Double bond rule
The double bond rule states that chemical elements with a principal quantum number greater than 2 for their valence electrons tend not to form multiple bonds with themselves or with other elements. 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. An example is the rapid polymerization that occurs upon condensation of disulfur, the heavy analogue of O2. Numerous violations to the rule exist.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 in 1932.
