Isomerism Quiz

Isomerism

• Isomers are molecules with the same molecular formula but different arrangements of atoms in space.
• Two main types of isomers: Structural and Stereoisomers.

Structural Isomerism

• Isomers with different structural arrangements of carbon chains or functional groups.
• Different arrangements of carbon skeletons
• Different positions of functional groups
• Different functional groups

Stereoisomerism

• Isomers with the same structural formula but different arrangements of atoms in space.
• Two main types of stereoisomers: Geometric and Optical Isomers.

Geometric Isomerism

• Also called cis-trans isomerism.
• Results from the restricted rotation about double bonds (or in cyclic molecules).
• Cis isomers: Identical groups are on the same side of the double bond.
• Trans isomers: Identical groups are on opposite sides of the double bond.
• Occurs in alkenes (C=C) and cyclic compounds due to the inability of free rotation around a double bond or single bond in a cyclic structure.

Optical Isomerism

• Isomers that rotate the plane of polarized light.
• Optical isomers are non-superimposable mirror images of each other.
• Optically active molecules: rotate the plane of polarized light.
• Chiral carbon: A carbon atom bonded to four different atoms or groups. Chiral carbons are necessary for optical isomerism.
• Enantiomers: Non-superimposable mirror images. They have the same physical properties (melting point, boiling point, density). However they rotate the plane of polarized light in opposite directions.
• Diastereomers: Non-superimposable stereoisomers that are not mirror images. They have different physical properties.
• Racemic mixture: An equal mixture of enantiomers that does not rotate plane-polarized light.

Terminology

• Plane of Symmetry: A plane that divides an object into two equal halves.
• Chiral/Dissymmetric: An object without a plane of symmetry.
• Chiral Carbon: A carbon atom bonded to four different groups, it is the key for optical isomerism.
• Enantiomers: Non-superimposable mirror images.
• Diastereomers: Non-superimposable stereoisomers that are not mirror images
• Racemic Mixture: Equal amounts of enantiomers, they don't rotate plane-polarized light,

Examples

• Lactic acid illustrates optical isomerism. It contains a chiral carbon atom with four different groups attached leading to enantiomers.
• 1-bromo-3-chlorocyclobutane shows geometric isomers. Double bond in alkenes will have geometric isomerism.

Further Details

• Geometric isomers in alkenes: Carbon atoms in C=C are sp2 hybridized. The two carbon atoms and the four atoms attached to them are in the same plane. C=C cannot rotate due to the pi bond.
• Geometrical isomers in cyclic compounds: No rotation around single bonds due to the ring structure. Cis and trans isomers are possible.