Stereoisomers

trans but-2-ene

cis but-2-ene

Stereoisomers are compounds that

have the same molecular formula
have the same atom-to-atom bonding sequence
have a different arrangement of the atoms in space

Compounds containing double bonds where each of the carbons of the double bond has two different groups can exist as stereoisomers.

But-2-ene is an example of an alkene that can exist as stereoisomers.

Enantiomers(optical isomers) are another class of stereoisomers. Compounds that can exist as enantiomers have non-superimposable mirror images due to being asymmetric (having no symmetry planes or centres).

Enantiomers are said to be "optically active" because they differ in only one physical property, namely, the direction in which a solution of the enantiomer rotates plane-polarised light.

One enantiomer rotates the plane of polarised light in one direction.
The other rotates it an equal amount in the opposite direction.

Compounds that have a carbon atom with four different substituents are asymmetric.

The carbon that has the four different groups is referred to as the stereogenic (chiral) centre.

The stereogenic carbon is marked with a red dot in the structure of pentan-2-ol.

To show the structures of the two enantiomers FIRST draw a three-dimensional representation of the molecule with the stereogenic carbon at the center.

The solid lines are IN the plane of the page. The solid wedge points to a group IN FRONT of the plane of the page. The hashed wedge points to a group BEHIND the plane of the page.


enantiomer 1	enantiomer 2

THEN reflect that structure in the plane of the page as shown at the right.

Enantiomers 1 and 2 are nonsuperimposable. If enantiomer 2 is rotated about the C-OH bond so that the OH and the CH₂CH₂CH₃ line up with enantiomer 1, the CH₃ and H do not line up.

Molecules such as pentan-2-ol that can exist as enantiomers are said to be chiral.