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). [1] The double bonds, 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 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 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 [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 | Dinitrogen, 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) | ||||||||
 |
Another unrelated double bond rule exists that relates to the enhanced reactivity of sigma bonds attached to an atom adjacent to 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. [14] [15]
In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to other atoms were fully ionic. It describes the degree of oxidation of an atom in a chemical compound. Conceptually, the oxidation state may be positive, negative or zero. While fully ionic bonds are not found in nature, many bonds exhibit strong ionicity, making oxidation state a useful predictor of charge.
In organometallic chemistry, organolithium reagents are chemical compounds that contain carbon–lithium (C–Li) bonds. These reagents are important in organic synthesis, and are frequently used to transfer the organic group or the lithium atom to the substrates in synthetic steps, through nucleophilic addition or simple deprotonation. Organolithium reagents are used in industry as an initiator for anionic polymerization, which leads to the production of various elastomers. They have also been applied in asymmetric synthesis in the pharmaceutical industry. Due to the large difference in electronegativity between the carbon atom and the lithium atom, the C−Li bond is highly ionic. Owing to the polar nature of the C−Li bond, organolithium reagents are good nucleophiles and strong bases. For laboratory organic synthesis, many organolithium reagents are commercially available in solution form. These reagents are highly reactive, and are sometimes pyrophoric.
The Suzuki reaction is an organic reaction, classified as a cross-coupling reaction, where the coupling partners are a boronic acid and an organohalide, and the catalyst is a palladium(0) complex. It was first published in 1979 by Akira Suzuki, and he shared the 2010 Nobel Prize in Chemistry with Richard F. Heck and Ei-ichi Negishi for their contribution to the discovery and development of palladium-catalyzed cross-couplings in organic synthesis. This reaction is also known as the Suzuki–Miyaura reaction or simply as the Suzuki coupling. It is widely used to synthesize polyolefins, styrenes, and substituted biphenyls. Several reviews have been published describing advancements and the development of the Suzuki reaction. The general scheme for the Suzuki reaction is shown below, where a carbon-carbon single bond is formed by coupling a halide (R1-X) with an organoboron species (R2-BY2) using a palladium catalyst and a base. The organoboron species is usually synthesized by hydroboration or carboboration, allowing for rapid generation of molecular complexity.
Organoboron chemistry or organoborane chemistry studies organoboron compounds, also called organoboranes. These chemical compounds combine boron and carbon; typically, they are organic derivatives of borane (BH3), as in the trialkyl boranes.
Organosulfur chemistry is the study of the properties and synthesis of organosulfur compounds, which are organic compounds that contain sulfur. They are often associated with foul odors, but many of the sweetest compounds known are organosulfur derivatives, e.g., saccharin. Nature is abound with organosulfur compounds—sulfur is vital for life. Of the 20 common amino acids, two are organosulfur compounds, and the antibiotics penicillin and sulfa drugs both contain sulfur. While sulfur-containing antibiotics save many lives, sulfur mustard is a deadly chemical warfare agent. Fossil fuels, coal, petroleum, and natural gas, which are derived from ancient organisms, necessarily contain organosulfur compounds, the removal of which is a major focus of oil refineries.
Diphosphene is a type of organophosphorus compound that has a phosphorus–phosphorus double bond, denoted by R-P=P-R'. These compounds are not common but are of theoretical interest. Normally, compounds with the empirical formula RP exist as rings. However, like other multiple bonds between heavy main-group elements, P=P double bonds can be stabilized by a large steric hindrance from the substitutions. The first isolated diphosphene bis(2,4,6-tri-tert-butylphenyl)diphosphene was exemplified by Masaaki Yoshifuji and his coworkers in 1981, in which diphosphene is stabilized by two bulky phenyl group.
Annulynes or dehydroannulenes are conjugated monocyclic hydrocarbons with alternating single and double bonds in addition to at least one triple bond.
A persistent carbene (also known as stable carbene) is a type of carbene demonstrating particular stability. The best-known examples and by far largest subgroup are the N-heterocyclic carbenes (NHC) (sometimes called Arduengo carbenes), for example diaminocarbenes with the general formula (R2N)2C:, where the four R moieties are typically alkyl and aryl groups. The groups can be linked to give heterocyclic carbenes, such as those derived from imidazole, imidazoline, thiazole or triazole.
Stannabenzene (C5H6Sn) is the parent representative of a group of organotin compounds that are related to benzene with a carbon atom replaced by a tin atom. Stannabenzene itself has been studied by computational chemistry, but has not been isolated.
Organogermanium chemistry is the science of chemical species containing one or more C–Ge bonds. Germanium shares group 14 in the periodic table with carbon, silicon, tin and lead. Historically, organogermanes are considered as nucleophiles and the reactivity of them is between that of organosilicon and organotin compounds. Some organogermanes have enhanced reactivity compared with their organosilicon and organoboron analogues in some cross-coupling reactions.
