# How can geometric isomerism arise due to steric factors?

I can see how a double bond and a ring structure can lead to geometric isomerism but can't see the same for steric factors. Could you help? Thank you!

• Do you mean with rings? 6 membered rings can't flip if they have large bulky steric groups attached. Its hard to visualise without a model though! – Nick Sep 3 '13 at 9:07

Let's take a biphenyl compound:

Due to steric factors, the rings cannot be planar. Instead, they have an approximate right angle between them:

(Image source)

Note that for the right angle to "flip" over to the other side, the $\ce{NO2}$ and $\ce{CH3}$ would have to pass. These groups are bulky enough to prevent that from happening1 unless sufficient energy is provided.

Now, try to look from it along the axis. You'll notice that for this particular molecule, the nitro group farther from your eyes is a clockwise step from the closer nitro group. This happens regardless of which side of the axis (left/right) you look at it from.

On the other hand, if we twisted the molecule so that the angle is again a right angle on the other side, there is now an anticlockwise step from the closer nitro to the farther nitro.

These two configurations of the compound are stereoisomers.

Since the steric barrier can be overcome with sufficient energy, such compounds usually racemize over time (though when the groups themselves are as bulky as a phenyl, this doesn't happen at any appreciable timescale, IIRC).

1. I actually had some trouble using ChemDoodle to draw the molecule without any overlapping bits because of that. Doesn't exactly prove the bulkiness, but one gets the idea.

• Where did you get that book from which you took the image? The link in your "image source". Are there more books? On chem or physics or computer science? – Alpha Mineron Oct 1 '17 at 12:10