In chemistry, the double bond rule states that elements with a principal quantum number (n) greater than 2 for their valence electrons (period 3 elements and higher) tend not to form multiple bonds (e.g. double bonds and triple bonds). Double bonds for these heavier elements, when they exist, are often weak due to poor orbital overlap between the n>2 orbitals of the two atoms. Although such compounds are not intrinsically unstable, they instead tend to dimerize or even polymerize. [1] Moreover, the multiple bonds of the elements with n=2 are much stronger than usual, because lone pair repulsion weakens their sigma bonding but not their pi bonding. [2] An example is the rapid polymerization that occurs upon condensation of disulfur, the heavy analogue of O2. Numerous exceptions to the rule exist. [3] Several exceptions of this rule has been already made. [4]
| B boron (n=2) | C carbon (n=2) | N nitrogen (n=2) | O oxygen (n=2) | Si silicon (n=3) | P phosphorus (n=3) | S sulfur (n=3) | |
|---|---|---|---|---|---|---|---|
| B | diborenes | alkylideneboranes | aminoboranylidenes, rare [5] | oxoboranes, rare, rapid oligomerization [6] | borasilenes (rare) [7] | boranylidenephosphanes, rare, stable compounds are known [8] | thioxoboranes, rare [9] |
| C | alkenes | imines | carbonyls | silenes | phosphaalkenes | thioketones | |
| N | azo compounds | nitroso compounds | silanimines, rare, easy oligomerization, observed only at low temp [10] | phosphazene (P=N) | sulfilimines | ||
| O | Singlet oxygen | silanones, Si=O bonds extremely reactive, oligomerization to siloxanes | numerous, e.g. phosphine oxides, phosphonates, phosphinates, phosphates | numerous, e.g. sulfuric acid, sulfates, sulfoxides (R-S(=O)-R′, compounds with a sulfinyl group), and sulfones (R-S(=O)2-R′, the sulfonyl group) | |||
| Si | disilenes | silylidenephosphanes a.k.a. phosphasilenes, rare [11] | silanethiones, rare, easy oligomerization [12] | ||||
| P | diphosphenes | common compounds such as thiophosphates and phosphine sulfides, for example, triphenylphosphine sulfide and certain dithiadiphosphetanes | |||||
| S | disulfur, thiosulfoxides | ||||||
| B boron (n=2) | C carbon (n=2) | N nitrogen (n=2) | O oxygen (n=2) | Si silicon (n=3) | P phosphorus (n=3) | S sulfur (n=3) | Ge germanium (n=4) | As arsenic (n=4) | |
|---|---|---|---|---|---|---|---|---|---|
| B | diborynes | Borataalkynes have been observed [13] | Observed in (t-Bu)BN(t-Bu) (an iminoborane) | ||||||
| C | alkynes | cyanides | Carbon monoxide (C≡O) | silynes | phosphaalkynes | Carbon monosulfide (C≡S) | arsaalkynes | ||
| N | Dinitrogen, Diazonium | Phosphorus mononitride (P≡N) | Arsa-diazonium [14] | ||||||
| O | Silicon monoxide has some triple-bond character | ||||||||
| Si | disilynes | ||||||||
| P | Diphosphorus | ||||||||
| S | Observed in (I2)2S2+2 [15] | ||||||||
| Ge | Digermyne | ||||||||
| As | Arsenic monophosphide (As≡P) | ||||||||
Another unrelated double bond rule exists that relates to the enhanced reactivity of sigma bonds attached to an atom adjacent to a double bond. [16] 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. [17] [18]