Bredt's Rule in Organic Chemistry

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

Bredt's rule came about due to an observation of bridged-cyclic molecules. In this lesson we will learn what Bredt's rule is, why it occurs, and discuss some applications.

Bredt's Rule

Have you ever tried getting together with someone, but your schedules just never seem to line up? You can keep trying to get together, but it won't ever work.

In organic chemistry, double bonds (pi bonds) are formed when two p-orbitals line up. If these two orbitals don't line up, a pi bond can't be formed between two molecules no matter how we rotate the molecule, even if they are already bound together with a single bond.

In the 1920s, German organic chemist Julius Bredt was experimenting with bicyclic compounds and rings with a bridge. He realized that, no matter what he did, these rings could never form a double bond at the bridge (or the bridgehead). Let's look at a simple bridged molecule, norbornane:

Norborane structure

Theoretically there are three possible places for a double bond to be located:

Norborane with double bonds

Just as a side note, it may seem as though there would be a lot more possibilities to put this double bond, but any of these other possibilities is actually the same molecule rotated.

Yet, Bredt realized that only the double bond between carbon 2 and carbon 3 would form. The other two isomers would never form. At the time he had no explanation for this phenomena, but this rule that double bonds can't form at the bridgehead became known as Bredt's rule.

Why Is Bredt's Rule True?

Today we have started to understand bonds a little more, and thus have an explanation as to why Bredt's rule occurs. Bredt's Rule is true because the strain on the ring pulls the p-orbitals into different planes, so they cannot line up to form a pi-bond (double bond).

Let's take a look at the 3D model of norbornane:

3D molecule

Notice that the carbon bridge atom isn't in the same plane as the carbon atoms around it. This means that the p-orbitals (the orbitals that form double bonds) aren't in the same plane:

P-orbitals on norborane

The p-orbital on carbon 1 is actually pointing behind the sheet of paper and out of the sheet of paper. The p-orbitals on carbon atoms 2, 3, and 7 are pointing up and down within the plane of the paper.

The p-orbital on carbon cannot line up with the p-orbital on carbon 2 or carbon 7. Thus a double bond can never form. But the p-orbitals on carbon 2 and carbon 3 do line up, so a double bond can form here.

Applications of Bredt's Rule

Typically amides are very stable molecules. This is due in part to the significant resonance ability for the carbon-nitrogen bond to form a double bond:

Resonance in amides

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