出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2019/06/22 19:20:56」(JST)
In organic
chemistry, an electrophile is an electron pair acceptor. Electrophiles are positively charged or neutral species having vacant orbitals that are attracted to an electron rich centre. It participates in a chemical reaction by accepting an electron pair in order to bond to a nucleophile. Because electrophiles accept electrons, they are Lewis acids (see acid-base reaction theories). 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. They appear to attract electrons as well and seem to behave as though they are partially empty. These partially empty substances thus require an electron rich center, and thus they are filled. Electrophiles can be observed as electron-sensitive or photo-sensitive.
The electrophiles are attacked by the most electron-populated part of one nucleophile. The electrophiles frequently seen in the organic syntheses are cations such as H+ and NO+, polarized neutral molecules such as HCl, alkyl halides, acyl halides, and carbonyl compounds, polarizable neutral molecules such as Cl2 and Br2, oxidizing agents such as organic peracids, chemical species that do not satisfy the octet rule such as carbenes and radicals, and some Lewis acids such as BH3 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;[1]
This process is called AdE2 mechanism. Iodine (I2), chlorine (Cl2), sulfenyl ion (RS+), mercury cation (Hg2+), and dichlorocarbene (:CCl2) 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 the alkene has a cation-stabilizing substituent like phenyl group. There is an example of the isolation of the bromonium ion 2.[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 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 the more stabilizing substituents) will form.
This process is called A-SE2 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.
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, the simultaneous protonation (by HCl) and attack of the alkyne by the nucleophile (Cl−) is believed to take place. This process is known as the AdE3 mechanism. 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 a reversibly-formed weak association of the alkyne and HCl. Such a mechanism is consistent with the predominantly anti addition of the hydrochlorination product and the termolecular rate law, Rate = k[alkyne][HCl]2.[3][4] In support of the proposed alkyne-HCl association, a T-shaped complex of an alkyne and HCl has been characterized crystallographically.[5]
In contrast, phenylpropyne reacts by the ip-AdE2 mechanism to give predominantly the syn product. In this case, the intermediate vinyl cation is favored because it is resonance-stabilized by the phenyl group. In this pathway, HCl reacts with the alkyne to form a vinyl cation-chloride anion ion pair, which immediately collapses to give the vinyl halide. 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.[6]
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 the OSO3H group is replaced by an OH group, forming an alcohol:
As can be seen, the H2SO4 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 is the fructose-derived organocatalyst used in the Shi epoxidation.[7] 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 the AB-ring segments of various natural products, including γ-rhodomycionone and α-citromycinone.[8]
Polymer-bound chiral selenium electrophiles effect asymmetric selenenylation reactions.[9] 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 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.
| Fluorine | 3.86 |
| Chlorine | 3.67 |
| Bromine | 3.40 |
| Iodine | 3.09 |
| Hypochlorite | 2.52 |
| Sulfur dioxide | 2.01 |
| Carbon disulfide | 1.64 |
| Benzene | 1.45 |
| Sodium | 0.88 |
| Some selected values [10] (no dimensions) | |
Several methods exist to rank electrophiles in order of reactivity[11] and one of them is devised by Robert Parr[10] with the electrophilicity index ω given as:
with the electronegativity and chemical hardness. This equation is related to the classical equation for electrical power:
where is the resistance (Ohm or Ω) and 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.[12] Strongly electrophilic radicals such as the halogens react with electron-rich reaction sites, and strongly nucleophilic radicals such as 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.[13] Superelectrophiles form as a doubly electron deficient superelectrophile by protosolvation of a cationic electrophile. As observed by Olah, 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 BF3 and HF. The responsible reactive intermediate is the CH3CO2H3 dication. Likewise, methane can be nitrated to nitromethane with nitronium tetrafluoroborate NO+
2BF−
4 only in presence of a strong acid like fluorosulfuric acid.
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-BF4.[14]
A method was uncovered by Dale E. O'Dell and associates for effecting intramolecular conjugate addition to α-enones of unactiviated carbon nucleophiles which proceeds through the mediation of novel alkyltin(IV) chemistry.[citation needed]
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