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]
| 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 [4] | oxoboranes, rare, rapid oligomerization [5] | borasilenes (rare) [6] | boranylidenephosphanes, rare, stable compounds are known [7] | thioxoboranes, rare [8] |
| C | alkenes | imines | carbonyls | silenes | phosphaalkenes | thioketones | |
| N | azo compounds | nitroso compounds | silanimines, rare, easy oligomerization, observed only at low temp [9] | 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 [10] | silanethiones, rare, easy oligomerization [11] | ||||
| 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 [12] | 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 [13] | ||||||
| O | Silicon monoxide has some triple-bond character | ||||||||
| Si | disilynes | ||||||||
| P | Diphosphorus | ||||||||
| S | Observed in (I2)2S2+2 [14] | ||||||||
| Ge | Digermyne | ||||||||
| As | Arsenic monophosphide (As≡P) | ||||||||
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Another unrelated double bond rule exists that relates to the enhanced reactivity of sigma bonds attached to an atom adjacent to a double bond. [15] 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. [16] [17]
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.
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 their properties have theoretical importance.
In chemistry, a phosphaalkyne is an organophosphorus compound containing a triple bond between phosphorus and carbon with the general formula R-C≡P. Phosphaalkynes are the heavier congeners of nitriles, though, due to the similar electronegativities of phosphorus and carbon, possess reactivity patterns reminiscent of alkynes. Due to their high reactivity, phosphaalkynes are not found naturally on earth, but the simplest phosphaalkyne, phosphaethyne (H-C≡P) has been observed in the interstellar medium.
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.
tert-Butylphosphaacetylene is an organophosphorus compound. Abbreviated t-BuCP, it was the first example of an isolable phosphaalkyne. Prior to its synthesis, the double bond rule had suggested that elements of Period 3 and higher were unable to form double or triple bonds with lighter main group elements because of weak orbital overlap. The synthesis of t-BuCP discredited much of the double bond rule and opened new studies into the formation of unsaturated phosphorus compounds.
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.
Digermynes are a class of compounds that are regarded as the heavier digermanium analogues of alkynes. The parent member of this entire class is HGeGeH, which has only been characterized computationally, but has revealed key features of the whole class. Because of the large interatomic repulsion between two Ge atoms, only kinetically stabilized digermyne molecules can be synthesized and characterized by utilizing bulky protecting groups and appropriate synthetic methods, for example, reductive coupling of germanium(II) halides.
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.
Carbones are a class of molecules containing a carbon atom in the 1D excited state with a formal oxidation state of zero where all four valence electrons exist as unbonded lone pairs. These carbon-based compounds are of the formula CL2 where L is a strongly σ-donating ligand, typically a phosphine (carbodiphosphoranes) or a N-heterocyclic carbene/NHC (carbodicarbenes), that stabilises the central carbon atom through donor-acceptor bonds. Carbones possess high-energy orbitals with both σ- and π-symmetry, making them strong Lewis bases and strong π-backdonor substituents. Carbones possess high proton affinities and are strong nucleophiles which allows them to function as ligands in a variety of main group and transition metal complexes. Carbone-coordinated elements also exhibit a variety of different reactivities and catalyse various organic and main group reactions.
1-Phosphaallenes is are allenes in which the first carbon atom is replaced by phosphorus, resulting in the structure: -P=C=C<.
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.
In organic chemistry, carboboration describes an addition of both a carbon and a boron moiety to certain carbon-containing double and triple bonds, such as alkenes, alkynes, and allenes.
A ketenyl anion contains a C=C=O allene-like functional group, similar to ketene, with a negative charge on either terminal carbon or oxygen atom, forming resonance structures by moving a lone pair of electrons on C-C-O bond. Ketenes have been sources for many organic compounds with its reactivity despite a challenge to isolate them as crystal. Precedent method to obtain this product has been at gas phase or at reactive intermediate, and synthesis of ketene is used be done in extreme conditions. Recently found stabilized ketenyl anions become easier to prepare compared to precedent synthetic procedure. A major feature about stabilized ketene is that it can be prepared from carbon monoxide (CO) reacting with main-group starting materials such as ylides, silylene, and phosphinidene to synthesize and isolate for further steps. As CO becomes a more common carbon source for various type of synthesis, this recent finding about stabilizing ketene with main-group elements opens a variety of synthetic routes to target desired products.
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.