lecture 4: chirality

isomers

set a molecules that have the same molecular formula, nut have a different arrangement of the atoms in space

constitutional isomers

atoms are arranged in a different order

• different physical and chemical properties

• distinctly different molecules

stereoisomers

atoms are arranged in the same order but in different 2-D orientation

line drawings in 3-D space

thick triangles: means the bond looks towards you

• sits in front of the paper plane

dotted triangles: means the bond looks away from you

• sits behind the paper plane

cis-trans isomers

• Depending on whether the substituents of highest priority at each end of the double bond are next to or opposite each other

• Occurs with alkenes

E and Z isomers

• the E, Z system must be used for tri- tetra-substituted alkenes (3 or 4 different bonded groups)

• To assign an E, Z configuration, first assign a priority to the substituents on each side of the double bond

• Priority based on atomic number

• Highest atomic number= highest priority

• number 1 group on opposite sides or same sides

chirality

• when a molecule is not identical to its reflection

• mirror image is not superimposable with itself

achirality

• when a molecule is identical to its reflection

• superimposable

• have a plane of symmetry (an imaginary plane passing through an object dividing it such that one half is the mirror image of the other half)

chiral center

a carbon atom that is bonded to 4 different groups

• stereogenic center

• induces chirality

• asymmetric carbon center

stereoisomers; enantiomers

direct mirror images of a molecule/pairs of mirror images of chiral molecules

• do not differ in physical characteristics

• interact differently with polarised light and with stereospecific environments (e.g. active sites, receptors/proteins on our tongue are chiral environments that can perceive the different enantiomers)

• one enantiomer will shift the light to one side of the plane, and vice versa

stereoisomers; naming enantiomers

use the Cahn, Ingold, Prelog systems of nomenclature

• used to designate configuration at a stereocenter (chiral center)

• labels R and S are used to differentiate between enantiomers

• priority of the 4 different substituents is assigned through atomic number; the next connected atom to the C is used for atomic priority numbering

steps for naming enantiomers

stereocentres and stereoisomers

• 1 stereocentre, 2¹=2 stereoisomers are possible

• 2 stereocentres, 2²=4 stereoisomers are possible

• general rule: for a molecule with n stereocentres, a max of 2ⁿ are possible

diastereoisomers

stereosiomers that are not mirror images and cannot be superimposed

meso compounds

• Tartaric acid has two chiral centres

• Therefore has 4 stereoisomers that exist

• Two of them are enantiomers

• The other enantiomers actually create an achiral molecule with an internal symmetry plane within the molecule

• Thus these are not enantiomers

• THIS IS CALLED THE MESOFORM

enzymes

chiral substances which only produce or react with substances that match their stereochemical requirements