Электрофилы: определение, свойства и реакции.

A chemical species that accepts an electron pair from a nucleophile
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. 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 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.

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:

Proton (H+) adds (by working as an electrophile) to one of the carbon atoms on the alkene to form cation 1. Chloride ion (Cl−) combines with the cation 1 to form the adducts 2 and 3. 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 is another example of an AdE2 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, the AdE2 mechanism is generally competitive with the AdE3 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, the formation of a sterically unencumbered, stabilized carbocation favors the AdE2 pathway, while a more nucleophilic bromide ion favors the AdE3 pathway to a greater extent compared to reactions involving the chloride ion. 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 the AdE2ip ("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 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.

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. 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. 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 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.

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, 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]2+ dication. Likewise, methane can be nitrated to nitromethane with nitronium tetrafluoroborate NOBF only in presence of 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 BF4.