Cycloalkenes and Bredt's Rule
Cycloalkenes
Most cycloalkenes contain cis-double bonds. The cis prefix for cycloalkenes does not need to be written. It is assumed. The prefix only needs written if it is trans because the only way it is possible to have a trans-double bond is to have at least eight carbon atoms in the ring.
Bredt's Rule
Let’s look at bicyclic molecules that are alkenes. A double bond cannot be attached to a bridgehead carbon of a bicyclic compound if the ring it is in does not have at least eight carbon atoms in it. This is Bredt’s Rule
Violation of Bredt's Rule
Not Violations of Bredt's Rule
Bredt's Guidelines?
I show you Bredt's Rule here as you may still encounter it in most organic chemistry textbooks. But, this is interesting because it shows how we need to be careful as scientist around these "rules". Bredt's Rule was widely accepted for over 100 years. It was so accepted that for many years, most organic chemists would never even try to make a bicyclic alkenes that "violated" Bredt's Rule. But, in 2004, researchers at UCLA did indeed make some.
Although these anti-Bredt alkenes are pretty unstable and react immediately, (because of the reasoning given above) they can be made. This is a great reminder to be wary of allowing others to impose limits on you based on what they think is possible.
8. State whether the following compounds violate Bredt’s rule.
a)
b)
c)
d)
e)
Answers
8. a) no b) yes c) no d) yes e) no