Isomerism

Definitions

functional group
reactive group within a compound

asymmetric (chiral) centre
atom which has 4 different atoms or
groups of atoms directly attached

optical isomers
molecules that can exist as
non-superimposable mirror images

optically active
sample which rotates the
plane of plane polarised light

racemic mixture
50:50 mixture of 2 optical isomers

homologous series
compounds with:

  • same general formula
  • similar chemical properties
  • gradation in physical properties
  • successive members differ by CH2

structural isomers
molecules that have same molecular
formula but different structural formula

geometric isomers
molecules w same structural formula but
different arrangement of atoms due to presence of 1 or more C=C bond

Naming

priority

  • carboxylic acid
  • aldehyde
  • ketone
  • alcohol
  • amine
  • alkene

suffix for alkene goes
before other suffixes

KEEP e (butane-...)when:

  • suffix starts w consonant
  • 2 or more functional groups (di/tri)

Structural
Isomerism

Stereoisomerism

GEOMETRIC

OPTICAL

CARBON CHAIN

POSITIONAL
CHAIN

FUNCTIONAL GROUP

D: different arrangement of
carbon atoms in molecule

D: functional group is on 2 or more different
positions in carbon chain

D: isomers have different
functional groups

stereoisomers
same structural formula but
different 3D spatial arrangements

C=C double bond consists of

  • sigma bond
  • pi bond

pi bond prevents atoms rotating
around double C=C bond
(no free rotation around double bond)

why E/Z isomers exist?

  • restricted rotation around C=C bond
  • different groups/atoms on each C of double bond

POLARITY of
Z (cis) isomer

  • since 2 highest priority groups are
  • on the same side of double bond
  • electron-withdrawing effects
  • give permanent dipole in molecule
  • results in increase in molecular forces
  • and higher boiling point

POLARITY of
E (trans) isomer

  • dipoles cancel out giving
  • no polarity
  • weaker intermolecular forces
  • lower boiling point

occurs in molecules with chiral centre

  • atom that has different atoms/groups atoms
  • attached tetrahedrally

every molecule has a MIRROR image

  • if mirror image not quite same =
  • cannot be superimposed

Optical Activity

distinguish solution racemic + single enantiomer?

racemic solution
equal concentrations of 2 optical isomers as one isomer rotates it to the right other rotates it to the left 2 opposite effects cancel out

  • allow plane polarised light to pass through it + racemate = no overall rotation of plane of plane polarised light
  • as + and - rotations cancel out
  • single enantiomer = rotation of the plane of plane polarised light

receptor sites in body are
STEREOSPECIFIC

  • only 1 optical isomer is the right shape
  • to bond with the receptor site
  • so only one optical isomer is useful in drug

plane polarised light
light which has been passed
through a polarising filter