Different compounds- different properties. geometric isomerism
Optical Isomerism Arises 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?
four groups of electrons around the central atom four bonding pairs TETRAHEDRAL shape bond angle exactly 109
imaginary mirrors alanine
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.
Left and right hands are an example of non-superimposable mirror images.
Enantiomers ? 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.
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
The CORNy rules for naming enantiomers. Optical isomers exist as L-enantiomers or D-enantiomers. CO R N R N L-enantiomerD-enantiomer
1.Imagine looking down on the molecule with the single H atom point straight up towards you. 2.Label the other three groups a.COOH = CO b.R-group = R c.NH 2 = N 3.If CORN is arranged in a CLOCKWISE direction it is the L-amino acid. 4.If CORN is arranged ANTI-CLOCKWISE it is a D-amino acid. The CORNy rules for naming enantiomers.
How 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.