Final answer:
The electrophilic site in a reaction involving a carbonyl compound is the carbon atom of the carbonyl group, as it becomes partially positive due to the electronegativity difference with oxygen, making it susceptible to nucleophilic attack.
Step-by-step explanation:
The electrophilic site in the reactants of the reaction is the carbon atom (C) of the carbonyl group. Oxygen (O) is more electronegative than carbon, resulting in polarized double bonds where the carbon becomes partially positive (δ+), making it an electrophile, which can be readily attacked by a nucleophile. In contrast, the oxygen bears a partial negative charge (δ-) and thus exhibits nucleophilic behavior. Electrophilic addition reactions in alkenes and alkynes occur at the region of increased electron density, typically at the carbon atoms of multiple bonds, where the loosely held π electrons can interact with electrophiles to form new σ bonds.
Electron withdrawing groups, such as halogens, enhance the reactivity of the electrophilic site by making the carbon more positive (δ+), while good leaving groups facilitate the collapse of intermediate structures and increase the rate of the reaction. In nucleophilic acyl substitution reactions, the carbonyl carbon is also an electrophilic site, where the nucleophile attacks and switches from a π to a σ bond. The nature of the substituents and the resulting stability of carbocations are factors that influence electrophilic reactivity and the course of the reaction.