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Reactive Intermediates
In chemistry, a reactive intermediate or an intermediate is a short-lived, high-energy, highly reactive molecule. When generated in a chemical reaction, it will quickly convert into a more stable molecule. Only in exceptional cases can these compounds be isolated and stored, e.g. low temperatures, matrix isolation. When their existence is indicated, reactive intermediates can help explain how a chemical reaction takes place. Most chemical reactions take more than one elementary step to complete, and a reactive intermediate is a high-energy, hence unstable, product that exists only in one of the intermediate steps. The series of steps together make a reaction mechanism. A reactive intermediate differs from a reactant or product or a simple reaction intermediate only in that it cannot usually be isolated but is sometimes observable only through fast spectroscopic methods. It is stable in the sense that an elementary reaction forms the reactive intermediate and the elementary reacti ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Chemistry
Chemistry is the scientific study of the properties and behavior of matter. It is a physical science within the natural sciences that studies the chemical elements that make up matter and chemical compound, compounds made of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they undergo during chemical reaction, reactions with other chemical substance, substances. Chemistry also addresses the nature of chemical bonds in chemical compounds. In the scope of its subject, chemistry occupies an intermediate position between physics and biology. It is sometimes called the central science because it provides a foundation for understanding both Basic research, basic and Applied science, applied scientific disciplines at a fundamental level. For example, chemistry explains aspects of plant growth (botany), the formation of igneous rocks (geology), how atmospheric ozone is formed and how environmental pollutants are degraded (ecology), the prop ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Aryne
In organic chemistry, arynes and benzynes are a class of highly Reactivity (chemistry), reactive chemical Chemical species, species derived from an aromatic ring by removal of two substituents. Arynes are examples of didehydroarenes (1,2-didehydroarenes in this case), although 1,3- and 1,4-didehydroarenes are also known. Arynes are examples of alkynes under high Ring strain, strain. Bonding in arynes The alkyne representation of benzyne is the most widely encountered. Arynes are usually described as having a strained triple bond (left), but resonance contributors include a cumulene form (middle) and biradical form (right): Geometric constraints on the triple bond in benzyne result in diminished overlap of in-plane p-orbitals, and thus weaker triple bond. The vibrational frequency of the triple bond in benzyne was assigned by Radziszewski to be 1846 cm−1, indicating a weaker triple bond than in unstrained alkyne with vibrational frequency of approximately 2150 cm−1 ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Enol
In organic chemistry, enols are a type of functional group or intermediate in organic chemistry containing a group with the formula (R = many substituents). The term ''enol'' is an abbreviation of ''alkenol'', a portmanteau deriving from "-ene"/"alkene" and the "-ol". Many kinds of enols are known. Keto–enol tautomerism refers to a chemical equilibrium between a "keto" form (a carbonyl, named for the common ketone case) and an enol. The interconversion of the two forms involves the transfer of an alpha hydrogen atom and the reorganisation of bonding electrons. The keto and enol forms are tautomers of each other. Enolization Organic esters, ketones, and aldehydes with an α-hydrogen ( bond adjacent to the carbonyl group) often form enols. The reaction involves migration of a proton () from carbon to oxygen: : In the case of ketones, the conversion is called a keto-enol tautomerism, although this name is often more generally applied to all such tautomerizations. Usua ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Ion-neutral Complex
An ion-neutral complex in chemistry is an aggregate of an ion with one or more neutral molecules in which at least one of the partners has a rotational degree of freedom about an axis perpendicular to the intermolecular direction ''Advances in Gas Phase Ion Chemistry'', Volume 4 by L.M. Babcock, N.G. Adams 2001 In chemistry, the dissociation of a molecule into two or more fragments can take place in the gas phase, provided there is sufficient internal energy for the requisite barriers to be overcome. For many years, it was assumed that the fragments of a gas phase dissociation simply fly apart. In 1958, Allan Maccoll suggested that the decomposition of alkyl halides (RX) might take place via the intermediacy of ion pairs, + X− in which the charged fragments were no longer covalently bonded but were held together by electrostatic attraction. Maccoll and coworkers subsequently examined chlorine isotope effects in the thermal decomposition of chloroethane and con ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Carbenoid
In chemistry a carbenoid is a reactive intermediate that shares reaction characteristics with a carbene. In the Simmons–Smith reaction the carbenoid intermediate is a zinc / iodine complex that takes the form of :I-CH2-Zn-I This complex reacts with an alkene to form a cyclopropane just as a carbene would do. Carbenoids appear as intermediates in many other reactions. In one system a carbenoid chloroalkyllithium reagent is prepared in situ from a sulfoxide and t-BuLi which reacts the boronic ester to give an ate complex. The ate complex undergoes a 1,2-metallate rearrangement to give the homologated product, which is then further oxidised to a secondary alcohol.Iterative Stereospecific Reagent-Controlled Homologation of Pinacol Boronates by Enantioenriched -Chloroalkyllithium Reagents Paul R. Blakemore and Matthew S. Burge J. Am. Chem. Soc.; 2007; 129(11) pp 3068 - 3069; (Communication) : The enantiopurity of the chiral sulfoxide is preserved in the ultimate product after ox ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Chemical Trap
