Electrophile

In chemistry, an electrophile (literally electron-lover) is a reagent attracted to electrons that participates in a chemical reaction by accepting an electron pair in order to bond to a nucleophile. Chemistry (from Egyptian kēme (chem meaning "earth") is the Science concerned with the composition structure and properties A reagent or reactant is a substance or compound consumed during a Chemical reaction. The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J lone pair is a (valence electron pair without bonding or sharing with other Atoms They are found in the outermost Electron shell of an atom so lone pairs A chemical bond is the physical process responsible for the attractive interactions between Atoms and Molecules and which confers stability to diatomic and polyatomic In Chemistry, a nucleophile (literally nucleus lover as in nucleus and phile) is a Reagent that forms a Chemical bond to Because electrophiles accept electrons, they are Lewis acids (see acid-base reaction theories). Most electrophiles are positively charged, have an atom which carries a partial positive charge, or have an atom which does not have an octet of electrons. Electric charge is a fundamental conserved property of some Subatomic particles which determines their Electromagnetic interaction.

The electrophiles attack the most electron-populated part of a nucleophile. In Chemistry, a nucleophile (literally nucleus lover as in nucleus and phile) is a Reagent that forms a Chemical bond to 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 of lewis acids such as BH₃ and DIBAL. Organic synthesis is a special branch of Chemical synthesis and is concerned with the construction of Organic compounds via Organic reactions Organic The haloalkanes (also known as halogenoalkanes or alkyl halides) are a group of Chemical compounds consisting of Alkanes such as Methane An acyl halide (also known as an acid halide) is a Chemical compound derived from an Oxoacid by replacing a Hydroxyl group with a In Organic chemistry, a carbonyl group is a Functional group composed of a Carbon Atom double-bonded to an Oxygen An oxidizing agent or oxidising agent (also called an oxidant, oxidizer or oxidiser) can be defined as either a Chemical compound A peroxy acid (often spelled as one word peroxyacid, and sometimes called peracid) is an Acid in which an acidic -OH group has been replaced by an -OOH The octet rule is a simple chemical Rule of thumb that states that Atoms tend to combine in such a way that they each have eight Electrons in In Chemistry, a carbene is a highly reactive Organic molecule containing a Carbon atom with six valence electrons and having the general formula In Chemistry, radicals (often referred to as free radicals) are atoms molecules or ions with Unpaired electrons on an otherwise Open shell In chemistry a borane is a chemical compound of Boron and Hydrogen. Diisobutylaluminium hydride, DIBAL DIBAL-H or DIBAH is a Reducing agent with the formula ( i -Bu2AlH2 where i -Bu represents

1 Electrophiles in organic chemistry
2 Electrophilicity scale
3 Superelectrophiles
4 See also
5 References

Electrophiles in organic chemistry

Alkenes

Electrophilic addition is one of the three main forms of reaction concerning alkenes. In Organic chemistry, an electrophilic addition reaction is an Addition reaction where in a Chemical compound, a Pi bond is removed by the creation In Organic chemistry, an alkene, olefin, or olefine is an unsaturated Chemical compound containing at least one Carbon They consist of:

Hydrogenation by the addition of hydrogen over the double bond. Hydrogenation is the Chemical reaction that results in addition of Hydrogen (H2
Electrophilic addition reactions with halogens and sulfuric acid. In Organic chemistry, an electrophilic addition reaction is an Addition reaction where in a Chemical compound, a Pi bond is removed by the creation
Hydration to form alcohols.

