A carbocation is a positively charged, electron-deficient ion of carbon with only 6 valence electrons from the preferred 8 for covalent bonding. Therefore, it acts as an electron-accepting reactive intermediate in many organic reactions.
After accepting two electrons, it can transform from a less stable to a more stable uncharged molecule.
Gaining two electrons from electron-rich nucleophiles would lead to a new covalent bond formation. The nucleophile may be a halide ion (X-), OH-, another negative ion, or a species bearing lone pair of electrons (e.g., NH3, H2O, etc.).
For example, a substitution reaction in benzene proceeds by a carbocation intermediate. The reagent alkyl halide is treated with AlCl3, an electron acceptor (Lewis acid). The AlCl3 removes the halogen to form a carbocation intermediate (the electrophile). The electron-rich Benzene (the nucleophile) can then pick up this intermediate. A new covalent bond is formed between the alkyl and the benzene to give alkyl benzene as the product.
In some reactions, the carbocation formed may lose a proton from an adjacent atom to form an alkene, creating an elimination product.
Therefore, the role of carbocation in reactions is to form a new bond.
