Most molecules are not a rigid species. They usually have free rotation about the single bonds, although in certain cases, this might not be the case. There is a strict rotation about double bonds, effectively fixing the atoms into fixed positions.
Two molecules with identical formulas may have their arrangements such that the relationship between the atoms is in some way different. These are called stereoisomers.
There are two types of stereoisomerism, geometric and optical.
20.6.2 Describe and explain geometrical isomerism in non-cyclic alkenes
Geometric isomerism is caused by the lack of rotation between the carbon carbon double bonds
If the alkene has two different substituents on each carbon atom then it is possible to produce two structures that have the same name and molecular formulas.
1,2 - dichloroethene is an example of geometrical isomers.
When both chlorine atoms are on the same side, it is represented with (cis-) and when chlorine atoms are on opposite sides, it is represented with (trans-)
20.6.3 Describe and explain geometrical isomerism in C3 and C4 cycloakanes
Cyclic molecules such as cyclopropane or cyclobutane also have restricted rotation about the carbon - carbon double bonds, as rotation would break the ring structure. This gives the possibility of geometric isomerism, with a difference between sites above the plane of the heterocycle and those below.
1,2 - dichlorocyclobutane
When both chlorine atoms are on the same side, it is represented with (cis-) and when chlorine atoms are on opposite sides, it is represented with (trans-)
20.6.4 Explain the difference in the physical and chemical properties of geometrical isomers
Geometric isomers have different physical and chemical properties
Physical properties
Cis - 1,2-dichloroethene has both of the chlorine atoms on the same side as each carbon - chlorine bond is polar the overall molecule itself is polar. However, the trans, the individual dipoles in the bonds then cancel out and the molecule is non-polar overall.
Cis - 1,2 - dichloroethane = 60.2
Trans - 1,2 - dichloroethane = 48.5
The two geometric isomers of butenedioic acid (maleic and fumaric acid) also demonstrate different physical properties. The melting points are very different as the close proximity of the two -COOH groups allows for the formation of intramolecular hydrogen bonds. This decreases the possibility of intermolecular hydrogen bonding and reduces the melting points.
Cis-butenedioic acid + 135
Trans-butenedioic acid = 287
Chemical properties
Action of heat allows cis to bond with each other and form a cyclo.
20.6.5 Describe and explain optical isomerism in simple organic molecules
Chiral carbons must be joined to four different atoms or groups, and are also as a description of the molecule itself.
Enantiomers are optical isomers. Racemic mixture = an equimolar mixture of two optical isomers whose opposite rotation effect on plane polarised light cancels out.
20.6.6 Outline the use of a polarimeter in distinguishing between optical isomers
Plane polarised light is light with which all light waves are vibrating in the same orientation. This can be achieved by passing normal light through a lens which contains thousands of vertical lines called polarised filters.
The lines that emerges from the polarised filter has only vertical orientated light.
When the light passes through the solution of an optical isomer, the angle rotated is dependent on three factors.
The type of enantiomers, path length and concentration of enantiomers
The light emerging from the sample solution can be analysed with another filter. The two enantiomers rotate the polarised light the same angle, only different direction.
20.6.7 Compare the physical and chemical properties of enatiomers
The physical properties of enantiomers are identical except for different angles on plane polarised light.
The chemical properties are identical as well except with enzymes. React differently with other optical isomers. This is because enzymes are used to optically active enantiomers.