Addition of halogens

These occur between alkenes and electrophiles, often halogens as in halogen addition reactions. A halogen addition reaction is a simple Organic reaction where a Halogen Molecule is added to the Carbon-carbon double bond of an Alkene Common reactions include use of bromine water to titrate against a sample to deduce the number of double bonds present. Titration is a common laboratory method of Quantitative chemical analysis that is used to determine the unknown Concentration of a known Reactant For example, ethene + bromine → 1,2-dibromoethane:

C₂H₄ + Br₂ → BrCH₂CH₂Br

This takes the form of 3 main steps shown below^[1];

Forming of a π-complex

The electrophilic Br-Br molecule interacts with electron-rich alkene molecure to form a π-complex 1. Structure This Hydrocarbon has four Hydrogen Atoms bound to a pair of Carbon atoms that are connected by a Double bond. The Dewar-Chatt-Duncanson model is a model in Organometallic chemistry which explains the type of Chemical bonding between an Alkene and a metal ( pi-complex
Forming of a three-membered bromonium ion

The alkene is working as an electron donor and bromine as an electrophile. The three-membered bromonium ion 2 consisted with two carbon atoms and a bromine atom forms with a release of Br⁻. A halonium ion in Organic chemistry is any onium compound (ion containing a bridged Halogen atom carrying a positive charge
Attacking of bromide ion

The bromonium ion is opened by the attack of Br⁻ from the back side. This yields the vicinal dibromide with an antiperiplanar configuration. Alkane stereochemistry concerns the Stereochemistry of linear Alkanes and the linear alkane Conformers The existence of more than one conformation is due When other nucleophiles such as water or alcohol are existing, these may attack 2 to give an alcohol or an ether.

This process is called Ad_E2 mechanism. Iodine (I₂), chlorine (Cl₂), sulfenyl ion (RS⁺), mercury cation (Hg²⁺), and dichlorocarbene (:CCl₂) also react through similar pathways. Iodine (ˈaɪədaɪn ˈaɪədɪn or /ˈaɪədiːn/ from ιώδης iodes "violet" is a Chemical element that has the symbol I and Atomic Chlorine (ˈklɔriːn from the Greek word 'χλωρóς' ( khlôros, meaning 'pale green' is the Chemical element with Atomic number 17 and A sulfenic acid is an Organosulfur compound and Oxoacid with the general formula R[[sulfur S]] O[[hydrogen H]] where R ≠ H Mercury (ˈmɜrkjʊri also called quicksilver or hydrargyrum, is a Chemical element with the symbol Hg ( Latinized hydrargyrum Dichlorocarbene is a Carbene commonly encountered in Organic chemistry. 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. A carbenium ion is a Carbocation of the trivalent and classical type R3C+ There is an example of the isolation of the bromonium ion 2. ^[2]

Addition of hydrogen halides

Hydrogen halides such as hydrogen chloride (HCl) adds to alkenes to give alkyl halide in hydrohalogenation. A hydrohalogenation reaction is the Electrophilic addition of Hydrohalic acids like Hydrogen chloride or Hydrogen bromide to Alkenes 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. In Chemistry, stereoselectivity is the property of a Chemical reaction that yields an unequal mixture of stereoisomers from a single Reactant At least, which of the two carbon atoms will be attacked by H⁺ is usually decided by Markovnikov's rule. In Chemistry, Markovnikov's rule or Markownikoff's rule is an Observation based on Zaitsev's rule. Thus, H⁺ attacks the carbon atom which carries the less number of substituents so as to the more stabilized carbocation (with the more stabilizing substituents) will form.

This process is called A-S_E2 mechanism. Hydrogen fluoride (HF) and hydrogen iodide (HI) react with alkenes similarly and Markovnikov-type products will be given. Structure HF forms orthorhombic crystals consisting of zig-zag chains of HF molecules Hydrogen bromide (HBr) also takes this pathway, but sometimes a radical process competes and a mixture of isomers may form.

Hydration

One of the more complex hydration reactions utilises sulfuric acid as a catalyst. In Organic chemistry, a hydration reaction is a Chemical reaction in which a Hydroxyl group (OH- and a Hydrogen Cation Sulfuric (or sulphuric acid, H 2 S[[oxygen O]]4 is a strong Mineral acid. Catalysis is the process in which the rate of a Chemical reaction is increased by means of a Chemical substance known 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:

C₂H₄ + H₂O → C₂H₅OH

As you can see the H₂SO₄ does not take part in the overall reaction, however it does take part but remains unchanged so is classified as a catalyst.

