Electrophile

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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.

Contents 1 Electrophiles in Organic Chemistry 1.1 Alkenes 1.2 Addition Reactions 1.3 Hydration 2 See also

One important type of electrophiles are carbocations, important in many organic chemistry reactions.

Because of their ionic reactivity, electrophilic molecules are prominent actors in toxication, the conversion of compounds to toxic metabolites. Cytochrome, for instance, metabolizes many drugs into toxic, electrophilic intermediates. Alternately, detoxication may involve conjugating electrophiles into less reactive species.

Electrophiles in Organic Chemistry

Alkenes

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

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

Addition Reactions

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, ethene + bromine:

C₂H₄ + Br₂ --> CH₂BrCH₂Br

This takes the form of 3 main steps;

1. Dipole induced in bromine
   The Br-Br molecule has even distribution of electrons over the covalent bond, but as it nears the ethene molecule, with its high electron density, a dipole is induced in the bromine molecule. Giving the Br atom closest to the ethene a δ+ (slight positive) charge, and the other atom a δ– charge.
2. Electrophilic attack
   The positively charged bromine atom acts as an electrophile, and the electrons in the ethene double bond begin to form a new carbon-bromine bond with the δ+ bromine.
3. Bromide ion forms
   The carbon-bromine bond is formed using an electron pair from the C=C bond, thus the other carbon in the double bond becomes part of an electron deficient Carbocation. The Br-Br bond breaks as the electrons move away from the new C-Br bond and a Br^- ion is produced. This ion then acts as a nucleophile and uses a lone pair of electrons to form a new bond with the carbocation.

This works in the same way for addition of H-Br and sulphuric acid

Hydration

One of the more complex reactions utilises sulphuric 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 molecule then attaches to the carbocation. Forming ethyl hydrogensulphate.
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 sulphuric acid.

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

This is an important reaction in industry as it produces ethanol.

See also

• Nucleophilear:شغوف بالإلكترون

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