In organic chemistry, an electrophile is a reagent attracted to electrons. 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. 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.
Addition of halogensThese 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: C2H4 + Br2 → BrCH2CH2Br This takes the form of 3 main steps shown below;Lenoir, D.; Chiappe, C. Chem. Eur. J. 2003, 9, 1036.
- Forming of a π-complex
- :The electrophilic Br-Br molecule interacts with electron-rich alkene molecule to form a π-complex 1.
- 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−.
- 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. When other nucleophiles such as water or alcohol are existing, these may attack 2 to give an alcohol or an ether.
Addition of hydrogen halidesHydrogen 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.
HydrationOne 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: C2H4 + H2O → C2H5OH 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:
- 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, forming ethyl hydrogensulphate (upper way on the above scheme).
- When water (H2O) is added and the mixture heated, 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.