Nucleophilic Substitution and the Sₙ2 Reaction

Substitution and Elimination Reactions

In this course Professor Max Majireck (Hamilton College) introduces us to substitution and elimination reactions, understanding the fundamental differences in features, mechanisms, and kinetics between S_N2, S_N1, E1 and E2. We begin by: (i) defining key terms, including nucleophile, nucleophilic substitution, leaving group, and substrate before moving on to the general mechanism of an S_N2 reaction, relating this to its kinetics and energetics; (ii) then moving to describe the key features of an S_N2 reaction, that are the variables of the reaction which affect the likelihood and rate of reaction; (iii) then relating this to the S_N1 reaction, comparing its mechanism to the S_N2 reaction, and how these differences affect kinetics and energetics in real-world examples; (iv) penultimately we discuss the E1 reaction, distinguishing between substitution and elimination reactions first before examining the mechanism for an E1 reaction, and the factors involved which affect the kinetics and thermodynamics of its reaction, and; (v) to finish off we talk about the E2 reaction, the last of the different types of substitution and elimination reactions, learning about its general mechanism and features, before finally looking at ways in which we can work around established rules in organic chemistry that can limit our flexibility.

Nucleophilic Substitution and the Sₙ2 Reaction

In this first mini-lecture we introduce the idea of nucleophilic substitution, an extremely common mechanism in organic chemistry in a variety of different forms. We begin with the general mechanism of the S_N2 reaction, the first of the types of reactions that we will be looking at that affect aliphatic compounds. We learn about some key definitions of this reaction (the nucleophile, the substrate, the solvent, the leaving group) to create a good foundational understanding for proceeding reactions. Next, we look at the kinetics and thermodynamics of this reaction. Which compounds are involved in the rate-determining step? And how does the bond breaking and making we see relate to the energy profile diagram of the reaction?