In chemistry , an electrophile is a chemical species that forms bonds with nucleophiles by accepting an electron pair . Because electrophiles accept electrons, they are Lewis acids . Most electrophiles are positively charged , have an atom that carries a partial positive charge, or have an atom that does not have an octet of electrons.
24-397: Dilithium , Li 2 , is a strongly electrophilic , diatomic molecule comprising two lithium atoms covalently bonded together. Li 2 has been observed in the gas phase . It has a bond order of 1, an internuclear separation of 267.3 pm and a bond energy of 102 kJ/mol or 1.06 eV in each bond. The electron configuration of Li 2 may be written as σ. Being
48-407: A mixture of acetic acid and boron trifluoride is able to remove a hydride ion from isobutane when combined with hydrofluoric acid via the formation of a superacid from BF 3 and HF. The responsible reactive intermediate is the [CH 3 CO 2 H 3 ] dication. Likewise, methane can be nitrated to nitromethane with nitronium tetrafluoroborate NO 2 BF 4 only in presence of
72-446: A reversibly-formed weak association of the alkyne and HCl. Such a mechanism is consistent with the predominantly anti addition (>15:1 anti : syn for the example shown) of the hydrochlorination product and the termolecular rate law, Rate = k [alkyne][HCl] . In support of the proposed alkyne-HCl association, a T-shaped complex of an alkyne and HCl has been characterized crystallographically. In contrast, phenylpropyne reacts by
96-476: A strong acid like fluorosulfuric acid via the protonated nitronium dication. In gitionic ( gitonic ) superelectrophiles, charged centers are separated by no more than one atom, for example, the protonitronium ion O=N =O —H (a protonated nitronium ion ). And, in distonic superelectrophiles, they are separated by 2 or more atoms, for example, in the fluorination reagent F-TEDA-BF 4 . Stereoselectivity Too Many Requests If you report this error to
120-464: Is voltage . In this sense the electrophilicity index is a kind of electrophilic power. Correlations have been found between electrophilicity of various chemical compounds and reaction rates in biochemical systems and such phenomena as allergic contact dermititis. An electrophilicity index also exists for free radicals . Strongly electrophilic radicals such as the halogens react with electron-rich reaction sites, and strongly nucleophilic radicals such as
144-453: Is called Ad E 2 mechanism ("addition, electrophilic, second-order"). Iodine (I 2 ), chlorine (Cl 2 ), sulfenyl ion (RS ), mercury cation (Hg ), and dichlorocarbene (:CCl 2 ) also react through similar pathways. The direct conversion of 1 to 3 will appear when the Br is large excess in the reaction medium. A β-bromo carbenium ion intermediate may be predominant instead of 3 if
168-715: Is more precise than any previously measured atomic oscillator strength. This lithium oscillator strength is related to the radiative lifetime of atomic lithium and is used as a benchmark for atomic clocks and measurements of fundamental constants. Electrophile Electrophiles mainly interact with nucleophiles through addition and substitution reactions. Frequently seen electrophiles in organic syntheses include cations such as H and NO , polarized neutral molecules such as HCl , alkyl halides , acyl halides , and carbonyl compounds , polarizable neutral molecules such as Cl 2 and Br 2 , oxidizing agents such as organic peracids , chemical species that do not satisfy
192-401: Is shown below: In this manner, the stereoselectivity of the product, that is, from which side Cl will attack relies on the types of alkenes applied and conditions of the reaction. At least, which of the two carbon atoms will be attacked by H is usually decided by Markovnikov's rule . Thus, H attacks the carbon atom that carries fewer substituents so as the more stabilized carbocation (with
216-496: Is the fructose -derived organocatalyst used in the Shi epoxidation . The catalyst can accomplish highly enantioselective epoxidations of trans -disubstituted and trisubstituted alkenes . The Shi catalyst, a ketone , is oxidized by stoichiometric oxone to the active dioxirane form before proceeding in the catalytic cycle. Oxaziridines such as chiral N-sulfonyloxaziridines effect enantioselective ketone alpha oxidation en route to
240-469: The Ad E 2 ip ("addition, electrophilic, second-order, ion pair") mechanism to give predominantly the syn product (~10:1 syn : anti ). In this case, the intermediate vinyl cation is formed by addition of HCl because it is resonance-stabilized by the phenyl group. Nevertheless, the lifetime of this high energy species is short, and the resulting vinyl cation-chloride anion ion pair immediately collapses, before
264-434: The electrophilicity index ω given as: with χ {\displaystyle \chi \,} the electronegativity and η {\displaystyle \eta \,} chemical hardness . This equation is related to the classical equation for electrical power : where R {\displaystyle R\,} is the resistance ( Ohm or Ω) and V {\displaystyle V\,}
SECTION 10
#1732858468751288-450: The octet rule such as carbenes and radicals , and some Lewis acids such as BH 3 and DIBAL . These occur between alkenes and electrophiles, often halogens as in halogen addition reactions . Common reactions include use of bromine water to titrate against a sample to deduce the number of double bonds present. For example, ethene + bromine → 1,2-dibromoethane : This takes the form of 3 main steps shown below; This process
