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. Because electrophiles accepts 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.

The electrophiles attack the most electron-populated part of a nuculeophile. The electrophiles frequently seen in the organic syntheses are cations such as H⁺ and NO^{+, polarlized neutral molecules such as HCl, alkyl halides, acyl halides, and carbonyl compounds, polarlizable 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.}

Electrophiles in organic chemistry

Alkenes

Electrophilic addition is one of the three main forms of reaction concerning alkenes. They consist of:

• Hydrogenation by the addition of hydrogen over the double bond.
• Electrophilic addition reactions with halogens and sulfuric acid.
• Hydration to form alcohols.

Addition of halogens

These occur between alkenes and electrophiles, often halogens. Common reactions include use of bromine water to titrate against a sample to deduce the number of double bonds present. For example, ethylene + 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.

2. 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⁻.

3. 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 anti-periplanar configulation. When other nucleofiles such as water or alcohol are exsiting, these may attack 2 to give an alcohol or an ether.

This proccess is called Ad_E2 mechanism. Iodine (I₂), chlorine (Cl₂), sulfenyl ion (RS⁺), marcury cation (Hg²⁺), and dichlorocarbene (:CCl_{2) also react through similar pathways. The direct convertion 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 insted 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.}

Addition of hydrogen halides

Hydrogen harides such as hydrogen chloride (HCl) adds to alkenes to give alkyl halide. For exmaple, the reaction of HCl with ethylene furnishes chloroethane. The reaction proceeds with a cation intermediate, being differnt from the above halogen addition. An example shown below:

1. Proton (H⁺) adds (by working as an electrophile) to one of the carbon atoms on the alkene to form cation 1.
2. 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 atack relies on the types of alkenes applied and conditions of the reaction. At least, which of the two carbon atoms will be atacked by H⁺ is usually decided by Markovnikov's rule. Thus, H⁺ atacks 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 similary 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.

Hydration

One of the more complex 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 you can see the H₂SO₄ does not take part in the overall reaction, however it does take part but remains unchanged so is clasified as a catalyst.

This is the reaction in more detail:

1. 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.
2. The remaining (negatively charged) ⁻OSO₃H ion then attaches to the carbocation. Forming ethyl hydrogensulphate (upper way on the above scheme).
3. When water (H₂O) is added and the mixture headed ethanol is produced (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 molecure 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.

See also

• Nucleophile