Question

In 1935, J. Bredt, a German chemist, proposed that a bicycloalkene could not have a double bond at a bridgehead carbon unless one of the rings contains at least eight carbons. This is known as Bredt’s rule. Explain why there cannot be a double bond at this position.

Short answer

The bridgehead carbon can't have the 120° bond angle required for the sp²carbon of the double bond, because if it did, the compound would be too strained to exist.

Step-by-step solution

Bredt’s rule

Bredt's rule is a consequence of the fact that having a double bond on a bridgehead, carbons from which three bonds radiate and which the rings share a single covalent bond, would be equivalent to having a trans double bond on a ring, which is not stable for small rings (fewer than eight atoms) due to a combination of ring strain, and angle strain (non planar alkene). The p orbitals of the bridgehead atom and adjacent atoms are orthogonal and thus are not aligned properly for the formation of pi bonds.

Final answer

The bridgehead carbon can't have the 120° bond angle required for the sp²carbon of the double bond, because if it did, the compound would be too strained to exist.