Double bond rule

The double bond rule states that chemical elements with a principal quantum number greater than 2 for their valence electrons (period 3 elements and lower) should not form multiple bonds (e.g. double bonds and triple 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. 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 disproven starting from 1981 with the isolation of crystalline samples of compounds with silicon–silicon and phosphorus–phosphorus double bonds. Double bonds that would ordinarily not form can often be stabilized with proper functional groups either electronically or sterically.

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

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