Problem-Solving Strategies

Possible relationships

Click a relationship to reveal its definition.

+ Identical

Identical drawings are superimposable because they represent the same molecule. Both drawings have the exact same IUPAC name, including the same stereodescriptors (e.g., R/S, cis/trans).

+ Enantiomers

Enantiomers are nonsuperimposable mirror images — but how do we recognize a pair of enantiomers when we see them? First of all, only a chiral molecule has an enantiomer. If a molecule has an internal plane of symmetry, then it is achiral and does NOT have an enantiomer.

When comparing enantiomers, you will find that they have opposite R/S configurations for ALL chiral centers. So if the first drawing represents a chiral molecule, there are two simple ways to identify its enantiomer: the second drawing must be the enantiomer if you see a mirror-image relationship, or if you determine that they have opposite configuration(s). The only difference between enantiomers is their 3D shape, so they have the exact same IUPAC name, but with opposite configurations of their chiral centers (R/S).

Note: sometimes the name of a chiral molecule starts with d, l, (+) or (−), which relates to its optical activity. Enantiomers have specific rotations that are equal in magnitude but opposite in sign, so one enantiomer is dextrorotatory (positive angle of rotation, α > 0) and the other is levorotatory (negative angle of rotation, α < 0).
+ Diastereomers

Diastereomers are stereoisomers that are not enantiomers, so they do NOT have a mirror-image relationship. If the molecule in the first drawing has multiple chiral centers, and some — but not all — of the configurations have been changed (inverted) in the second drawing, then the two drawings represent diastereomers.

If the only difference between two compounds is that one is cis and the other is trans, then they are diastereomers. Like all stereoisomers, diastereomers have the same IUPAC name but with different stereodescriptors (e.g., R/S, cis/trans).

+ Constitutional Isomers

Constitutional isomers have the same molecular formula but different connectivity. As such, they do NOT have the same IUPAC names. They can look wildly different, with different functional groups — their only relationship is having the same number of C, H, N, O… atoms.

For the number of hydrogen atoms to match, both compounds must have the same HDI (degrees of unsaturation). Rather than counting the number of H's, you can count up the number of rings and pi bonds in each compound. If the degrees of unsaturation don't match, then the two compounds can't have the same molecular formula and they are not constitutional isomers.

+ Unrelated

Unrelated compounds do not have the same molecular formula.

A systematic approach

Work through these steps when you're comparing two drawings.

  1. A good first step is to see if the two drawings represent the same molecule. Mentally flip and/or rotate one of the drawings to see if the position of every atom matches the other. If needed, rotate single bonds to help with the comparison. Remember that flipping a molecule like a pancake will turn wedges into dashes. If the drawings match exactly, then they are identical.
  2. If the drawings don't superimpose exactly, then how do they differ?
    1. Do both compounds have the same functional groups attached at the same positions to the same carbon chains, which would give the same IUPAC name? If so, the only difference is their three-dimensional shape, and they are stereoisomers — either enantiomers or diastereomers. If you can't easily determine the relationship between two chiral centers, label each as R or S to see whether or not they match.
    2. If the connectivity of the atoms is different (e.g., the functional groups are different or at different positions on the carbon chain), then you have constitutional isomers — assuming the molecular formula is the same for both. If the molecules do not have the same molecular formula, then they are unrelated.
  3. Chair conformations are difficult to evaluate, so you can try redrawing cyclohexane rings as simple hexagons. Groups in an “up” position should be drawn as wedges, and groups in a “down” position should be drawn as dashes.