Question: Label the electrophilic and nucleophilic sites in each molecule.

a.

b.

c.

d.

Electrophiles are also called Lewis acids. They are electron-deficient species and, therefore are electron-loving species.

The bonded atom (less electronegative) with an electronegative atom is always an electron-deficient species.

For example, the carbon in the carbonyl group (C=O) is an electrophile.

Nucleophiles are also called Lewis bases. They are electron-rich species and, therefore are called electron-hating species. A nucleophile contains atleast one lone pair of electrons. Example: oxygen with three lone pairs of electrons, $C{l}^{-}$, $I^{-}$etc.

a. Iodine (I) is a more electronegative atom than carbon. Therefore, the carbon adjacent to the iodine is an electrophile. Iodine is a halogen. Therefore, it has seven valence electrons. Hence, iodine contains three lone pairs of electrons. Hence, it is a nucleophile.

Representation of electrophile and nucleophile

b. Oxygen (O) is a more electronegative atom than carbon. Therefore, the carbon atoms adjacent to oxygen are electrophiles. Oxygen is a sixteenth group member. Therefore, it has six valence electrons. Hence, oxygen contains two lone pairs of electrons and it is a nucleophile.

Representation of electrophile and nucleophile

c. The pi bond always acts as a nucleophile because the pi electrons are weakly bonded.

Representation of electrophile and nucleophile

d. The carbon in the carbonyl group (C=O) is an electrophile due to the high electronegativity of oxygen. Therefore, carbon acts as an electrophile, and oxygen is the nucleophile. The chlorine atom in the molecule is a halogen. Therefore, it has three lone pairs of electrons and is also a nucleophile.

Representation of electrophile and nucleophile