Nucleophiles are chemical species that donate the electron pairs to make the chemical bonds during the reactions. All ions or molecules having free pair of electrons, or having at least one pi bond can act as nucleophiles. Nucleophiles are defined as Lewis bases as they donate the electrons. The word nucleophile means the nucleus lover and they have an affinity for the nuclei. The term nucleophile was first time introduced in 1933 by the Christopher Krek Ingold. Some examples of the nucleophiles are compounds with lone pairs such as ammonia and anions (Cl–).
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The atoms of nucleophiles are electron rich, and they are capable to donate the pair of electrons for making the new covalent bonds. In biological organic chemistry and the laboratory, the most relevant nucleophilic atoms are Sulphur, nitrogen, oxygen, and the common functional groups of the nucleophiles are alcohol, phenols, water, amines, thiols and occasionally carboxylates as well. Specifically, in the laboratory reactions, azide and halide anions are commonly seen as acting as the nucleophiles. Carbons are also nucleophiles, otherwise, the synthesis of large organic molecules such as fatty acids and DNA was not possible. The most common carbon nucleophiles are the enolate ions which are used in the biochemical reactions. Cyanide is also carbon nucleophile and it is commonly used in the laboratory. Bases can be the nucleophiles and nucleophiles can be the bases. Most of the trends for the basicity are also applied to the nucleophiles.
Generally, in the rows of the periodic table, as the ions become more basic, they become more reactive as nucleophiles. In the series of nucleophiles, if the attacking element is the same such as the oxygen, the order of nucleophilicity follows basicity. Sulfur is a better nucleophile than oxygen. Nucleophilicity is dependent on the many factors such as solvent, basicity, charge, nature of the substituents and the polarizability. With an increase in the negative charge, the nucleophilicity is also increased. Nucleophiles are negatively charged or they may be neutral. Whatever the case is nucleophile should be a good Lewis base means that it should have electrons for sharing.
The protonation state of the nucleophilic atom has a significant effect on its nucleophilicity. For nucleophilicity, there are predictable trends in the periodic table. Moving horizontally across the second row of the periodic table, nucleophilicity trend becomes parallel to the trend of basicity and the reason behind both of the trends is the same. The elements that have more electronegativity, hold the electrons in their shells more tightly and their ability to donate the electrons for the formation of new bonds is very less. By following the horizontal trend, the amines are more nucleophilic than the alcohols although both of these groups act as nucleophiles in the biochemical reactions and the laboratory as well.
Experimentally, it has been proven that the nucleophiles having the reactive atom with a negative charge is much better than the nucleophile containing the neutral reactive atom. With an increase in the atomic size, the phenomenon of nucleophilicity is increased. Some of the nucleophiles are sterically hindered and react too much slowly.