In
chemistry, the double bond rule states that
elements with a
principal quantum number greater than 2 for their
valence electron
In chemistry and physics, a valence electron is an electron in the outer shell associated with an atom, and that can participate in the formation of a chemical bond if the outer shell is not closed. In a single covalent bond, a shared pair form ...
s (
period 3 elements and higher) tend not to form multiple bonds (e.g.
double bonds and
triple bond
A triple bond in chemistry is a chemical bond between two atoms involving six bonding electrons instead of the usual two in a covalent single bond. Triple bonds are stronger than the equivalent single bonds or double bonds, with a bond order o ...
s). The double bonds, when they exist, are often weak due to poor
orbital overlap. Although such compounds are not intrinsically unstable, they instead tend to
polymerize
In polymer chemistry, polymerization (American English), or polymerisation (British English), is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are many for ...
. An example is the rapid polymerization that occurs upon condensation of
disulfur, the heavy analogue of . Numerous violations to the rule exist.
Other meanings
Another unrelated double bond rule exists that relates to the enhanced reactivity of
sigma bonds attached to an atom adjacent to a double bond. In bromoalkenes, the C–Br bond is very stable, but in an
allyl
In organic chemistry, an allyl group is a substituent with the structural formula , where R is the rest of the molecule. It consists of a methylene bridge () attached to a vinyl group (). The name is derived from the scientific name for garlic, . ...
bromide, this bond is very reactive. Likewise, bromobenzenes are generally inert, whereas
benzylic bromides are reactive. The first to observe the phenomenon was
Conrad Laar
Conrad Peter Laar (22 March 1853 – 11 February 1929) was a German chemist. He coined the expression tautomerism
Tautomers () are structural isomers (constitutional isomers) of chemical compounds that readily interconvert.
The chemical re ...
in 1885. The name for the rule was coined by Otto Schmidt (1874–1943) in 1932.
References
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Chemical bonding