Organic Chemistry: Concepts and Applications - Headley Allan D. 2020

Elimination Reactions of Organic Chemistry
12.1 Introduction

Elimination reactions are another very important category of reactions of organic chemistry. As the name suggests, these are reactions in which atoms (or groups of atoms) are removed from a reactant molecule to form a new product. These reactions are the opposite of addition reactions that were covered in Chapter 8. Typically, the atoms or groups of atoms are eliminated from two adjacent atoms and, as a result, are referred to as β-elimination, since β is the second letter of the Greek alphabet. Another term that is used for this type of elimination is a 1,2-elimination since the atoms or group of atoms that leave are originally on carbons 1 and 2 of the reactant. The product of an elimination reaction is an unsaturated functionality (alkene or alkyne). Typically, the atoms that are removed are a proton and a good leaving group (a weak conjugate base) with its pair of bonding electrons, as shown in Reaction (12-1). Since ions of halogens (especially bromine and iodine) are weak conjugate bases, they are typically good leaving groups for these types of reactions. Remember that a proton is really a hydrogen atom without its electron, a positively charged electrophile; and a leaving group is one that has a pair of electrons, a Lewis base.

(12-1)

Of course, one could imagine different ways (mechanisms) in which the elimination of the atoms (H⁺ and X⁻) can take place to give the alkene product. In this chapter, we will examine possibilities (mechanisms) for elimination reactions and apply that knowledge to design the synthesis of different target molecules.