This is the reaction in more detail:

The H-OSO₃H molecule has a δ+ charge on the initial H atom, this is attracted to and reacts with the double bond in the same way as before.
The remaining (negatively charged) ⁻OSO₃H ion then attaches to the carbocation. A carbocation (ˌkɑrboʊˈkætaɪɒn is an Ion with a positively-charged Carbon Atom. Forming ethyl hydrogensulphate (upper way on the above scheme).
When water (H₂O) is added and the mixture headed ethanol (C₂H₅OH) is produced, the "spare" hydrogen atom from the water goes into "replacing" the "lost" hydrogen and thus reproduces sulfuric acid. Another pathway in which water molecule combines directly to the intermediate carbocation (lower way) is also possible. This pathway become predominant when aqueous sulfuric acid is used.

Overall this process adds a molecule of water to a molecule of ethene.

This is an important reaction in industry as it produces ethanol, which is the alcohol having various purposes including fuels and starting material for other chemicals.

Electrophilicity scale

Electrophilicity index
Fluorine	3. Fluorine, fluorum meaning "to flow" is the Chemical element with the symbol F and Atomic number 9 86
Chlorine	3. Chlorine (ˈklɔriːn from the Greek word 'χλωρóς' ( khlôros, meaning 'pale green' is the Chemical element with Atomic number 17 and 67
Bromine	3. 40
Iodine	3. Iodine (ˈaɪədaɪn ˈaɪədɪn or /ˈaɪədiːn/ from ιώδης iodes "violet" is a Chemical element that has the symbol I and Atomic 09
Hypochlorite	2. The hypochlorite Ion is Cl[[Oxygen O]]− A hypochlorite compound is a Chemical compound containing this group with chlorine in Oxidation 52
sulfur dioxide	2. 01
Carbon disulfide	1. 64
Benzene	1. Benzene, or benzol, is an organic Chemical compound and a known Carcinogen with the molecular formula C 6 H 6 45
Sodium	0. Sodium (ˈsoʊdiəm is an element which has the symbol Na( Latin natrium, from Arabic natrun) atomic number 11 atomic mass 22 88
Some selected values ^[3] (no dimensions)

Several methods exist to rank electrophiles in order of reactivity ^[4] and one of them is devised by Robert Parr ^[3] with the electrophilicity index ω given as:

$\omega = \frac{\chi^2}{2\eta}\,$

with $\chi\,$ the electronegativity and $\eta\,$ chemical hardness. Robert Ghormley Parr (born September 22, 1921) is a theoretical chemist. OMEGA is the premier Counter-terrorism unit of Latvia. Founded in 1992 OMEGA cooperates with many other counter-terrorism units over the world " Electronegativity " is the opposite of " Electropositivity," which describes an element's ability to donate electrons The HSAB concept is an Acronym for 'hard and soft Acids and bases. This equation is related to classical equation for electrical power:

$P = \frac{V^2}{R}\,$

where $R\,$ is the resistance (Ohm or Ω) and $V\,$ is voltage. Electric power is defined as the rate at which Electrical energy is transferred by an Electric circuit. Electrical resistance is a ratio of the degree to which an object opposes an Electric current through it measured in Ohms Its reciprocal quantity is The ohm (symbol Ω) is the SI unit of Electrical impedance or in the Direct current case Electrical resistance, Electrical tension (or voltage after its SI unit, the Volt) is the difference of electrical potential between two points of an electrical 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.

A electrophilicity index also exists for free radicals ^[5]. In Chemistry, radicals (often referred to as free radicals) are atoms molecules or ions with Unpaired electrons on an otherwise Open shell 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. In Organic chemistry, butyl is a four- Carbon Alkyl substituent with Chemical formula -C4H9.

