Bredt's Rule
No double bond can be generated on a bridgehead carbon of bicyclic compounds unless size of ring is sufficiently large. This rule was proposed in 1924 by organic chemist Julius Bredt.
It is not possible for rings with fewer than eight atoms to generate a double bond at bridgehead. This impossibility is due to a combination of ring strain and angle strain for nonplanar alkene. The p-orbitals of the bridgehead atom and adjacent atoms are orthogonal. Thus, they are not appropriately aligned for the formation of pi bonds.
Examples of showing and not showing bridgehead double bond
Applications of Bredt's Rule
Bredt's rule can be useful to predict which isomer is obtained from an elimination reaction in a bridged ring system. It can also be applied to reaction mechanisms that go through carbocations and, to a lesser degree, via free radicals, because these intermediates, like carbon atoms involved in a double bond, prefer to have a planar geometry with 120° angles and sp2 hybridization. The rule also allows the rationalisation of observations. For example, bicyclo[5.3.1] undecane-11-one-1-carboxylic acid undergoes decarboxylation on heating to 132°C, but the similar compound bicycle[2.2.1] heptan-7-one-1-carboxylic acid remains stable beyond 500°C, despite both being β-keto acids with the carbonyl group on a one-carbon bridge and the carboxylate group on the bridgehead.
