In chemistry, a conjugated system is a system of connected p orbitals with delocalized electrons in a molecule, which in general lowers the overall energy of the molecule and increases stability. It is conventionally represented as having alternating single and multiple bonds. Lone pairs, radicals or carbenium ions may be part of the system, which may be cyclic, acyclic, linear or mixed. The term "conjugated" was coined in 1899 by the German chemist Johannes Thiele.
In chemistry, aromaticity is a property of cyclic (ring-shaped), typically planar (flat) molecular structures with pi bonds in resonance that gives increased stability compared with other geometric or connective arrangements of the same set of atoms. Aromatic rings are very stable and do not break apart easily. Organic compounds that are not aromatic are classified as aliphatic compounds—they might be cyclic, but only aromatic rings have enhanced stability.
Organolithium reagents are organometallic compounds that contain carbon–lithium 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.
Cyclobutadiene is an organic compound with the formula C4H4. It is very reactive owing to its tendency to dimerize. Although the parent compound has not been isolated, some substituted derivatives are robust and a single molecule of cyclobutadiene is quite stable. Since the compound degrades by a bimolecular process, the species can be observed by matrix isolation techniques at temperatures below 35 K. It is thought to adopt a rectangular structure.
In organic chemistry, Hückel's rule predicts that a planar ring molecule will have aromatic properties if it has 4n + 2 π electrons, where n is a non-negative integer. The quantum mechanical basis for its formulation was first worked out by physical chemist Erich Hückel in 1931. The succinct expression as the 4n + 2 rule has been attributed to W. v. E. Doering (1951), although several authors were using this form at around the same time.
Antiaromaticity is a characteristic of a cyclic molecule with a π electron system that has higher energy due to the presence of 4n delocalised electrons in it. Unlike aromatic compounds, which follow Hückel's rule and are highly stable, antiaromatic compounds are highly unstable and highly reactive. To avoid the instability of antiaromaticity, molecules may change shape, becoming non-planar and therefore breaking some of the π interactions. In contrast to the diamagnetic ring current present in aromatic compounds, antiaromatic compounds have a paramagnetic ring current, which can be observed by NMR spectroscopy.
1,3,5,7-Cyclooctatetraene (COT) is an unsaturated derivative of cyclooctane, with the formula C8H8. It is also known as [8]annulene. This polyunsaturated hydrocarbon is a colorless to light yellow flammable liquid at room temperature. Because of its stoichiometric relationship to benzene, COT has been the subject of much research and some controversy.
Corannulene is a polycyclic aromatic hydrocarbon with chemical formula C20H10. The molecule consists of a cyclopentane ring fused with 5 benzene rings, so another name for it is [5]circulene. It is of scientific interest because it is a geodesic polyarene and can be considered a fragment of buckminsterfullerene. Due to this connection and also its bowl shape, corannulene is also known as a buckybowl. Corannulene exhibits a bowl-to-bowl inversion with an inversion barrier of 10.2 kcal/mol (42.7 kJ/mol) at −64 °C.
Homoaromaticity, in organic chemistry, refers to a special case of aromaticity in which conjugation is interrupted by a single sp3 hybridized carbon atom. Although this sp3 center disrupts the continuous overlap of p-orbitals, traditionally thought to be a requirement for aromaticity, considerable thermodynamic stability and many of the spectroscopic, magnetic, and chemical properties associated with aromatic compounds are still observed for such compounds. This formal discontinuity is apparently bridged by p-orbital overlap, maintaining a contiguous cycle of π electrons that is responsible for this preserved chemical stability.
[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.
Group 2 organometallic chemistry refers to the chemistry of compounds containing carbon bonded to any group 2 element. By far the most common group 2 organometallic compounds are the magnesium-containing Grignard reagents which are widely used in organic chemistry. Other organmetallic group 2 compounds are rare and are typically limited to academic interests.
Boroles represent a class of molecules known as metalloles, which are heterocyclic 5-membered rings. As such, they can be viewed as structural analogs of cyclopentadiene, pyrrole or furan, with boron replacing a carbon, nitrogen and oxygen atom respectively. They are isoelectronic with the cyclopentadienyl cation C5H+5(Cp+) and comprise four π electrons. Although Hückel's rule cannot be strictly applied to borole, it is considered to be antiaromatic due to having 4 π electrons. As a result, boroles exhibit unique electronic properties not found in other metalloles.
Organomolybdenum chemistry is the chemistry of chemical compounds with Mo-C bonds. The heavier group 6 elements molybdenum and tungsten form organometallic compounds similar to those in organochromium chemistry but higher oxidation states tend to be more common.
In chemistry, the cyclooctatetraenide anion or cyclooctatetraenide, more precisely cyclooctatetraenediide, is an aromatic species with a formula of [C8H8]2− and abbreviated as COT2−. It is the dianion of cyclooctatetraene. Salts of the cyclooctatetraenide anion can be stable, e.g., Dipotassium cyclooctatetraenide or disodium cyclooctatetraenide. More complex coordination compounds are known as cyclooctatetraenide complexes, such as the actinocenes.
In organometallic chemistry, a transition metal alkyne complex is a coordination compound containing one or more alkyne ligands. Such compounds are intermediates in many catalytic reactions that convert alkynes to other organic products, e.g. hydrogenation and trimerization.
Octalene is a polycyclic hydrocarbon composed of two fused cyclooctatetraene rings.
Bicyclo[6.2.0]decapentaene is a bicyclic organic compound and an isomer of naphthalene and azulene.
In chemistry, compounds of palladium(III) feature the noble metal palladium in the unusual +3 oxidation state. Compounds of Pd(III) occur in mononuclear and dinuclear forms. Palladium(III) is most often invoked, not observed in mechanistic organometallic chemistry.
Phosphasilenes or silylidenephosphanes are a class of compounds with silicon-phosphorus double bonds. Since the electronegativity of phosphorus (2.1) is higher than that of silicon (1.9), the "Si=P" moiety of phosphasilene is polarized. The degree of polarization can be tuned by altering the coordination numbers of the Si and P centers, or by modifying the electronic properties of the substituents. The phosphasilene Si=P double bond is highly reactive, yet with the choice of proper substituents, it can be stabilized via donor-acceptor interaction or by steric congestion.
Atsuhiro Osuka is a research professor of organic chemistry in the Department of Chemistry, Graduate School of Science, Kyoto University (Japan). He is recognized in the fields of porphyrinoid chemistry for his works in extended π-electron systems and its tunable aromatic behaviors.
