5. Stereoisomers

What are stereoisomers?

Stereoisomers have the same molecular formula, connectivity, and bond multiplicity but differ in the spatial arrangement of atoms.

What are the two major classes of stereoisomers?

Conformational isomers and configurational isomers.

How do conformational isomers differ from configurational isomers?

Conformational isomers can rotate around single bonds without breaking them, whereas configurational isomers require bond-breaking and reforming to interconvert.

What are the two types of configurational isomers?

Cis–trans isomers and optical isomers.

What are cis–trans isomers?

Isomers in which the same substituents are on the same side (cis) or opposite sides (trans) of a double bond or ring system.

Why can’t propene exhibit cis–trans isomerism?

Because one of its carbon atoms has two identical hydrogen atoms, preventing the formation of distinct isomers.

What is the reference plane in cis–trans isomerism?

The horizontal axis of the double bond, used to determine whether an isomer is cis or trans.

What is optical isomerism?

A type of configurational isomerism where molecules with chiral centers exist as enantiomers that rotate plane-polarized light.

What is a chiral carbon?

A carbon atom bonded to four different atoms or groups, also known as a stereocenter or asymmetric center.

Why is penicillamine optically active?

Because it has a chiral carbon and can exist as two different enantiomers.

How do enantiomers affect our senses?

Different enantiomers of the same compound can have different odors and tastes.

How do enantiomers interact with plane-polarized light?

One enantiomer rotates light clockwise (designated +), while the other rotates it counterclockwise (designated -).

What is a racemic mixture?

A 50:50 mixture of two enantiomers that does not rotate plane-polarized light.

Stereoisomers

Molecules with the same molecular formula and connectivity but different spatial arrangements of atoms.

Conformational Isomers (Conformers)

Stereoisomers that differ by rotation around a single bond.

Configurational Isomers

Stereoisomers that can only be interconverted by breaking and reforming bonds.

Cis–Trans Isomers

A type of configurational isomerism where identical groups are positioned either on the same (cis) or opposite (trans) sides of a double bond or ring.

Reference Plane

The horizontal axis of a double bond or the flat face of a ring structure, used to determine cis–trans isomerism.

Chiral Carbon

A carbon atom bonded to four different atoms or groups, making the molecule optically active.

Optical Isomers (Enantiomers)

Non-superimposable mirror-image isomers that rotate plane-polarized light in opposite directions.

Optical Activity

The ability of chiral molecules to rotate plane-polarized light.

(+)-Enantiomer

The enantiomer that rotates plane-polarized light clockwise.

(-)-Enantiomer

The enantiomer that rotates plane-polarized light counterclockwise.