FLOW CHART FOR DETERMINING THE RELATIONSHIP BETWEEN TWO STRUCTURES

Determining stereochemical relationships between two structures:

Two structures that have the same atom-to-atom connections may be configurational diastereomers, configurational enantiomers, or homomers. (Two homomers may differ in their conformation, of course.)

Two compounds that have the same configuration at all of their stereocenters are homomers.

Two compounds that differ at some but not all of their sp³ stereocenters or at any of their sp² stereocenters are configurational diastereomers.

Two compounds that differ at all of their sp³ stereocenters and at none of their sp² stereocenters are configurational enantiomers (if they are chiral) or homomers (if they are achiral).

NOTE: Allenes and iminoketenes, R₂C=C=CR₂ and R₂C=C=NR, in which the two R groups on each of the terminal C atoms are different, are treated as a single sp³ stereocenter.

http://chemistry.umeche.maine.edu/CHY431/Isomers.html

Determining the chirality of a compound:

A compound that has no sp³ stereocenters must be achiral overall. However, it may have chiral conformers, and those chiral conformers may be lowest in energy, and sometimes there may even be a large steric barrier to interconversion of the conformational enantiomers.

A compound that has one sp³ stereocenter must be chiral.

A compound that has more than one sp³ stereocenter may or may not be chiral. If it has a plane of symmetry, it is achiral. If it lacks a plane of symmetry, it is probably chiral.

NOTE: Allenes and iminoketenes, R₂C=C=CR₂ and R₂C=C=NR, in which the two R groups on each of the terminal C atoms are different, are treated as a single sp³ stereocenter.

Discussion: Words in chemistry often have very precise meanings that are meant to convey specific concepts or relationships. It is important for you to be able to define these words in a precise and accurate manner, and provide examples where relevant. Understanding is much more useful than rote memorization.

Exercises: Provide a precise definition for each word.

• Achiral

• Chiral

• Constitutional isomer
• Dextrorotatory
• Diastereomer
• Enantiomer

• Isomer
• Levorotatory
• Meso compound
• Optically active
• Plane polarized light
• Polarimeter
• Racemic mixture
• Resolution
• Specific rotation
• Stereocenter
• Stereoisomer

You might want to look at some definitions of stereoisomers.

Or you might want to look at some examples of the different kinds of stereoisomers.

Or you might find it useful to see a glossary of stereochemical terms.