In Organic chemistry, a reaction Nucléophile is carried out between an unspecified substrate and a nucleophilic reagent.

An addition, of chemistry, consists of the addition of an atom or groups atoms on a substrate having an unsaturated connection. Nucleophilic is an atom or a molecule which presents a strong affinity for the substances électrophiles. In the particular context of the nucleophilic addition, the substrate is slightly électrophile and the reagent is strongly nucleophilic, or the substrate must be modified (before the nucleophilic reaction of addition can take place) to become strongly électrophile if one tries to make it react with nucleophilic who is weak.

The carbonyl derivatives such as aldehydes and ketones react by nucleophilic addition with various nucleophilic giving place to a transformation of double connection C=O into (R1) (R2) (Z) C-OH (in acid medium), where R1 is the chain present on the one hand of the molecule substrate, where R2 is the chain present on other behalf of the molecule substrate, and where Z is the nucleophilic reagent. The mechanism in is simple: a doublet of electrons of nucleophilic Z attacks the carbon of the carbonyl group of the substrate, which creates a bond sigma between this carbon and the assailing atom of nucleophilic Z. Simultaneously, the pair of electrons forming bond pi between the aforementioned atom of carbon and the atom of oxygen of the carbonyl group is aspired by this oxygen atom. It results from it that this oxygen atom takes a negative charge (if the reaction is made in acid medium or that the circumstances lend themselves to it, the lone electrons causing this negative charge will tend to carry out a acido-basic reaction with acid hydrogen atoms available, from where the transformation of the C=O into C-OH).

In short and in a passably simplified way, here the general mechanism of the nucleophilic addition:

However, of many alternatives of the mechanism exist; they are determined by particular properties of certain nucleophilic reagents. Inter alia, when the nucleophilic one is rather weak (for example, if an alcohol is this nucleophilic), one needs initially protoner the substrate to increase the reactivity by it (a doublet of electrons of the oxygen atom of the substrate attacks an acid hydrogen); then, the positive load thus appeared on the oxygen atom because of the creation of a bond of co-ordination number involves a system of resonance which makes it possible to obtain a résomère carbocation. Consequently, the nucleophilic weak one (here, an alcohol) can attack carbon carrying the positive load, which involves the formation of a hemi-acetal (a function alcohol and a function ether connected to the same carbon). The hydrogen atom creating, because of the bond of co-ordination number which it then makes with the oxygen atom of the function ether, a positive load instabilisante on the aforementioned oxygen atom, it tends to being easily recapté by nucleophilic unspecified, even weak. The acid hydrogen collected at the beginning was thus only one catalyst, since it is expelled of the substrate at the end of the reaction. Let us mention with the fact that if the operation is repeated on hemi-acetal as a substrate and this, under the same conditions, therefore with an alcohol for nucleophilic and in the presence of acid hydrogen atoms, one attends the formation of an acetal (two functions ether on same carbon, therefore the function alcohol of hemi-acetal is replaced by a function ether) by a mechanism identical to that which has just been described for the formation of hemi-acetals.

It is necessary to distinguish the normal nucleophilic addition, described here, of nucleophilic the addition known as “on the carbonyl group”, also called “addition-elimination”, which can normally be done only when the substrate is a carboxylic acid or a derivative (anhydride of acid, halide of acyle, ester, amide, etc), since in these functions a group O=C-X is found, where X is a strongly electronegative atom (generally, it is about oxygen). This great electronegativity then tends to strongly attract the electronic cloud, which weakens the connection sigma between C and O. It results from it that after the attack of nucleophilic Z, one witnesses not only one absorption of the pair of electrons forming the connection pi between C and O by this same O, but rather with such an absorption, followed immediately rejection of those to reform connection pi with the identical one, which is accompanied by the expulsion of the best group therefore surrounding the atom of carbon.