Isomerism. Extended materials Jelena Marinković. First class: Isomerism-revision.
The existence of chemical compounds that have the same molecular formulae but different molecular structures or different arrangements of atoms in space is called isomerism. In structural isomerism the molecules have different types of compound or they make simply differ in the position of the functional group in the molecule. Structural isomerism generally have different physical and chemical properties. In stereoisomerism , the isomers have the same formula and functional groups, but differ in the arrangement of groups in space. Optical isomerism is one form of this. Another type is cis-trans isomerism, in which the isomers have different positions of groups with respect to a double bond or ring or central atom.
Structural Isomers StereoisomersDifferent bonding Different shapes
Students should explain different types of isomerism, and how that influence to the physical and chemical properties of compounds.
Because these molecules are so similar, there is very little difference in their physical and chemical properties. In fact the only difference is that they have differing effects on polarised light, one isomer rotating the plane of polarisation clockwise, the other anticlockwise . Biological systems are much more sensitive to the shape of the molecule and so the different enantiomers usually have different biological effects, for example, one isomer of aspartame tastes sweet, but the other enantiomer tastes bitter.
If a molecule has four different groups attached to a single carbon atom, then the compound can exist as a pair of enantiomers. The only difference in the properties of these compounds is in their interaction with plane polarised light. Plane polarised light can be considered to be light in which the oscillation of the wave is restricted to one plane, say the vertical. This can be achieved by passing the light through a polarising filter. If the light is now passed through a second polarising filter orientated in the same direction (e.g. vertical) then there is virtually 100% transmission . If the second polarising filter has its axis at right angles to the first (e.g. horizontal) then no light will pass. A pure enantiomer placed between the two filters will rotate the plane of polarisation in one direction (say clockwise) so that maximum transmission is no longer when the second filter is aligned with the first one. The second enantiomer will rotate the plane of polarisation by exactly the same amount but in the opposite direction (anticlockwise). Substances that affect polarised light in this way are said to be optically active. An instrument containing two polarising filters that can be rotated relative to each other, so as to allow the angle between their orientations to be measured is called a polarimeter.
The other enantiomer would rotate it an equal amount but anticlockwise. Apart from this the properties of enantiomers are identical. Chemically the behaviour of the enantiomers is identical unless the reaction also involves a pure enantiomer.