REACTIONS

ROTATION OF PLANE POLARISED LIGHT

Light waves travel in peaks and troughs in all planes

polarised light travels in only one plane

Optical isomers are OPTICALLY ACTIVE

"They rotate plane polarised light"

one enantiomer rotates the light clockwise

one enantiomer rotates the light anticlockwise

Using a polarimetre it is possible to measure the angle the light is rotated and identify the enantiomer.

The direction and degree by which the light is rotated depends on

The nature of the enantiomer

The concentration of the enantiomer in solution

A RACEMIC MIXTURE

contains equal amounts of each enantiomer

It will HAVE NO EFFECT ON POLARISED LIGHT

(the CW rotation of one enantiomer will be cancelled out by the ACW rotation of the other)

Most chemical reactions give rise to racemix mixtures