Organic chemistry: chirality and stereochemistry

constitutional isomers

• same atoms but different connectivities

• same molecular formula but different structural formula

configurational isomers

• need to make/break bonds to interchange between isomers

• a different configuration is a different molecule

conformational isomers

• interchange through rotation around a single bond

• rotating about the bonds not breaking them

• conformations readily interconvertible and are all the same molecule

• synperiplanar > synclinal/anticlinal > antiperiplanar

stereoisomers

• same molecular formula but different spatial arrangement of atoms

pro-chiral

one step away from being chiral

chirality

• chiral - geometric property of a rigid object is non-superimposable on its mirror image

• asymmetric centre - atom with 4 different groups attached

• homochiral - all one enantiomer and not other

• racemate - 50:50 mixture of 2 enantiomers

enantiomers

• structures that are not identical but are mirror images of eachother

• have identical physical and chemical properties in achiral environments

• all chiral centres are inverted

• rotate the plane of plane polarised monochromatic light in equal and opposite directions

diastereoisomers

• stereoisomers that are not mirror images of eachother (not enantiomers)

• can be chiral or achiral

• when molecule has 2 or more chiral centres they have a mixture of inverted and non-inverted combinations - differ at one or more but not all chiral centres

• can have different physical and chemical properties

meso compounds

• compounds which contain stereogenic centres but are achiral

absolute configuration

1. assign priority to each group bonded to chiral centre

2. arrange molecule so that lowest priority group is facing away from you

3. mentally move substituent priority from highest to lowest - clockwise manner - R, anticlockwise manner - S

look along axis to see whether lowest priority group facing into/out of plane

if multiple bonds, multiply up number of bonds - say that that atom involved in bonding twice if a double bond

if 2 groups are same, look at what the atoms are bonded to

resolution

• way of separating enantiomers

• add resolving agent (must be enantiomerically pure) to racemic mixture

• this separates enantiomers into diastereoisomers so they have different physical and chemical properties

• cleave the resolving agent and recycle if possible

• separate diastereoisomers and convert them back into enantiomeric form

enantiomeric purity: optical activity - specific rotation

• plane polarised light passed through polarimeter tube

• polarimeter tube contains solution of the chiral compound

• rotation of plane polarised light after passing through polarimeter measured

• angle between end point and starting point measured (alpha)

• alpha positive - S

• alpha negative - R

• no rotation - racemic mixture

• longer tube or higher conc = can rotate sample more

enantiomeric purity - chiral HPLC

• mixture of enantiomers reacted with resolving agent in chiral column so they are separated into diastereoisomers

• retention time of diastereoisomers measured