Stereo Isomerism

• There are two main types of isomerism, structural isomerism and stereo isomerism.
• Structural isomerism is concerned with the order in which the atoms are bonded and is made up of chain isomerism, position isomerism and functional group isomerism.
• Stereo isomerism is concerned with the layout of the atoms in space; it is made up of geometric isomerism (e.g. cis-trans), and optical isomerism.

Geometric Isomerism

• But-2-ene is a structural isomer of but-1-ene.
• However, if you look at how the but-2-ene can be formed, then you will realize that there are two variations of the molecule:
• To turn the first molecule into the second; on end of the molecule must be rotated; however this would involve breaking one of the bonds in the C=C double bond.
• This would require a chemical reaction, and so the two variations of but-2-ene are isomers.
• They are cis and trans isomers.

Cis and Trans Isomers

• Cis and trans isomers are molecules with the same atoms within them, but a different layout of the atoms. The order of the atoms in cis trans isomers is the same, but the geometries are different.
• This form of isomerism is sometimes called geometric isomerism, and is a type of stereo-isomerism.
• Stereo-isomers have the same molecular formula, and the atoms are bonded together in the same order; however the arrangement of the atoms in space is different in each isomer.
• Cis and trans isomers are given different names- if the substituent groups are on adjacent sides of the double bond to each other, the prefix cis is added. If they are on opposite sides, the suffix trans is added.
• They must have different groups on each C of the double bond.
• Cis and Trans isomers can also be represented by skeletal formula.
• As cis and trans isomers are different compounds, they have different physical properties.

Optical Isomerism

• When four single bonds are formed around a central carbon atom, the molecule formed is tetrahedral in shape.
• If the four groups are all different, the molecule can exist in two isomeric forms. This can be seen with the amino acid, alanine:
• The way that they are different is that they are mirror images of each other which can’t be superimposed onto each other.
• In the two molecules, the H atom, the C atom and the CH₃ groups will coincide (in the same planes as each other); however, the COOH and NH₂ groups are in the wrong places; no amount of twisting will allow them to be superimposed onto each other.
• The only way to make them the same is to break the C-COOH and N-NH₂ bonds and swap the groups over; this involves bond breaking and bond making and so is a chemical reaction.
• All molecules have mirror images; however some of them don’t exist as non super-imposable images:
• The mirror images of glycine can be superimposed onto each other; the molecule just has to be flipped vertically and the isomer is made.

D/L enantiomers

• Molecules that show this kind of isomerism are called enantiomers.
• Any central carbon atom that is surrounded by four different atoms/groups will form two enantiomers.
• The arrangement of atoms like this is described as a chiral centre (our hands and feet are also examples of chiral centres, which is where the word chiral comes from- Greek for “handedness”).
• As many proteins have four different groups around a central carbon atom, they also show chirality.
• All the proteins within our body are built up from L-enantiomers (they have the same geometric arrangement around the central carbon atom).
• L-enantiomers can be identified using the CORN rule: with the H atom pointing upwards, looking down from the H atom to the carbon and moving clockwise, the L-amino acid has the order COOH, R, NH₂, i.e.
• Enantiomers behave identically in all chemical reactions and most of their physical properties are the same.
• The only difference comes when they are in the presence of other chiral molecules.
• D and L enantiomers will react differently with our enzymes (like trying to put a left hand glove onto your right hand).
• Examples of different effects include:
  • Enantiomers react differently with chiral taste-buds (D amino acids are sweet and L-amino acids are bitter/tasteless).
  • Enantiomers can smell differently.
  • Many enantiomers are beneficial medicines; however their isomers can have drastic effects (e.g. thalidomide).
• D-amino acids do exist in nature, for example penicillin works by breaking down the peptide links in D-alanine; these occur in the cell walls of bacteria but not in humans. Thus penicillin only affects bacterial cells.

Test for enantiomers

• A plane of polarised light is rotated in opposite directions by each enantiomer.
• If a solution containing equal amounts of each enantiomer is placed in a polarimeter, the solution will be optically inactive.

Useful books for revision