A nucleophilic substitution reaction is when a nucleophile donates an electron pair to an electrophile, forming a bond, but also resulting in one of the bonds in the electrophile breaking to produce a small molecule or leaving group.
🔎 Formula: Nu + R-X → R-Nu + X⁻

• Nucleophile (Nu): Donates an electron pair.
• Electrophile (R-X): Has an electron-deficient atom (R) bonded to a leaving group (X). In the reaction, the nucleophile's electron pair forms a bond with R, and X becomes the leaving group.

Imagine the nucleophile as a friendly neighbor (let's call her Nancy) wanting to give you a gift (electron pair). You, being the busy electrophile, take the gift but also hand over a box (leaving group) that you no longer need. That's a nucleophilic substitution!

• Reactants: Chloroethane (a halogenoalkane) + Aqueous sodium hydroxide
• Reaction: CH₃CH₂Cl(g) + OH⁻(aq) → CH₃CH₂OH(aq) + Cl⁻(aq)
• What Happened
  • The highly electronegative chlorine in chloroethane creates a partial positive charge on the carbon.
  • Carbon, now being electron-deficient, is like a magnet for our nucleophile.
  • Hydroxide (OH^-) jumps in, donates an electron pair, and forms a bond with carbon.
  • As they bond, the carbon-chlorine bond breaks, and Cl⁻, the chlorine, decides to leave (as our leaving group).