Isomerism

Isomerism

Compounds with the same molecular formula but different structural formulas or arrangements of atoms.

Types of isomerism

Structural (or constitutional) isomerism and stereoisomerism.

Structural isomerism

Isomers that differ in the connectivity of atoms (how atoms are bonded).

Types of structural isomerism

Chain isomerism, positional isomerism, functional group isomerism.

Chain isomerism

Isomers differ in the carbon chain arrangement (e.g., straight vs branched).

Positional isomerism

Isomers differ in the position of a functional group on the carbon chain.

Functional group isomerism

Isomers have different functional groups but the same molecular formula.

Stereoisomerism

Isomers with the same molecular formula and connectivity but different spatial arrangement of atoms.

Types of stereoisomerism

E/Z (geometric) isomerism and optical isomerism.

E/Z isomerism

Caused by restricted rotation around the C=C double bond and different groups attached to the carbons.

Assigning E and Z configurations

Assign priorities to groups on each carbon using atomic numbers; Z (zusammen) means highest priority groups on the same side; E (entgegen) means highest priority groups on opposite sides.

E/Z isomerism limitation

Alkenes with two identical groups on a carbon cannot show E/Z isomerism because priority groups are identical.

Optical isomerism

Isomerism due to molecules being non-superimposable mirror images (enantiomers), usually caused by a chiral carbon.

Chiral carbon

A carbon atom bonded to four different groups.

Enantiomers

Pairs of optical isomers that are mirror images but not superimposable.

Effect of enantiomers on plane-polarised light

Each enantiomer rotates plane-polarised light in opposite directions (one clockwise, one anticlockwise).

Racemic mixture

An equal mixture of both enantiomers that does not rotate plane-polarised light.

Optical isomerism and drug activity

Different enantiomers can have different biological effects; one may be active and the other inactive or harmful.

Technique to separate enantiomers

Chiral chromatography or using enzymes that selectively react with one enantiomer.

Cis-trans isomerism

A special case of E/Z where each carbon of the double bond has one hydrogen and one other group.

Difference between cis and trans isomers

Cis: same groups on the same side of the double bond; Trans: same groups on opposite sides.

Isomerism in cycloalkanes

They can exhibit cis-trans isomerism due to restricted rotation around the ring.

Conformational isomerism

Different spatial orientations of a molecule due to rotation around single bonds (not tested deeply in AQA).

Meso compound

A molecule with chiral centers but is superimposable on its mirror image due to an internal plane of symmetry (optically inactive).

Importance of stereoisomerism in AQA Chemistry

Because it affects physical and chemical properties, especially in organic molecules.

Isomerism shown by but-2-ene

E/Z (geometric) isomerism.

Difference between optical isomers and E/Z isomers

Optical isomers differ by chirality and rotate plane-polarised light; E/Z differ by spatial arrangement around a double bond.

Recognising optical isomerism

Look for a carbon with four different groups attached (a chiral center).

Priority assignment for E/Z isomerism

The atom with the highest atomic number bonded directly to the double-bonded carbon gets higher priority.

Example of functional group isomerism

C₂H₆O can be ethanol (an alcohol) or dimethyl ether (an ether).