Knowing More On Nucleophilicity

In organic chemistry, the charge and the electronic constituents play a very important role. Ions having negative or positive charges determine how much the species will be reactive.  Nucleophiles and electrophiles are the commonly employed charged species playing a role in the reactions of organic chemistry.  Nucleophiles or nucleus loving species are the ones that have an extra pair of electrons and thus are in search of a nucleus that can enhance their stability. Electrophiles or electron loving species are in need of a donor of electrons since they have a positive charge.  

Nucleophilicity

A nucleophile is an entity (atom/ion/molecule) that has an electron pair that can be donated in order to form a covalent bond with an electrophile. An electrophile is just the opposite of a nucleophile in the sense that it is an electron-deficient species (atom/ion/molecule) that accepts a pair of electrons to form a covalent bond with a nucleophile. The literal meaning of nucleophiles is “nucleus loving”, and that fits in their role, too, since they have extra pair/s of electrons, so they are in search of some positive charge to become more stable. 

Some of the commonly employed nucleophiles are the halogen anions (I–, Cl–, Br–), the hydroxide ion (OH–), the cyanide ion (CN–), ammonia (NH3), and water. 

The ease by which a nucleophile can make a covalent bond with an electrophile by donation of a pair of electrons is usually referred to as the nucleophilicity of that species. In other words, it measures the ability of a nucleophile to react at an electron-deficient centre. The nucleophilicity of a species can be determined by the availability of its electron pairs, as the more available the electrons of a nucleophile are, the better the nucleophile is. The most reactive nucleophiles are thought to be of high nucleophilicity. 

Factors Affecting Nucleophilicity 

• Charge- Increasing the negative charge increases the nucleophilicity of the chemical species. 
• Electronegativity- increasing electronegativity reduces the nucleophilicity of the species. 
• Solvent
• Steric hindrance 

Trends Of Nucleophilicity In The Periodic Table

• Across a period, the nucleophilicity decreases since the electron pairs become less available due to the reducing size of the atom or close proximity of the nucleus. I- > Br- > Cl- >F-
• If the same central atom is compared with its ionic form, the species with high electron density will be a better nucleophile.  For example, an anion will have more nucleophilicity than an atom of the same.
• Moving across a group in the periodic table, as one goes down, the nucleophilicity increases due to the increasing polarisation of the nucleophile, which enhances the ability to form new C-X bonds. The size of the atom increases down a group, so for larger atoms, the electron density is more easily distorted or polarised since the electrons are farther away from the nucleus.

Below is a general table shown for reference for the order of nucleophilicity of various chemical species. 

Conclusion

In conclusion, it can be said that nucleophiles are negatively charged species capable of reacting with electrophiles and forming covalent bonds with them. The nucleophilicity of various species is determined by various factors, including the size of the atom, its electronegativity, the Solvent system used and if it has any steric hindrances.