Useful Concepts

There is no simple way to know how many isomers exist for a given molecular formula, and therefore, it is important to have a systematic method to derive them.

Important preliminary steps in the process are:

1. determine the IHD to see what types of structures you should be considering
2. list the functional groups that are possible based on the molecular formula and IHD.

Once you have done that, here are a few suggestions to aid the drawing process.....

• start with the longest continuous chain (or ring) first
• move substituents systematically along the chain, remember to consider all possible relative positions if you have more than one substituent.
• if you have C=C then consider E- and Z- possibilities or cis- and trans- for ring systems
• decrease the the chain length one step at a time and introduce new alkyl groups
• with alkyl substituents, remember that two methyl groups are equivalent to an ethyl and an H-
• watch out that you don't repeat structures
• if chiral centers are present then need to consider R and S stereoisomers, do this by identifying the chiral centers.

© Dr. Ian Hunt, Department of Chemistry