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Double bond rule
The double bond rule states that chemical elements with a principal quantum number
greater than 2 (e.g. Period 3 elements and lower) do not form multiple bonds (e.g. double bonds and triple bonds) with themselves or with other elements.〔''New Element-Carbon (p-p) Bonds '' Doz. Dr. Peter Jutzi Angewandte Chemie International Edition in English Volume 14 Issue 4, Pages 232 - 245 1975 〕 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. This effect is manifested by the rapid polymerization that occurs upon condensation of disulfur, the heavy analogue of O2.
This rule was challenged and ultimately revised starting from 1981 with the isolation of crystalline samples of compounds with silicon–silicon and phosphorus–phosphorus double bonds.〔''Multiple bonds to silicon: 20 years later '' Robert West Polyhedron Volume 21, Issues 5-6, 15 March 2002, Pages 467-472 〕 Double bonds that would ordinarily not form can be stabilized with proper functional groups either electronically or sterically.
== Other meanings==
Another unrelated double bond rule exists that relates to the enhanced reactivity of sigma bonds once removed from a double bond. In bromoalkenes the C-Br bond is very stable but in an allyl 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 in 1885. The name for the rule was coined by Otto Schmidt (1874–1943) in 1932 〔 ''A History of the Double-Bond Rule'' Bernard E. Hoogenboom Journal of Chemical Education 1998 75 (5), 596 〕 〔Schmidt, O. Z. Phys. Chem. 1932, A159, 337–356.〕
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