Inchemistry, anelectrophile is achemical species that forms bonds withnucleophiles by accepting anelectron pair.[1] Because electrophiles accept electrons, they areLewis acids.[2] Most electrophiles are positivelycharged, 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 throughaddition andsubstitution reactions. Frequently seen electrophiles inorganic syntheses includecations such asH+ andNO+, polarized neutral molecules such asHCl,alkyl halides,acyl halides, andcarbonyl compounds, polarizable neutral molecules such asCl2 andBr2,oxidizing agents such as organicperacids, chemical species that do not satisfy theoctet rule such ascarbenes andradicals, and some Lewis acids such asBH3 andDIBAL.
These occur between alkenes and electrophiles, often halogens as inhalogen addition reactions. Common reactions include use of bromine water totitrate 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;[3]
This process is calledAdE2 mechanism ("addition, electrophilic, second-order").Iodine (I2),chlorine (Cl2),sulfenyl ion (RS+),mercury cation (Hg2+), anddichlorocarbene (:CCl2) also react through similar pathways. The direct conversion of1 to3 will appear when the Br− is large excess in the reaction medium. A β-bromocarbenium ion intermediate may be predominant instead of3 if the alkene has a cation-stabilizing substituent like phenyl group. There is an example of the isolation of the bromonium ion2.[4]
Hydrogen halides such as hydrogen chloride (HCl) adds to alkenes to give alkyl halides inhydrohalogenation. 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, thestereoselectivity 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 byMarkovnikov'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 anAdE2 mechanism.[5]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,[6] the AdE2 mechanism is generally competitive with theAdE3 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.[7]
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 mechanistic pathway is known by the Ingold labelAdE3 ("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 a reversibly-formed weak association of the alkyne and HCl. Such a mechanism is consistent with the predominantlyanti addition (>15:1anti:syn for the example shown) of the hydrochlorination product and the termolecular rate law, Rate =k[alkyne] [HCl]2.[8][9] In support of the proposed alkyne-HCl association, a T-shaped complex of an alkyne and HCl has been characterized crystallographically.[10]
In contrast, phenylpropyne reacts by theAdE2ip ("addition, electrophilic, second-order, ion pair") mechanism to give predominantly thesyn product (~10:1syn: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 ofsyn addition.[7]
One of the more complexhydration reactions utilisessulfuric acid as acatalyst. 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 producesethanol, whose purposes include fuels and starting material for other chemicals.
Many electrophiles arechiral and optically stable. Typically chiral electrophiles are also optically pure.
One suchreagent is thefructose-derived organocatalyst used in theShi epoxidation.[11] The catalyst can accomplish highly enantioselective epoxidations oftrans-disubstituted and trisubstitutedalkenes. The Shi catalyst, aketone, is oxidized by stoichiometricoxone to the activedioxirane form before proceeding in the catalytic cycle.
Oxaziridines such as chiralN-sulfonyloxaziridines effect enantioselective ketone alpha oxidation en route to the AB-ring segments of variousnatural products, including γ-rhodomycionone and α-citromycinone.[12]
Polymer-bound chiralselenium electrophiles effect asymmetric selenenylation reactions.[13] 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 usingorganotin 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[14] (no dimensions) | |
Several methods exist to rank electrophiles in order of reactivity[15] and one of them is devised byRobert Parr[14] with theelectrophilicity indexω given as:
with theelectronegativity andchemical hardness. This equation is related to the classical equation forelectrical power:
where is theresistance (Ohm or Ω) and isvoltage. 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 forfree radicals.[16] Strongly electrophilic radicals such as the halogens react with electron-rich reaction sites, and strongly nucleophilic radicals such as the 2-hydroxypropyl-2-yl andtert-butyl radical react with a preference for electron-poor reaction sites.[citation needed]
Superelectrophiles are defined as cationic electrophilic reagents with greatly enhanced reactivities in the presence ofsuperacids. These compounds were first described byGeorge A. Olah.[17] Superelectrophiles form as a doubly electron deficient superelectrophile by protosolvation of a cationic electrophile. As observed by Olah, a mixture ofacetic acid andboron trifluoride is able to remove a hydride ion fromisobutane when combined withhydrofluoric acid via the formation of asuperacid from BF3 and HF. The responsiblereactive intermediate is the [CH3CO2H3]2+ dication. Likewise,methane can be nitrated tonitromethane withnitronium tetrafluoroborate NO+
2BF−
4 only in presence of a strong acid likefluorosulfuric acid via the protonated nitronium dication.[citation needed]
Ingitionic (gitonic) superelectrophiles, charged centers are separated by no more than one atom, for example, the protonitronium ion O=N+=O+—H (a protonatednitronium ion). And, indistonic superelectrophiles, they are separated by 2 or more atoms, for example, in the fluorination reagentF-TEDA-BF4.[18]
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