In chemistry, a chemical trap is a chemical compound that is used to detect unstable compounds. The method relies on efficiency of bimolecular reactions with reagents to produce a more easily characterize trapped product. In some cases, the trapping agent is used in large excess. Case studies Cyclobutadiene A famous example is the detection of cyclobutadiene released upon oxidation of cyclobutadieneiron tricarbonyl. When this degradation is conducted in the presence of an alkyne, the cyclobutadiene is trapped as a bicyclohexadiene. The requirement for this trapping experiment is that the oxidant (ceric ammonium nitrate) and the trapping agent be mutually compatible. : Diphosphorus Diphosphorus is an old target of chemists since it is the heavy analogue of N2. Its fleeting existence is inferred by the controlled degradation of certain niobium complexes in the presence of trapping agents. Again, a Diels-Alder strategy is employed in the trapping: : Silylene Another classic but e ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Transition State
In chemistry, the transition state of a chemical reaction is a particular configuration along the reaction coordinate. It is defined as the state corresponding to the highest potential energy along this reaction coordinate. It is often marked with the double dagger (‡) symbol. As an example, the transition state shown below occurs during the SN2 reaction of bromoethane with a hydroxide anion: The activated complex of a reaction can refer to either the transition state or to other states along the reaction coordinate between reactants and products, especially those close to the transition state. Peter Atkins and Julio de Paula, ''Physical Chemistry'' (8th ed., W.H. Freeman 2006), p.809 According to the transition state theory, once the reactants have passed through the transition state configuration, they always continue to form products. History of concept The concept of a transition state has been important in many theories of the rates at which chemical re ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Resonance Structure
In chemistry, resonance, also called mesomerism, is a way of describing bonding in certain molecules or polyatomic ions by the combination of several contributing structures (or ''forms'', also variously known as ''resonance structures'' or ''canonical structures'') into a resonance hybrid (or ''hybrid structure'') in valence bond theory. It has particular value for analyzing delocalized electrons where the bonding cannot be expressed by one single Lewis structure. The resonance hybrid is the accurate structure for a molecule or ion; it is an average of the theoretical (or hypothetical) contributing structures. Overview Under the framework of valence bond theory, resonance is an extension of the idea that the bonding in a chemical species can be described by a Lewis structure. For many chemical species, a single Lewis structure, consisting of atoms obeying the octet rule, possibly bearing formal charges, and connected by bonds of positive integer order, is sufficient for des ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Conjugated System
In physical organic 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 Chemical stability, stability. It is Resonance (chemistry), conventionally represented as having alternating single and multiple covalent bond, bonds. Lone pairs, radical (chemistry), radicals or carbenium ions may be part of the system, which may be Cyclic molecule, cyclic, acyclic, Linear molecular geometry, linear or mixed. The term "conjugated" was coined in 1899 by the German chemist Johannes Thiele (chemist), Johannes Thiele. Conjugation is the orbital overlap, overlap of one p-orbital with another across an adjacent Sigma bond, σ bond (in transition metals, d-orbitals can be involved). A conjugated system has a region of overlapping p-orbitals, bridging the interjacent locations that simple diagrams illustrate as not having a π bond. They allow a delocalization of pi el ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Cage Effect (chemistry)
In chemistry, the cage effect (also known as geminate recombination) describes how the properties of a molecule are affected by its surroundings. First introduced by James Franck and Eugene Rabinowitch in 1934, the cage effect suggests that instead of acting as an individual particle, molecules in solvent are more accurately described as an encapsulated particle. The encapsulated molecules or radicals are called cage pairs or geminate pairs. In order to interact with other molecules, the caged particle must diffuse from its solvent cage. The typical lifetime of a solvent cage is 10 seconds. Many manifestations of the cage effect exist. In free radical polymerization, radicals formed from the decomposition of an initiator molecule are surrounded by a cage consisting of solvent and/or monomer molecules. Within the cage, the free radicals undergo many collisions leading to their recombination or mutual deactivation. This can be described by the following reaction: : R\!-\!R ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Chemical Compound
A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element is therefore not a compound. A compound can be transformed into a different substance by a chemical reaction, which may involve interactions with other substances. In this process, bonds between atoms may be broken or new bonds formed or both. There are four major types of compounds, distinguished by how the constituent atoms are bonded together. Molecular compounds are held together by covalent bonds; ionic compounds are held together by ionic bonds; intermetallic compounds are held together by metallic bonds; coordination complexes are held together by coordinate covalent bonds. Non-stoichiometric compounds form a disputed marginal case. A chemical formula specifies the number of atoms of each element in a compound molecule, usin ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Chemical Decomposition
Chemical decomposition, or chemical breakdown, is the process or effect of simplifying a single molecular entity, chemical entity (normal molecule, reaction intermediate, etc.) into two or more fragments. Chemical decomposition is usually regarded and defined as the exact opposite of chemical synthesis. In short, the chemical reaction in which two or more products are formed from a single reactant is called a decomposition reaction. The details of a decomposition process are not always well defined. Nevertheless, some activation energy is generally needed to break the involved bonds and as such, higher temperatures generally accelerates decomposition. The net reaction can be an endothermic process, or in the case of spontaneous decompositions, an exothermic process. The stability of a chemical compound is eventually limited when exposed to extreme environmental conditions such as heat, radiation, humidity, or the acidity of a solvent. Because of this chemical decomposition is often ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
