Nucleophilic Reactions A nucleophilic reaction involves a nucleophile attacking an electrophile. A nucleophile (from "nucleus-loving") is an electron-rich species that donates a pair of electrons to form a new covalent bond. Nucleophiles are Lewis bases and typically possess lone pairs of electrons or $\pi$ bonds. Examples include $\text{OH}^-$, $\text{CN}^-$, and $\text{NH}_3$. An electrophile (from "electron-loving") is an electron-poor species that accepts a pair of electrons from the nucleophile. Electrophiles are Lewis acids and often have a positive charge, a partial positive charge, or an incomplete octet. Examples include carbocations, the carbon atom of a carbonyl group ($\text{C}=\text{O}$), and alkyl halides. In a nucleophilic reaction, the nucleophile uses its high electron density to attack an electron-deficient center on the electrophile, forming a new bond. This process often leads to the displacement of a leaving group or the addition across a multiple bond. Example: Nucleophilic addition to a carbonyl group The carbon atom in a carbonyl group ($\text{C}=\text{O}$) is electrophilic due to the electronegativity of oxygen. A nucleophile, such as $\text{CN}^-$, attacks this electron-deficient carbon. $$ \text{R}_2\text{C}=\text{O} + \text{CN}^- \longrightarrow \text{R}_2\text{C}(\text{O}^-)\text{CN} $$ Electrophilic Reactions An electrophilic reaction involves an electrophile attacking an electron-rich center, which acts as a nucleophile. An electrophile (from "electron-loving") is an electron-poor species that accepts a pair of electrons from an electron-rich center. Electrophiles are Lewis acids. Examples include $\text{H}^+$, $\text{Br}^+$, and $\text{NO}_2^+$. The electron-rich species, often an alkene, alkyne, or aromatic ring, acts as a nucleophile by donating its $\pi$ electrons to the electrophile. In an electrophilic reaction, the electrophile seeks out and attacks a region of high electron density. The $\pi$ electrons of a double or triple bond, or the delocalized $\pi$ electrons of an aromatic ring, are particularly susceptible to electrophilic attack. Example: Electrophilic addition to an alkene The $\pi$ bond in an alkene is a region of high electron density. An electrophile, such as $\text{H}^+$ (from $\text{HBr}$), attacks the $\pi$ bond, forming a carbocation intermediate. $$ \text{R}_2\text{C}=\text{CR}_2 + \text{H}^+ \longrightarrow \text{R}_2\text{C}^+-\text{CHR}_2 $$ This carbocation then reacts with a nucleophile (e.g., $\text{Br}^-$) to complete the addition. That's 2 down. 3 left today — send the next one.