(N/A) In an organic reaction,the organic molecule (also referred to as a substrate) reacts with an appropriate attacking reagent,leading to the formation of one or more intermediates $(I)$ and finally products $(P)$.
Organic molecule (Substrate) $\xrightarrow[\text{Reagent}]{\text{Attacking}}$ Intermediate $(I) \rightarrow$ Product $(P)$.
Substrate: The reactant that supplies carbon to the new bond is called the substrate.
Reagent: The other reactant is called the reagent. If both reactants supply carbon to the new bond,the choice is arbitrary,and the molecule on which attention is focused is called the substrate.
Mechanism of reaction: In such a reaction,a covalent bond between $2$ carbon atoms or a carbon and some other atom is broken,and a new bond is formed. $A$ sequential account of each step,describing details of electron movement,energetics during bond cleavage and bond formation,and the rates of transformation of reactants into products (kinetics) is referred to as the reaction mechanism.
Importance of reaction mechanism: The knowledge of reaction mechanism helps in $(i)$ understanding the reactivity of organic compounds and $(ii)$ in planning strategies for their synthesis.