A2 Nomenclature and isomerism

Where compounds have the same structural formulas but differ in the arrangement of the bonds in space

1) E-Z isomerism 2) Optical isomerism

When there are four different substituents attached to one carbon atom.

Molecules that are non- superimposable mirror images of one another.

• They differ in the way that they rotate the plane of polarisation of polarised light. • This can be clockwise ((+)- isomer) or anti-clockwise ((-)-isomer)

• Enantiomers are optical isomers- they are non-superimposable mirror images of each other. • They have similar chemical properties but differ in their ability to rotate plane polarised light and in their biological activity.

The carbon atom bearing four different substituents. It can also be called the chiral carbon/ asymmetric carbon.

When a compound and its mirror image are non-superimposable

• The ability of a single optical isomer to rotate the plane of polarisation of plane-polarised monochromatic light. • An optically active molecule has no planes of symmetry.

Vibrating electric and magnetic fields that oscillate in all planes perpendicular to the direction of travel.

Monochromatic light that oscillates only in one plane

A filter that cuts out all light vibrations except for those in one plane.

A device that converts unpolarised light to plane-polarised light

• A device for measuring an angle of rotation caused by a chemical. • Plane polarised light passes through the analyser.

An equimolar mixture of enantiomers of a chiral compound.

No

A racemic mixture doesn’t rotate the plane of polarisation of plans-polarised light because the two enantiomers cancel out each others light rotating effect.

1) Polarised light is passed through two solutions of the same concentration, each containing a different optical isomer of the same substance. 2) One solution will rotate the plane of polarisation through a clockwise angle ((+)-isomer) 3) The other will rotate the plane of polarisation by the same angle anti-clockwise. This is the ((-)-isomer)

• Often, when made synthetically, a compound ends up as a mixture of optical isomers. • However when the same compound is made naturally in living systems, only one of the isomers is formed.

Many naturally occurring molecules are made using enzyme catalysts which cause one of the isomers to be formed.

• Many drugs work by binding with the active site or receptors of a cell which have specific three dimensional structures. • This means that sometimes only one optical isomer of a drug will fit and therefore on it one isomer will be effective.

1) Separate the two isomers- this may be difficult and expensive as optical isomers have very similar properties. 2) Sell the mixture as a drug- this is wasteful as half of it is inactive. 3) Design an alternative synthesis of the drug that makes only the required isomer.