Humans engage in welfare activities with the aim of improving the well-being of others, tackling inequality, and creating stronger bonds. This same principle can also be observed in organic chemistry. Some species are rich in electrons, and they help their electron-deprived counterparts by donating their electron richness and balancing out their electron imbalance. This act results in the formation of new bonds between the electron-rich and electron-deficient species.
The electron-rich species that donate the electrons are called nucleophiles. These nucleophiles can derive their electron prosperity from three different sources: lone pairs, pie bonds, and, on rare occasions, sigma bonds. In addition, nucleophiles can be either neutral or negatively charged.
Therefore, a nucleophile can be defined as an electron-rich species that can donate an available pair of electrons to an electrophile, resulting in the formation of a covalent bond.
The few examples are-
Role of a Nucleophile in a Chemical Reaction
In a chemical reaction, the nucleophile constantly seeks its complementary partner, the electrophile. The nucleophile then donates its two electrons to the electrophile and forms a new covalent bond.
Electrophiles are electron-deficient due to multiple factors; the most common is electronegativity.
Due to electronegativity, an electronegative atom or group of atoms takes away a covalent bond’s electron density, due to which one atom in the bond is richer than the other in electrons. This creates two terminals: electron-rich and electron-poor, called the dipoles.
For example, in a molecule of CH32-CH21-Br, the Bromine (Br) is more electronegative than Carbon 1 to which it is directly attached. The Bromine pulls most of the electron density of the bond and makes the adjacent Carbon 1 more electrophilic. This loss in electron density is denoted with a partial positive charge on Carbon as (δ+), and the gain is represented with Bromine (δ-).
CH3-δ+CH2-δ-Br
The carbon 1 with the partial positive charge becomes the electrophilic center. In a chemical reaction, the nucleophile is attracted to such an electron-deficient center, where it will donate the electrons and form a new bond.
The electronegative atom that earlier caused the bond to weaken due to the electron pull is lost as a leaving group (here, Br-), taking the two bond electrons with it.
Note that a curly arrow always travels with two electrons. (Read more about the use of double-headed curly arrow in Types of Arrows in Chemistry).
Some of the other factors responsible for creating electrophilic centers are- electron delocalization (resonance), presence of empty p-orbitals, Carbon atom next to positively charged Oxygen and Nitrogen, etc. The nucleophiles approach these centers in bond formation reactions.
Blogs - Common mistakes while learning about the nucleophiles
Techniques to identify a nucleophile – Visual Inspection Method - Part 1
Techniques to identify a nucleophile in organic reactions – Following the Arrow Trail