312-444: The 2-hydroxypropyl-2-yl and tert-butyl radical react with a preference for electron-poor reaction sites. Superelectrophiles are defined as cationic electrophilic reagents with greatly enhanced reactivities in the presence of superacids . These compounds were first described by George A. Olah . Superelectrophiles form as a doubly electron deficient superelectrophile by protosolvation of a cationic electrophile. As observed by Olah,
336-462: The AB-ring segments of various natural products , including γ-rhodomycionone and α-citromycinone. Polymer-bound chiral selenium electrophiles effect asymmetric selenenylation reactions. The reagents are aryl selenenyl bromides, and they were first developed for solution phase chemistry and then modified for solid phase bead attachment via an aryloxy moiety. The solid-phase reagents were applied toward
360-487: The Ad E 2 mechanism is generally competitive with the Ad E 3 mechanism (described in more detail for alkynes, below), in which transfer of the proton and nucleophilic addition occur in a concerted manner. The extent to which each pathway contributes depends on the several factors like the nature of the solvent (e.g., polarity), nucleophilicity of the halide ion, stability of the carbocation, and steric effects. As brief examples,
384-639: The OSO 3 H group is replaced by an OH group, forming an alcohol: As can be seen, the H 2 SO 4 does take part in the overall reaction, however it remains unchanged so is classified as a catalyst. This is the reaction in more detail: Overall, this process adds a molecule of water to a molecule of ethene. This is an important reaction in industry, as it produces ethanol , whose purposes include fuels and starting material for other chemicals. Many electrophiles are chiral and optically stable . Typically chiral electrophiles are also optically pure. One such reagent
408-607: The X-state, a-state, A-state, c-state, B-state, 2d-state, l-state, E-state, and the F-state. The most reliable of these potential energy curves are of the Morse/Long-range variety (see entries in the table below). Li 2 potentials are often used to extract atomic properties. For example, the C 3 value for atomic lithium extracted from the A-state potential of Li 2 by Le Roy et al. in
432-424: The alkene has a cation-stabilizing substituent like phenyl group. There is an example of the isolation of the bromonium ion 2 . Hydrogen halides such as hydrogen chloride (HCl) adds to alkenes to give alkyl halides in hydrohalogenation . For example, the reaction of HCl with ethylene furnishes chloroethane. The reaction proceeds with a cation intermediate, being different from the above halogen addition. An example
456-430: The chloride ion has a chance to leave the solvent shell, to give the vinyl chloride. The proximity of the anion to the side of the vinyl cation where the proton was added is used to rationalize the observed predominance of syn addition. One of the more complex hydration reactions utilises sulfuric acid as a catalyst . This reaction occurs in a similar way to the addition reaction but has an extra step in which
480-399: The formation of a sterically unencumbered, stabilized carbocation favors the Ad E 2 pathway, while a more nucleophilic bromide ion favors the Ad E 3 pathway to a greater extent compared to reactions involving the chloride ion. In the case of dialkyl-substituted alkynes (e.g., 3-hexyne), the intermediate vinyl cation that would result from this process is highly unstable. In such cases,
504-428: The more stabilizing substituents) will form. This is another example of an Ad E 2 mechanism. Hydrogen fluoride (HF) and hydrogen iodide (HI) react with alkenes in a similar manner, and Markovnikov-type products will be given. Hydrogen bromide (HBr) also takes this pathway, but sometimes a radical process competes and a mixture of isomers may form. Although introductory textbooks seldom mentions this alternative,
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#1732858468751528-400: The selenenylation of various alkenes with good enantioselectivities. The products can be cleaved from the solid support using organotin hydride reducing agents. Solid-supported reagents offers advantages over solution phase chemistry due to the ease of workup and purification. Several methods exist to rank electrophiles in order of reactivity and one of them is devised by Robert Parr with
552-463: The simultaneous protonation (by HCl) and attack of the alkyne by the nucleophile (Cl ) is believed to take place. This mechanistic pathway is known by the Ingold label Ad E 3 ("addition, electrophilic, third-order"). Because the simultaneous collision of three chemical species in a reactive orientation is improbable, the termolecular transition state is believed to be reached when the nucleophile attacks
576-468: The third-lightest stable neutral homonuclear diatomic molecule (after dihydrogen and dihelium ), dilithium is an extremely important model system for studying fundamentals of physics, chemistry, and electronic structure theory . It is the most thoroughly characterized compound in terms of the accuracy and completeness of the empirical potential energy curves of its electronic states. Analytic empirical potential energy curves have been constructed for
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