Superelectrophiles

Superelectrophiles are defined as cationic electrophilic reagents with greatly enhanced reactivities in the presence of superacids. A superacid is an Acid with an Acidity greater than that of 100% Sulfuric acid, which has a Hammett acidity function ( H 0 These compounds were first described by George A. Olah ^[6]. George Andrew Olah (born May 22, 1927 in Budapest, as Oláh György) is a Hungarian -born American Chemist. 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 deprotonate isobutane when combined with hydrofluoric acid via the formation of a superacid from BF₃ and HF. Acetic acid, also known as ethanoic acid, is an organic chemical compound, giving Vinegar its sour taste Boron trifluoride is the Chemical compound with the formula BF3 Deprotonation is a Chemistry term that refers to the removal of a Proton ( Hydrogen Cation H+ from a Molecule, forming Isobutane, also known as methylpropane or 2-methylpropane, is an Alkane, Isomeric with Butane. Hydrofluoric acid is a Solution of Hydrogen fluoride in Water. A superacid is an Acid with an Acidity greater than that of 100% Sulfuric acid, which has a Hammett acidity function ( H 0 The responsible reactive intermediate is the CH₃CO_H dication. A reaction intermediate or an intermediate is a Molecular entity that is formed from the reactants (or preceding intermediates and reacts further to give the directly Likewise methane can be nitrated to nitromethane with nitronium tetrafluoroborate NO₂⁺BF₄^- only in presence of a strong acid like fluorosulfuric acid. Methane is a Chemical compound with the molecular formula. It is the simplest Alkane, and the principal component of Natural gas. Nitromethane is an Organic compound with the chemical formula CH3NO2 Tetrafluoroborate is the Anion BF4− This tetrahedral species is Isoelectronic with many stable and important species including tetrafluoromethane Fluorosulfuric acid is FSO3H it is one of the strongest acids commercially available

In gitionic 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 ^[7]

References

^ Lenoir, D. The nitronium ion (improperly called nitryl ion, because it is not a radical 2+ is a generally unstable Cation created by the removal of an F-TEDA-BF4 is a Reagent in Chemistry used as a Fluorine donor In Chemistry, a nucleophile (literally nucleus lover as in nucleus and phile) is a Reagent that forms a Chemical bond to ; Chiappe, C. Chem. Eur. J. 2003, 9, 1036.
^ Brown, R. S. Acc. Chem. Res. 1997, 30, 131.
^ ^a ^b Electrophilicity Index Parr, R. G. ; Szentpaly, L. v. ; Liu, S. J. Am. Chem. Soc.; (Article); 1999; 121(9); 1922-1924. The Journal of the American Chemical Society (usually abbreviated as J doi:10.1021/ja983494x
^ Electrophilicity Index Chattaraj, P. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document. K. ; Sarkar, U. ; Roy, D. R. Chem. Rev.; (Review); 2006; 106(6); 2065-2091. Chemical Reviews (usually abbreviated as Chem Rev) is a peer-reviewed Scientific journal, published since 1924 by the American Chemical Society doi:10.1021/cr040109f
^ Electrophilicity and Nucleophilicity Index for Radicals Freija De Vleeschouwer, Veronique Van Speybroeck, Michel Waroquier, Paul Geerlings, and Frank De Proft Org. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document. Lett. ; 2007; 9(14) pp 2721 - 2724; (Letter) DOI: 10. 1021/ol071038k
^ Electrophilic reactions at single bonds. XVIII. Indication of protosolvated de facto substituting agents in the reactions of alkanes with acetylium and nitronium ions in superacidic media George A. Olah, Alain Germain, Henry C. Lin, David A. Forsyth J. Am. Chem. Soc.; 1975; 97(10); 2928-2929. The Journal of the American Chemical Society (usually abbreviated as J doi:10.1021/ja00843a067 10.1021/ja00843a067
^ Knorr Cyclizations and Distonic Superelectrophiles Kiran Kumar Solingapuram Sai, Thomas M. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document. Gilbert, and Douglas A. Klumpp J. Org. Chem. 2007, 72, 9761-9764 doi:10.1021/jo7013092

The Journal of Organic Chemistry (abbreviated as J Org Chem or JOC) is a Scientific journal for original contributions of fundamental A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.

Dictionary

-noun

(chemistry) a compound or functional group that is attractive to, and accepts electrons, especially accepting an electron pair from a nucleophile to form a bond

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