Aromatic compounds, also known as "mono- and polycyclic aromatic hydrocarbons", are organic compounds containing one or more aromatic rings. The parent member is benzene. Heteroarenes are closely related, since at least one carbon atom of CH group is replaced by one of the heteroatoms oxygen, nitrogen, or sulfur. Examples of non-benzene compounds with aromatic properties are furan, a heterocyclic compound with a five-membered ring that includes a single oxygen atom, and pyridine, a heterocyclic compound with a six-membered ring containing one nitrogen atom. Hydrocarbons without an aromatic ring are called aliphatic.
In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond.
In organic chemistry a diene is a covalent compound that contains two double bonds, usually among carbon atoms. They thus contain two alkene units, with the standard prefix di of systematic nomenclature. As a subunit of more complex molecules, dienes occur in naturally occurring and synthetic chemicals and are used in organic synthesis. Conjugated dienes are widely used as monomers in the polymer industry. Polyunsaturated fats are of interest to nutrition.
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or exemplified by the odors of gasoline and lighter fluid. They occur in a diverse range of molecular structures and phases: they can be gases, liquids, low melting solids or polymers. In the fossil fuel industries, hydrocarbon refers to the naturally occurring petroleum, natural gas and coal, and to their hydrocarbon derivatives and purified forms. Combustion of hydrocarbons is the main source of the world's energy. Petroleum is the dominant raw-material source for organic commodity chemicals such as solvents and polymers. Most anthropogenic (human-generated) emissions of greenhouse gases are carbon dioxide from the burning of fossil fuels, and methane released from natural gas handling and from agriculture.
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms. Study of structure determines their structural formula. Study of properties includes physical and chemical properties, and evaluation of chemical reactivity to understand their behavior. The study of organic reactions includes the chemical synthesis of natural products, drugs, and polymers, and study of individual organic molecules in the laboratory and via theoretical study.
In theoretical 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 to saturated compounds having single bonds, and other geometric or connective non-cyclic arrangements with 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.
Unsaturated hydrocarbons are hydrocarbons that have double or triple covalent bonds between adjacent carbon atoms. The term "unsaturated" means more hydrogen atoms may be added to the hydrocarbon to make it saturated. The configuration of an unsaturated carbons include straight chain, such as alkenes and alkynes, as well as branched chains and aromatic compounds.
A cycloalkene or cycloolefin is a type of alkene hydrocarbon which contains a closed ring of carbon atoms and either one or more double bonds, but has no aromatic character. Some cycloalkenes, such as cyclobutene and cyclopentene, can be used as monomers to produce polymer chains. Due to geometrical considerations, smaller cycloalkenes are almost always the cis isomers, and the term cis tends to be omitted from the names. trans-Cycloalkenes with 7 or fewer carbons in the ring will not occur under normal conditions because of the large amount of ring strain needed. In larger rings, cis–trans isomerism of the double bond may occur. This stability pattern forms part of the origin of Bredt's rule, the observation that alkenes do not form at the bridgehead of many types of bridged ring systems because the alkene would necessarily be trans in one of the rings.
Butene, also known as butylene, is an alkene with the formula C4H8. The word butene may refer to any of the individual compounds. They are colourless gases that are present in crude oil as a minor constituent in quantities that are too small for viable extraction. Butene is therefore obtained by catalytic cracking of long-chain hydrocarbons left during refining of crude oil. Cracking produces a mixture of products, and the butene is extracted from this by fractional distillation.
Annulenes are monocyclic hydrocarbons that contain the maximum number of non-cumulated or conjugated double bonds. They have the general formula CnHn or CnHn+1. The IUPAC naming conventions are that annulenes with 7 or more carbon atoms are named as [n]annulene, where n is the number of carbon atoms in their ring, though sometimes the smaller annulenes are referred to using the same notation, and benzene is sometimes referred to simply as annulene.
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.
In organic chemistry, a rearrangement reaction is a broad class of organic reactions where the carbon skeleton of a molecule is rearranged to give a structural isomer of the original molecule. Often a substituent moves from one atom to another atom in the same molecule, hence these reactions are usually intramolecular. In the example below, the substituent R moves from carbon atom 1 to carbon atom 2:
In organic chemistry, hyperconjugation refers to the delocalization of electrons with the participation of bonds of primarily σ-character. Usually, hyperconjugation involves the interaction of the electrons in a sigma (σ) orbital with an adjacent unpopulated non-bonding p or antibonding σ* or π* orbitals to give a pair of extended molecular orbitals. However, sometimes, low-lying antibonding σ* orbitals may also interact with filled orbitals of lone pair character (n) in what is termed negative hyperconjugation. Increased electron delocalization associated with hyperconjugation increases the stability of the system. In particular, the new orbital with bonding character is stabilized, resulting in an overall stabilization of the molecule. Only electrons in bonds that are in the β position can have this sort of direct stabilizing effect — donating from a sigma bond on an atom to an orbital in another atom directly attached to it. However, extended versions of hyperconjugation can be important as well. The Baker–Nathan effect, sometimes used synonymously for hyperconjugation, is a specific application of it to certain chemical reactions or types of structures.
A cyclic compound is a term for a compound in the field of chemistry in which one or more series of atoms in the compound is connected to form a ring. Rings may vary in size from three to many atoms, and include examples where all the atoms are carbon, none of the atoms are carbon, or where both carbon and non-carbon atoms are present. Depending on the ring size, the bond order of the individual links between ring atoms, and their arrangements within the rings, carbocyclic and heterocyclic compounds may be aromatic or non-aromatic; in the latter case, they may vary from being fully saturated to having varying numbers of multiple bonds between the ring atoms. Because of the tremendous diversity allowed, in combination, by the valences of common atoms and their ability to form rings, the number of possible cyclic structures, even of small size numbers in the many billions.
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.
In organic and physical organic chemistry, Clar's rule is an empirical rule that relates the chemical stability of a molecule with its aromaticity. It was introduced in 1972 by the Austrian organic chemist Erich Clar in his book The Aromatic Sextet. The rule states that given a polycyclic aromatic hydrocarbon, the resonance structure most important to characterize its properties is that with the largest number of aromatic π-sextets.
Trimethylenemethane cycloaddition is the formal [3+2] annulation of trimethylenemethane (TMM) derivatives to two-atom pi systems. Although TMM itself is too reactive and unstable to be stored, reagents which can generate TMM or TMM synthons in situ can be used to effect cycloaddition reactions with appropriate electron acceptors. Generally, electron-deficient pi bonds undergo cyclization with TMMs more easily than electron-rich pi bonds.
Calicene or triapentafulvalene is a hydrocarbon of the fulvalene class with chemical formula C8H6, composed of a cyclopentadiene ring and a cyclopropene ring linked by a double bond. Its name is derived from the Latin calix meaning "goblet", from its shape.
