How Are These Molecules Related?

by Kiley Lynch

Here’s a common type of midterm question: you’ll be asked to distinguish whether two molecules are constitutional isomers,  enantiomers, diastereomers, or some other kind of relationship – including their being the same.

Here’s a checklist for this.

  1. Figure out the connectivity of each molecule: are the connectivities the same? If connectivity is different, check molecular formula. If molecular formula is the same, but connectivity is different, they are constitutional isomers. If connectivity is different, and molecular formula is different, they are not isomers. If connectivity is the same, go to 2.
  2. Look for double bonds. If double bonds are not present, go to 3. If double bonds are present, are the orientations the same? If yes, go to 3. If orientations are different, they are diastereomers.
  3. Look for stereocenters. If there’s no stereocenters, and everything is the same so far, then these molecules are the same. If there are stereocenters, determine R/S of each, and evaluate.
  4. If one stereocenter is present: If R/S values are the same for each, the molecules are the same. If R/S values are opposite for each, they are enantiomers.

Things get a little more squirrelly when more than one stereocenters are present. Let’s talk about that tomorrow.

In the meantime, here’s some tips.

  • it can really help if you can figure out the name. This will help to determine the connectivity. To take a (too simple) example, the names 1-iodobutane and 2-iodobutane denote that these two molecules have different connectivity, for instance. Molecules with the same molecular formula but different connectivity are constitutional isomers.
  • if two molecules have the same numbering (e.g. 2-butene)  but differ by the arrangement on their double bonds Z (cis) or E (trans) then they are diastereomers.
  • if two molecules have the same connectivity, but you aren’t sure about their stereocenters, figure out R and S for each one.
  • Figuring out R and S for each stereocenter is the quickest, fastest way to tell if two molecules are enantiomers, diastereomers, or the same. It’s mucheasier than trying to rotate the bonds. That’s why I strongly recommend that you spend a lot of time developing the skill of determining R/S quickly. It will serve you well for the rest of the course.

Tomorrow: let’s explore this a bit more, except with diastereomers thrown in.

 Thanks for reading! James