Double bond rule
In
polymerize. An example is the rapid polymerization that occurs upon condensation of disulfur, the heavy analogue of O2. Numerous exceptions to the rule exist.[2]
| 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[3] | oxoboranes, rare, rapid oligomerization[4] |
borasilenes (rare)[5] | boranylidenephosphanes, rare, stable compounds are known[6] | thioxoboranes, rare[7] |
| C | alkenes | imines | carbonyls |
silenes | phosphaalkenes | thioketones | |
| N | azo compounds | nitroso compounds | silanimines, rare, easy oligomerization, observed only at low temp[8] | phosphazene (P=N) | sulfilimines | ||
| O | Singlet oxygen | silanones, Si=O bonds extremely reactive, oligomerization to siloxanes |
numerous, e.g. phosphine oxides, phosphonates, phosphinates, phosphates |
sulfinyls
| |||
| Si | disilenes | silylidenephosphanes a.k.a. phosphasilenes, rare[9] |
silanethiones, rare, easy oligomerization[10] | ||||
| P | diphosphenes | common compounds such as dithiadiphosphetanes
| |||||
| S | disulfur, thiosulfoxides | ||||||
Triple bonds
| 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[11] | Observed in ( t-Bu)BN(t-Bu) (an iminoborane ) |
||||||
| C | alkynes | cyanides | Carbon monoxide (C≡O) | silynes |
phosphaalkynes | Carbon monosulfide (C≡S) | arsaalkynes | ||
| N | Diazonium |
Phosphorus mononitride (P≡N) | Arsa- diazonium[12]
| ||||||
| O | Silicon monoxide has some triple-bond character | ||||||||
| Si | disilynes | ||||||||
| P | Diphosphorus | ||||||||
| S | Observed in (I2)2S2+2[13] | ||||||||
| Ge | Digermyne | ||||||||
| As | Arsenic monophosphide (As≡P) | ||||||||
Other meanings
 |
Another unrelated double bond rule exists that relates to the enhanced reactivity of
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.[14][15]