Therefore, since charges of opposite signs attract each other, nucleophiles must be negative species; or at least, possessing highly negative regions due to a local or scattered concentration of electrons.
Thus, a nucleophile can be represented by the letters Nu, as in the image above. The double dots in blue correspond to a pair of electrons, which they donate to the electrophile; that is, the electron-deficient atom or molecule. Note that the nucleophile can be neutral or anionic, however both donate pairs of electrons.
Nucleophiles look for nuclei or electrophiles, which means they go after charges or positive regions of a molecule. The following chemical equation represents nucleophilic attack:
Nu : + R-LG → R-Nu + LG :
The nucleophile Nu: donates its pair of electrons to R, which is bonded to an electronegative leaving group LG. In doing so, the R-LG link is broken, LG: exits or migrates, and the new R-Nu link is formed. This is the basis for many organic reactions.
It will be seen in the next section that a nucleophile can even be an aromatic ring, the electron density of which is dispersed in its center. Also, a nucleophile can become a sigma bond, which means that electrons from it migrate or jump to nearby nuclei.
Types of nucleophiles
There are several types of nucleophiles, but the vast majority consist of species with pairs of free electrons, double bonds, or sigma bonds that participate in molecular mechanisms.
Species with free electron pairs
When we speak of species with pairs of free electrons, we mean anions, or molecules with electronegative atoms, such as oxygen, nitrogen and sulfur. In the examples section you will see many nucleophiles of this type, apart from the OH – anion already mentioned.
A nucleophile can have double bonds, which are responsible for nucleophilic attack. However, they must be double bonds with an appreciable electron density, so not just any molecule that possesses them will be considered a strong nucleophile; that is, it will not have a high nucleophilicity.
For example, consider the benzene ring in the following alkylation reaction (Friedel-Crafts reaction):
The presence of an AlCl 3 – (CH 3 ) 2 CHCl mixture gives rise to the isopropyl carbocation. Its positive charge and instability strongly attracts electrons from one of the benzene double bonds, which attack the carbocation, as represented by the arrow.
In the process, a brief cationic and aromatic intermediate is formed, which eventually transforms into the product on the right.
As with benzene, other substances with double bonds can act as nucleophiles, provided the reaction conditions are the most appropriate. Likewise, there must be atoms located near the double bond that donate electron density, so that they “recharge” with electrons.
Sigma bonds as such are not nucleophiles; but it can behave as such once a reaction starts and the mechanism begins. Consider the following example:
The sigma CH bond adjacent to the carbocation moves towards it behaving like a nucleophile (see movement of the curved arrow). In this sense, the result is that the H – anion moves to the neighboring carbon, but so rapidly that the sigma bond and its pair of electrons are considered as the nucleophilic agent of this mechanism.
Examples of nucleophiles
In this section several examples of the first type of nucleophiles will be mentioned, which are very abundant and important in organic and inorganic chemistry.
Halides (F – , Cl – , Br – and I – ) are nucleophiles. They have to donate one of any of their four pairs of valence electrons. Depending on how quickly one of these halides attacks the electrophile, it will have more or less nucleophilicity.
In general, I – is a better nucleophile than F – and the other halides, since it makes it easier for it to form a covalent bond because it is more polarizable; that is, more voluminous and with less tenacity to give up its pair of electrons.
Molecules with electronegative atoms
Water, HOH, is a nucleophile, because the oxygen atom has high negative density and pairs of free electrons to donate and form a covalent bond. Likewise, alcohols, ROH, are nucleophiles, for the same reasons as those of water.
Small nitrogenous molecules, such as ammonia,: NH 3 , also tend to be nucleophiles. This is because nitrogen can donate its lone pair of electrons. Similarly, amines, RNH 2 , are also nucleophiles.
And in addition to small molecules with oxygen or nitrogen, sulfurized ones also count as nucleophiles. This is the case of hydrogen sulfide, H 2 S, and thiols, RSH.
Sulfur is a better nucleophile than oxygen and nitrogen because it is less “clinging” to its electron pair, making it easier for you to donate. To this fact it must also be added that its atom is more voluminous, that is, more polarizable, and therefore capable of forming covalent bonds with less difficulty.
Oxygenated, nitrogenous, sulfurized anions, and in general several of them, are strong nucleophiles. This is because they now have a negative charge that further intensifies the presence of the pair of electrons they will donate.
Consider for example the following anions arranged in decreasing order of nucleophilicity:
: CH 3 – >: NH 2 – >: OH – >: F –
The carboanion CH 3 – is the strongest nucleophile because the carbon atom does not stabilize the negative charge, whose electrons are “desperate” for nearby nuclei. This is not the case with the amide, NH 2 – , whose nitrogen atom better stabilizes the negative charge and gives up the electron pair more easily than OH – or F – .
Nucleophilicity defines how strong the nucleophilic character of a species is. This depends on many factors, but the most important are steric hindrance during nucleophilic attack and solvent action.
The smaller the nucleophile, the faster and more effective it will attack the electrophile. Also, the smaller the interactions between the solvent and the nucleophile, the faster it will attack the electrophile. Therefore, according to this, I – has higher nucleophilicity than F – .