W3L5: Isomerism in organic chemistry

Structural isomerism

• Isomers explain why there are so many different carbon compounds: a lot are variations with the same formula

• Structural isomers are compounds with the same molecular formula, but atoms are bonded in different orders. Aka configurational isomers

• These are structural isomers of one another: contain the same atoms, but butane is linear, methylpropane is branched

• Different structures BUT different physical properties

Structural isomers of heptane: C7H16

Structural isomers of C4H10O

Stereoisomerism

Stereoisomers are compounds with identical formulae, atoms bonded in the same order,

but in a different spatial arrangement

Geometric isomerism

Geometric isomers are stereoisomers that arise due to restricted rotation within a molecule

Optical isomerism

Optical isomers are stereoisomers that arise due to reflectional symmetry

Optical activity

• Chiral molecules rotate plane-polarised light: this is called optical activity

• A mixture that contains more of one enantiomer than the other is optically active

ALL isomers of C7H16

ALL isomers of C4H10O

ALL isomers of C4H8

We have partly discussed these already: there is one DBE here

• we need to consider double bonds and rings

Multiple stereogenic centres

Each additional stereogenic centre generates additional isomers

Generally speaking, n stereogenic centres gives 2n stereoisomers.

The exception to this is when meso forms exist:

Molecules with no stereogenic centres, which do display optical isomerism

Rings can display both geometric and optical isomerism