[n]Radialenes are alicyclic organic compounds containing n cross-conjugated exocyclic double bonds. The double bonds are commonly alkene groups but those with a carbonyl (C=O) group are also called radialenes. For some members the unsubstituted parent radialenes are elusive but many substituted derivatives are known.
Carbene analogs in chemistry are carbenes with the carbon atom replaced by another chemical element. Just as regular carbenes they appear in chemical reactions as reactive intermediates and with special precautions they can be stabilized and isolated as chemical compounds. Carbenes have some practical utility in organic synthesis but carbene analogs are mostly laboratory curiosities only investigated in academia. Carbene analogs are known for elements of group 13, group 14, group 15 and group 16.
Germylenes are a class of germanium(II) compounds with the general formula :GeR2. They are heavier carbene analogs. However, unlike carbenes, whose ground state can be either singlet or triplet depending on the substituents, germylenes have exclusively a singlet ground state. Unprotected carbene analogs, including germylenes, has a dimerization nature. Free germylenes can be isolated under the stabilization of steric hindrance or electron donation. The synthesis of first stable free dialkyl germylene was reported by Jutzi, et al in 1991.
In chemistry, an oxoborane is any chemical compound containing a boron atom with a terminal oxygen atom. The compound class is of some relevance to academic research. The parent compound, HBO, itself called "oxoborane", together with derivatives FBO, ClBO, BrBO, HOBO and MeBO have been detected in matrix isolation or in the gaseous phase at high temperature. In these compounds the boron and oxygen form a triple bond prone to cyclotrimerization to boroxines.
A silanone in chemistry is the silicon analogue of a ketone. The general description for this class of organic compounds is R1R2Si=O, with silicon connected to a terminal oxygen atom via a double bond and also with two organic residues (R). Silanones are extremely reactive and until 2013 were only detected by argon matrix isolation or in the gas phase but not isolated. A synthesis of a stable silanone was reported in 2014. Silanones are of some interest to academic research, with their reactivity being of some relevance to the double bond rule.
A borylene is the boron analogue of a carbene. The general structure is R-B: with R an organic moiety and B a boron atom with two unshared electrons. Borylenes are of academic interest in organoboron chemistry. A singlet ground state is predominant with boron having two vacant sp2 orbitals and one doubly occupied one. With just one additional substituent the boron is more electron deficient than the carbon atom in a carbene. For this reason stable borylenes are more uncommon than stable carbenes. Some borylenes such as boron monofluoride (BF) and boron monohydride (BH) the parent compound also known simply as borylene, have been detected in microwave spectroscopy and may exist in stars. Other borylenes exist as reactive intermediates and can only be inferred by chemical trapping.
Holger Braunschweig is Head and Chair of Inorganic Chemistry at the Julius-Maximilians-University of Würzburg in Würzburg, Germany. He is best known for founding the field of transition metal-boron multiple bonding, the synthesis of the first stable compounds containing boron-boron and boron-oxygen triple bonds, the isolation of the first non-carbon/nitrogen main-group dicarbonyl, and the first fixation of dinitrogen at an element of the p-block of the periodic table. By modifying a strategy pioneered by Prof. Gregory Robinson of the University of Georgia, Braunschweig also discovered the first rational and high-yield synthesis of neutral compounds containing boron-boron double bonds (diborenes). In 2016 Braunschweig isolated the first compounds of beryllium in the oxidation state of zero.
Plumbylenes (or plumbylidenes) are divalent organolead(II) analogues of carbenes, with the general chemical formula, R2Pb, where R denotes a substituent. Plumbylenes possess 6 electrons in their valence shell, and are considered open shell species.
Heteroatomic multiple bonding between group 13 and group 15 elements are of great interest in synthetic chemistry due to their isoelectronicity with C-C multiple bonds. Nevertheless, the difference of electronegativity between group 13 and 15 leads to different character of bondings comparing to C-C multiple bonds. Because of the ineffective overlap between p𝝅 orbitals and the inherent lewis acidity/basicity of group 13/15 elements, the synthesis of compounds containing such multiple bonds is challenging and subject to oligomerization. The most common example of compounds with 13/15 group multiple bonds are those with B=N units. The boron-nitrogen-hydride compounds are candidates for hydrogen storage. In contrast, multiple bonding between aluminium and nitrogen Al=N, Gallium and nitrogen (Ga=N), boron and phosphorus (B=P), or boron and arsenic (B=As) are less common.
Aluminylenes are a sub-class of aluminium(I) compounds that feature singly-coordinated aluminium atoms with a lone pair of electrons. As aluminylenes exhibit two unoccupied orbitals, they are not strictly aluminium analogues of carbenes until stabilized by a Lewis base to form aluminium(I) nucleophiles. The lone pair and two empty orbitals on the aluminium allow for ambiphilic bonding where the aluminylene can act as both an electrophile and a nucleophile. Aluminylenes have also been reported under the names alumylenes and alanediyl.