Question

Which substance in the reaction serves as the electrophile?'?

Answer 1

Electrophiles are reagents attracted to electrons.

Electrophiles tend to be electron-deficient and carry partial positive charges. They are attracted to species with lone pairs of electrons. For example, protons $\text{H}^+$ have no electrons and tend to share ones with other species, hence behaving as electrophiles in aqueous reactions. In the reaction between $\text{H}^+$ and ammonia $\text{NH}_3$, protons would be attracted to lone electron pairs on nitrogen atoms in ammonia molecules, which carry partial positive charges.

The Lewis Acid-base theory define Acids as species that accept electron pairs in a particular acid-base reaction. Electrophiles, by definition, tend to accept electrons. Lewis acids thus behaves as electrophiles in acid-base reactions. In the previous example, $\text{H}^+$ demonstrates acidic behavior and can be inferred as an electrophile.