Nucleophiles are electron-rich molecules and anions that donate an electron pair to the electron-deficient counterparts, the electrophiles, and form covalent bonds with them.
These nucleophiles are immensely valuable in organic chemistry since they commonly insert functional groups by substitution or addition reactions.
However, identifying a nucleophile can get tricky amongst the substrates in chemical reactions, especially if the nucleophile is neutral in nature where the contributing factor, like the lone pair, is hidden from plain sight.
If you remember the periodic table, you will recall that amongst the p-block elements on the right, it was Carbon that could form the highest four bonds, and the ability decreased from Carbon onwards. Nitrogen can form three, Oxygen two, and Halogens one bond. What happened was that the number of electrons that participated in bond formation started to decrease. The number of electrons completely uninterested in bond formation, called the lone pairs, increased. So, we went from zero lone pairs for Carbon to one for Nitrogen, two for Oxygen, and three for halogens. What we observe is that the electron density on the atoms called the heteroatoms starts to increase. Such heteroatoms start to become the obvious choice for nucleophiles.
Read about valency.
However, not all lone pairs in the bonded state of an atom become good nucleophiles. The lone pairs on the halogen atoms in the bonded state, as in HF, HCl, HBr, and HI molecules, are poor nucleophiles. The halogen atoms' electronegativity prevents them from being good electron donors. However, after undergoing heterolytic bond cleavage, the same electronegativity helps the halogen atoms retain the bond electrons to easily stabilize and accommodate the negative charges as - F-, Cl-, Br-, and I-. The halogen atoms' electron density increases their potency to act as nucleophile rises manifold. The same applies to other charge-containing monoatomic or polyatomic molecules formed by heterolytic bond cleavage. For example, -CN, -OH, etc. As a rule, the negatively charged monoatomic or polyatomic molecules are better nucleophiles than neutral molecules.
In the chapter Fundamentals of the Organic Reactions, I also mentioned that most organic chemistry reactions encounter three types of nucleophiles- lone pair (neutral or charged), pie bond containing, and sigma bond donors.
One common observation from these types is that these species act as nucleophiles because there is an electron-richness due to excess electrons. The excess of electrons can be inherent for molecules due to pie bonds like alkene, alkyne, and benzene.
Sigma bond donors are a special case. Only some atoms of Group 13, like Boron and Aluminum, can accommodate an extra pair of electrons in their trivalent state (since they have an available space in their p-orbital after forming three bonds) as a covalent bond. Due to the electron gain, the central atom becomes tetravalent and negatively charged. It can then donate the extra covalent bond, act as a nucleophile, and return to the neutral state.
Our first technique is visually inspecting and locating electron-rich atoms and molecules containing negative charges, pie bonds, and lone pairs. Only these species will have an appetite for donating electrons and forming new bonds.
Technique 1- Visually Inspecting the Nucleophiles - Examples
Please keep in mind that in the examples mentioned above, we have only examined potential nucleophiles. Whether they can function as efficient or inefficient nucleophiles based on factors like steric bulk, reactivity, stability, solvent, and more is a subject that requires extensive study. Types of reactions nucleophiles undergo and a special type of ambident nucleophiles is part of the chapter, Fundamentals of Organic Reactions, in the Introductory Organic Chemistry Course.
