Which of the following has the highest nucleophilicity?

What are electrophiles and nucleophiles? Explain with examples.

$(a)$ Explain the following terms and provide the difference between them:
$(i)$ Reagent species
$(ii)$ Substrate species
$(b)$ $(i)$ What is the mechanism of a reaction?
$(ii)$ What are the uses of a reaction mechanism?
$(c)$ Depict the general reaction that occurs between a substrate and a reagent in an organic reaction.

Which of the following represents the correct order of nucleophilicity?

Which among the following is a set of nucleophiles?

Which one of the following series contains only electrophiles?