What makes an electrophile in need of electrons? It must have an electron deficiency, either manifested by a partial or total positive charge, or by an electronic vacancy product of an incomplete valence octet. That is, we have several types of electrophiles, although they all accept pairs of electrons from negative species.
Two types of electrophiles are generally represented above. Both are symbolized by the letter E. The one on the left corresponds to a positively charged electrophile, E + . Meanwhile, the one on the right corresponds to an electrophile with electronic vacancy, represented by a grayish oval that indicates the absence of electrons in a valence orbital.
Electrophiles react by accepting pairs of electrons from atoms with high negative densities, that is, rich in electrons. These negative atoms or molecules are called nucleophiles, Nu – , which donate a pair of electrons to the electrophile E or E + :
Nu – + E + → Nu-E
This is the theoretical basis for many organic reactions, such as electrophilic aromatic substitution. The nucleophile does not necessarily have to be an anion, but it can also be a neutral molecule with an electronegative atom, such as nitrogen.
Examples of electrophiles
Lewis acids are electrophiles, since by definition they accept pairs of electrons. The metal cations, M n + , attract the negative regions of the neighboring polar molecules in the solvation processes. Thus, M n + ends up surrounding itself with negative charges, even accepting electrons to form coordination complexes.
The Cu 2+ cation , for example, is a Lewis acid and an electrophile because it coordinates with the oxygen atoms in water to form a complex aqueous, Cu (OH 2 ) 6 2+ . The same happens with other cations.
Not all Lewis acids are cations: some are neutral molecules or atoms. For example, BF 3 is a Lewis acid and an electrophile because it seeks to accept electrons from nearby negative species to complete its valence octet.
Another electrophile is the nitronium ion, NO 2 + , which is a very strong electrophilic agent formed in the nitration reactions of benzene. In that ion, the nitrogen atom has a positive charge, so it quickly accepts the electrons from benzene.
Some Brönsted acids are also electrophiles. For example, the hydronium cation, H 3 O + , is an electrophile because the oxygen atom has a positive charge. Being very electronegative, it will seek to gain electrons by donating one of its hydrogens to transform itself into a water molecule.
Another Brönsted acid such as sulfuric acid, H 2 SO 4 , is also an electrophile. The sulfur atom is highly oxidized, and it will seek to gain electrons by donating its two hydrogens.
Halogens (F 2 , Cl 2 , Br 2 and I 2 ) are electrophiles. Its atoms do not present electronic deficiencies; however, their bonds are unstable, since both atoms, XX, attract electrons to them with great force.
Therefore, halogens react as oxidizing agents, behaving as electrophiles and accepting pairs of electrons to become halide anions (F – , Cl – , Br – and I – ).
However, halogens don’t just gain electrons in this way. They can also bond with atoms less electronegative than themselves in order to obtain a net gain of electrons. For example, this is the reason why they can be added to the double bonds of alkenes or olefins.
Halogens represent a different type of electrophile than the two that were introduced at the beginning. However, its behavior in the end is the same as for all electrophiles: accepting pairs of electrons.
Alkyl and hydrogen halides
The alkyl and hydrogen halides are electrophiles in which the atom bonded to the halogen has a strong electronic deficiency represented by the symbol δ +. This is because the highly electronegative halogen draws the electron density of the neighboring atom towards it.
For alkyl halides, RX, R will have an electronic deficiency while X will have an excess of electrons, R δ + -X δ- . Thus, if a very negative species approaches RX, it will attack R to bind to it and cause X to come out as an anion.
Likewise, in hydrogen halides, HX, hydrogen has an electronic deficiency or a positive partial charge, H δ + -X δ- . Therefore, the negative species will give up their electrons to this hydrogen and it, as an electrophile, will accept them.
Acids, halogens, and halides are not the only molecules that can be classified as electrophiles. Although it may not seem like it, carbon dioxide, CO 2 , is an electrophile, since the central carbon atom is highly oxidized, O = C δ + = O.
Therefore, when CO 2 reacts, it will do so by accepting pairs of electrons, either becoming the carboxylate group, COOH, or the carbonate anion, CO 3 2- .
In addition to CO 2 , carbonyl compounds such as ketones, aldehydes and esters are also examples of electrophiles, since in them carbon has a positive partial charge and tends to accept electrons from very negative species.
Carbocations are extremely strong Lewis acids. There are tertiary (R 3 C + ), secondary (R 2 HC + ) or primary (RH 2 C + ). Carbon always forms four bonds, so this particular cation will figure out how to accept electrons anyway.
Not all electrophiles are equally “hungry” for electrons. Some are more reactive than others. The greater the electronic deficiency of an electrophile, the greater its sensitivity to nucleophilic attacks from negative species. That is, it will present higher electrophilicity, which is the relative measure of how reactive the electrophile in question is.
For example, carbocations, nitronium, and fluorine have high electrophilicity; while carbon dioxide or some cations such as sodium, have low electrophilicity.