20.6 – Stereoisomerism
20.6.1 – Describe stereoisomers as compounds with the same structural formula but with different arrangements of atoms in space
Stereoisomers have the same molecular and structural formula, but have different arrangements of atoms in space. Their 3-dimensional arrangement is different. This is then further categorised in to geometrical (cis-trans) and optical isomers.
20.6.2 – Describe and explain geometrical isomerism in non-cyclic alkenes
Some molecules are constrained in terms of how much they can rotate in space due to multiple bonds or a cyclic structure. Any functional groups that are attached to the carbon atoms that have restricted rotation will form geometric isomers.
To identify where the functional groups are situated, the prefixes cis and trans are used. A cis isomer is when the groups are on the same side on the fixed carbon atoms. On the other hand, trans isomers are when the functional groups are on opposite sides of the atoms.
20.6.3 – Describe and explain geometrical isomerism in C3 and C4 cycloalkanes
Geometrical isomerism frequently occurs in both alkenes and cycloalkanes.
Derivatives of Cyclopropane
This has two isomers, which are distinguished based on the location of the methyl (CH3) groups. The cyclic nature of the molecule means that the bonds cannot rotate as they do in propane.
Derivatives of Cyclobutane
Dichlorocyclobutane also has two isomers.
20.6.4 – Explain the difference in the physical and chemical properties of geometrical isomers
Physical Properties
The physical properties change according to the shape and polarity of the molecules. The dipole-dipole attractions in polar molecules create stronger intermolecular forces and, as a result, have high boiling points. Since cis-isomers are not symmetrical, they will be polar and therefore have higher boiling points. Conversely, the trans– isomers are influenced only by Van der Waal’s forces, giving them a lower boiling point.
On the other hand, melting point is related to how the molecules pack together in the solid state. Furthermore, when there are bulky groups attached to the molecule, the symmetrical trans-isomers can pack closer together, and will have the higher melting point.
Chemical Properties
In many cases, the chemical properties of cis- and trans-isomers remain very similar. However, there are still some exceptions. Butenedioic acid has a different reactivity, depending on which isomer it is.
20.6.5 – Describe and explain optical isomerism in simple organic molecules
Optical isomers are mirror images of each other, which means that they cannot be superimposed. This applies to molecules with a carbon atom surrounded by four different substituents tetrahedrally arranged around it. The two isomers are asymmetrical.
The central carbon is called the chiral centre of the molecule, and a chiral molecule is one that exhibits optical isomerism. Two non-superimposable optical isomers are called enantiomers. When the different optical isomers are present together in equal proportions, it is called a racemic mixture.
20.6.6 – Outline the use of a polarimeter in distinguishing between optical isomers
White light enters a polariser, and only the waves of light that oscillate at a specific plane will be able to pass through, and the rest are blocked. Plane-polarised light is the beam of light that emerges after it has been passed through a polariser.
When plane-polarised light is passed through solutions of some organic compounds, the plane of polarisation is rotated. These compounds are known as optically active. After the light passes through the solution, it enters a second polariser called the analyser. This is rotated until the light can pass through, indicating the angle between the original and new planes of light.
If the compound rotates the light clockwise, to the right, then it is called dextrorotatory, and given a (+) sign. If the compound rotates the light anticlockwise, to the left, then it is called levorotatory and given a (-) sign. Two enantiomers will have the same angle of polarised light, but in opposite directions.
A mixture of different enantiomers is called a racemic mixture. If they are present in equal proportions, then the solution will be optically inactive, since they cancel each other out.
20.6.7 – Compare the physical and chemical properties of enantiomers
Different enantiomers still have the same melting point, solubility, density and spectroscopic properties.
The only physical property that changes is their direction of rotation of plane-polarised light, as described above.
Similarly, enantiomers have the same chemical properties. The only exception is when they interact with other optically active compounds. This is seen in the body in the interactions of enantiomers with specific receptors and enzymes. In biological systems, there is usually only one enantiomer found.