Isomers are compounds that have the same molecular formula, but in which the connectivity of the atoms differ. There are different types of isomers: constitutional isomers, positional isomers, conformational isomers, configurational isomers (geometric and optical) and functional group isomers. In this lab, we consider positional isomers (regioisomers), isomers that have the same molecular formula but differ in the substitution position of a functional group; and constitutional isomers, molecules having the same molecular formula but different carbon connectivity.

An easy example to consider is isomers of C3H8O. This molecular formula has three possible stable isomers, two alcohols and an ether. Placing an alcohol group on the terminal, or 1, position of propane (C3H8) yields the primary alcohol 1-propanol (propan-1-ol). Another isomer is 2-propanol (propan-2-ol), it is formed by placing the alcohol in the 2 position on the carbon backbone, making it a secondary alcohol. If we look at all of the isomers of the molecular formula C3H8O, then we also need to write down the structural formula for ethyl methyl ether (methoxyethane). Ethyl methyl ether is a constitutional isomer, while the propanol isomers are positional isomers.

About the author

Dean Antic, Ph.D., is a Senior NMR Applications Scientist, organic chemist and spectroscopist at Thermo Fisher Scientific, San Diego, CA. Formerly, Dean was an adjunct professor of chemistry at Northeastern Illinois University and a certified 9-12 chemistry instructor.