2Types of isomerism Isomerism Structural isomerism Stereoisomerism Geometric isomerismOptical isomerism
3Different compounds- different properties. geometric isomerism
4Optical IsomerismArises because of the different ways you can arrange four different groups around a carbon atom.Hang on!! Aren’t -amino acids a group of compounds that have four different groups around a carbon atom?
5four groups of electrons around the central atom four bonding pairsTETRAHEDRAL shapebond angle exactly 109
7The only way to make these two superimpose is to break & reform bonds. All molecules have mirror images – however they don’t all exist as two isomers.What makes an isomer is the fact that the mirror image and the original molecule are non-superimposable!The only way to make these two superimpose is to break & reform bonds.
8Left and right hands are an example of non-superimposable mirror images.
9Enantiomers ?Molecules such as alanine that exist in these two forms = optical isomers or enantiomers.We distinguish between the two enantiomers of a molecule by +/-, D/L or more correctly R/S.A 50/50 mixture of the two enantiomers is called a racemic mixture or a racemate.
10chiral centre of alanine Some more key words …Molecules that are not superimposable on their mirror images are called chiral molecules.A carbon surrounded by 4 different groups is called a chiral centre.chiral centre of alanine
11The CORNy rules for naming enantiomers. Optical isomers exist as L-enantiomers orD-enantiomers.L-enantiomerD-enantiomerRRCOCONN
12The CORNy rules for naming enantiomers. Imagine looking down on the molecule with the single H atom point straight up towards you.Label the other three groupsCOOH = COR-group = RNH2 = NIf CORN is arranged in a CLOCKWISE direction it is the L-amino acid.If CORN is arranged ANTI-CLOCKWISE it is a D-amino acid.
13How do enantiomers differ? Behave identically in ordinary test-tube chemical reactions.Have same physical properties.BUT!!Behave differently in presence of other chiral molecules.e.g. Taste-buds are ‘chiral’ D-amino acids taste sweet, L-amino acids are tasteless or bitter.