Bredt's rule is an empirical observation in organic chemistry

Organic chemistry

Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...

 that states that a double bond

Double bond

A double bond in chemistry is a chemical bond between two chemical elements involving four bonding electrons instead of the usual two. The most common double bond, that between two carbon atoms, can be found in alkenes. Many types of double bonds between two different elements exist, for example in...

 cannot be placed at the bridgehead of a bridged ring system, unless the rings are large enough. The rule is named after Julius Bredt

Julius Bredt

Julius Bredt was a German organic chemist. He was the first to determine, in 1893, the correct structure of camphor. Bredt also discovered that a double bond cannot be placed at the bridgehead of a bridged ring system, a statement now known as Bredt's rule.- Further reading :...

.

For example, two of the following isomers of norbornene

Norbornene

Norbornene or norbornylene or norcamphene is a bridged cyclic hydrocarbon. It is a white solid with a pungent sour odor. The molecule consists of a cyclohexene ring bridged with a methylene group in the para position...

 violate Bredt's rule, which makes them too unstable to prepare:
In the figure, the bridgehead atoms involved in Bredt's rule violation are highlighted in red.

Bredt's rule is a consequence of the fact that having a double bond on a bridgehead would be equivalent to having a trans double bond on a ring, which is not possible for small rings (fewer than eight atoms) due to ring strain

Ring strain

In organic chemistry, ring strain is the tendency of a cyclic molecule, such as cyclopropane, to destabilize when its atoms are in non-favorable high energy spatial orientations...

, and angle strain

Angle strain

Angle strain, also called Baeyer strain in cyclic molecules, is the resistance associated with bond angle compression or bond angle expansion. It occurs when bond angles deviate from the ideal bond angles to achieve maximum bond strength in a specific chemical conformation...

 in particular.

Bredt's rule can be useful for predicting which isomer

Isomer

In chemistry, isomers are compounds with the same molecular formula but different structural formulas. Isomers do not necessarily share similar properties, unless they also have the same functional groups. There are many different classes of isomers, like stereoisomers, enantiomers, geometrical...

 is obtained from an elimination reaction

Elimination reaction

An elimination reaction is a type of organic reaction in which two substituents are removed from a molecule in either a one or two-step mechanism...

 in a bridged ring system. It can also be applied to reaction mechanism

Reaction mechanism

In chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical change occurs.Although only the net chemical change is directly observable for most chemical reactions, experiments can often be designed that suggest the possible sequence of steps in...

s that go via carbocation

Carbocation

A carbocation is an ion with a positively-charged carbon atom. The charged carbon atom in a carbocation is a "sextet", i.e. it has only six electrons in its outer valence shell instead of the eight valence electrons that ensures maximum stability . Therefore carbocations are often reactive,...

s and, to a lesser degree, via free radicals

Radical (chemistry)

Radicals are atoms, molecules, or ions with unpaired electrons on an open shell configuration. Free radicals may have positive, negative, or zero charge...

, because these intermediates, like carbon atoms involved in a double bond, prefer to have a planar geometry with 120 degree angles and sp² hybridization

Orbital hybridisation

In chemistry, hybridisation is the concept of mixing atomic orbitals to form new hybrid orbitals suitable for the qualitative description of atomic bonding properties. Hybridised orbitals are very useful in the explanation of the shape of molecular orbitals for molecules. It is an integral part...

.

An anti-bredt molecule is one that is found to exist and be stable (within certain parameters) despite this rule. A recent (2006) example such a molecule is 2-quinuclidonium tetrafluoroborate.

History

The first publication of what would later become known as Bredt's rule was in an article by Julius Bredt

Julius Bredt

Julius Bredt was a German organic chemist. He was the first to determine, in 1893, the correct structure of camphor. Bredt also discovered that a double bond cannot be placed at the bridgehead of a bridged ring system, a statement now known as Bredt's rule.- Further reading :...

 in 1924 about the chemistry of naturally occurring bicyclic terpene

Terpene

Terpenes are a large and diverse class of organic compounds, produced by a variety of plants, particularly conifers, though also by some insects such as termites or swallowtail butterflies, which emit terpenes from their osmeterium. They are often strong smelling and thus may have had a protective...

s. For an extensive review of this topic, see the article by Shea.