20.6 - Stereoisomerism
20.6.1: Describe stereoisomers as compounds with the same structural formula but with different arrangements of atoms in space.
Molecules are not rigid species. They usually have free rotation about single bonds, although in certain cases, for example cyclic molecules, this is not always the case. There is also restricted rotation about double bonds, effectively fixing the atoms into their relative positions.
Two molecules with identical molecular formula may have their arrangements of atoms such that the relationship between the atoms is in some way different. These are called stereoisomers.
The key to recognising stereoisomers is to identify non-superimposible structures, i.e. they structures that cannot fit (in an imaginary way) into the same space, such that all of the atoms in the second structure occupy identical positions as the first structure.
There are two forms of stereoisomerism, geometric and optical.
20.6.2: Describe and explain geometric isomerism in non-cyclic alkenes. Include the prefixes cis and trans and the term restricted rotation.
Geometric isomerism in alkenes is caused by the lack of rotation about the carbon - carbon double bond.
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 formula, but which are non-superimposible.
An example is 1,2-dichloroethene, CHCl=CHCl.
When the two chlorines are on the same side (laterally) of the double bond the structure is denominated using the prefix 'cis-'. When they are on opposite sides of the double bond (crossing the double bond from one to the other) then are designated the prefix 'trans-'.
20.6.3: Describe and explain geometric isomerism in C3 and C4 cycloalkenes. Include the dichloro derivatives of cyclopropane and cyclobutane
Cyclic molecules such as cyclopropane or cyclobutane also have restricted rotation about the carbon-carbon 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.
An example of this is 1,2-dichlorocyclobutane.
When the two chlorine atoms are on the same side of the ring structure, i.e. both above or both below, the molecule is designated using the prefix 'cis-'. Once again, when the substituents are on opposite sides the molecule is disignated using the prefix 'trans-'.
20.6.4: Explain the difference in physical and chemical properties of geometric isomers. Include cis- and trans- 1,2-dichloroethene as examples with different boiling points, and cis- and trans- but-2-ene-1,4-dioic acid dicarboxylic acid as examples that react differently when heated.
Geometric isomers have different physical and chemical properties
Cis-1,2-dichloroethene has both of the chlorine atoms on the same side of the double bond. As each C-Cl bond is polar the overall molecule itself is polar. However, trans-1,2-dichloroethene has the chlorine atoms on different sides of the double bond. The individual dipoles in the bonds then cancel out and the molecule is non-polar overall.
This means that the boiling point of the cis-isomer is greater that that of the trans-isomer.
- Cis-1,2-dichloroethene - boiling point = 60.2ºC
- Trans-1,2-dichloroethene - boiling point = 48.5ºC
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 point.
- Cis-butenedioic acid - melting point = 135ºC
- Trans-butenedioic acid - melting point = 287ºC
The action of heat on the cis and trans isomers of but-2-ene-1,4-dioic acid can be used to demonstrate differing chemical properties.
The two carboxylic acid groups in the cis isomer are in close proximity to one another. When heated the cis-isomer can condense the two -COOH groups together forming a cyclic anhydride (butenoic anhydride) and eliminating water.
Due to the restricted rotation of the double bond, the two -COOH groups in the trans isomer are held too far apart for this to happen.
20.6.5: Describe and explain optical isomerism in simple organic molecules. Include examples such as butan-2-ol and 2-bromobutane.
Asymmetric can be used to describe a carbon atom joined to four different atoms or groups.
Chiral (literally 'handedness' from the Greek word for hand 'kheir') can be used to describe a carbon atom joined to four different atoms or groups, and also as a description of the molecule itself.
Enantiomer = an optical isomer. In other words, the two optical isomers of a chiral molecule are called enantiomers.
Racemic mixture = an equimolar mixture of two optical isomers whose opposite rotation effect on plane polarised light cancels out. This is known as external compensation.
20.6.6: Outline the use of a polarimeter in distinguishing between optical isomers. Include the meaning of the term plane-polarised light.
Plane polarised light is light in which all of the light waves are vibrating in the same orientation. This can be achieved by passing normal white light through a lens which contains tens of thousands of vertical lines. This is called a polarising filter.
The light that emerges from the polarising filter has only vertically oriented light. This is now said to be plane polarised light.
When this light passes through a solution of an optical isomer it is rotated through an angle that depends on three factors.
- 1 The type of enantiomer
- 2 The path length
- 3 The concentration of the enantiomer
The light emerging from the sample solution can be analysed by using another polarising filter and seeing how much it must be rotated to allow the plane polarised light to pass through.
The two enantiomers rotate plane polarised light through the same angle, but in opposite directions, clockwise and anticlockwise.
20.6.7: Compare the physical and chemical properties of enantiomers.
The physical properties of enantiomers are identical apart from how they cause plane polarised light to rotate, (there is also the occasional exception of forming crystals which themselves are mirror images)
The chemical properties of optical isomers are identical except when reacting with other optically active molecules. Great use is made of this in biochemistry, with enzymes having optically active sites that can accept only one of the two optically active isomers.
The 21 naturally occuring amino acids are all optically active enantiomers. The enantiomeric pair are designated the letters 'D and 'L' to distinguish them. These letters are assigned according to the CORN convention.
In this convention the molecule is orientated with the hydrogen atom directly behind the chiral carbon atom. The other groups are then examined. The groups are carboxylic acid (CO), the alkyl group(R) and the nitrogen (amine) group (N). If the groups read 'CORN' in a clockwise direction the amino acid is designated using the letter 'L'. If the ward CORN is found by readiong anticlockwise then the amino acid is designated 'D'.
All of the naturally occuring amino acids are L-configuration but does not mean that they all rotate plane polarised light in the same way.