Bredt's rule is an empirical observation in organic chemistry that states that a double bond cannot be placed at the bridgehead of a bridged ring system, unless the rings are large enough. Organic chemistry is a specific discipline within the subject of chemistry. ... Covalent bonding is a form of chemical bonding characterized by the sharing of one or more pairs of electrons between atoms, in order to produce a mutual attraction, which holds the resultant molecule together. ...

For example, two of the following isomers of norbornene violate Bredt's rule, which makes them too unstable to prepare: Norbornene or norbornylene or norcamphene is a bridged cyclic hydrocarbon. ...

Image File history File links Bredts_rule. ...

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 (less than eight atoms) due to ring strain, and angle strain in particular. In chemistry, geometric isomerism is a form of stereoisomerism and describes the orientation of functional groups at the ends of a bond around which no rotation is possible. ... Ring strain is an organic chemistry term that describes the destabilization of a cyclic molecule—such as a cycloalkane—due to the non-favorable high energy spatial orientations of its atoms. ... The presence of angle strain in a molecule indicates that in a specific chemical conformation there exists a bond angle that deviates from the bond angle required to archieve maximum bond strength. ...

Bredt's rule can be useful for predicting which isomer is obtained from an elimination reaction in a bridged ring system. It can also be applied to reaction mechanisms that go via 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 degree angles and sp² hybridization. In chemistry, isomers are molecules with the same chemical formula and often with the same kinds of bonds between atoms, but in which the atoms are arranged differently. ... 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 chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical change occurs. ... A carbocation is an ion with a positively-charged carbon atom. ... For other uses, see radical. ... four sp³ orbitals three sp² orbitals In chemistry, hybridisation or hybridization (see spelling differences) is the mixing of atomic orbitals belonging to a same electron shell to form new orbitals suitable for the qualitative description of atomic bonding properties. ...

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. Quinuclidones are a class of bicyclic organic compounds with chemical formula C7H11NO with two structural isomers for the base skeleton 3-quinuclidone and 2-quinuclidone. ...

History

The first publication of what would later become known as Bredt's rule was in an article by Julius Bredt in 1924 about the chemistry of naturally occurring bicyclic terpenes.^[1] For an extensive review of this topic, see the article by Shea.^[2] Terpenes are a large and varied class of hydrocarbons, produced primarily by a wide variety of plants, particularly conifers, though also by some animals. ...

References

1. ↑  J. Bredt, H. Thouet and J. Schnitz Liebigs Ann. 1924, 437, 1.
2. ↑  K. J. Shea, "Recent developments in the synthesis, structure and chemistry of bridgehead alkenes", Tetrahedron 1980, 36, 1683-1715. http://dx.doi.org/10.1016/0040-4020(80)80067-6