Bredt's rule

In organic chemistry, Bredt's rule is the empirical observation that bridged molecules tend only to possess a double bond at the bridgehead in large ring systems. The rule is named after Julius Bredt, who first discussed it in 1902 and codified it in 1924.

An anti-Bredt molecule is a moecule which does not follow Bredt's rule. For example, two of the following norbornene isomers violate Bredt's rule and are too unstable to prepare:

Causes and consequences

Bredt's rule results from geometric strain: a double bond at a bridgehead atom necessarily must be trans in at least one ring. For small rings (fewer than eight atoms), a trans alkene cannot be achieved without substantial angle and ring and angle strain (the p orbitals are improperly aligned for a π bond through twisting and pyramidalization). Bredt's rule also applies to carbocations and, to a lesser degree, free radicals, because these intermediates also prefer a planar geometry with 120° angles and sp2 hybridization. It generally does not apply to hypervalent heteroatoms, although they are commonly written with a formal double bond.

Bredt's rule can predict the viability of competing elimination reactions in a bridged system. For example, the metal alkyl complexes usually decompose quickly via beta elimination, but Bredt strain prevents tetranorbornyl complexes from doing so. Bicyclo[5.3.1]undecane-11-one-1-carboxylic acid undergoes decarboxylation on heating to 132 °C, but the similar compound bicyclo[2.2.1]heptan-7-one-1-carboxylic acid remains stable beyond 500 °C, because the decarboxylation proceeds through an anti-Bredt enol.