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

Addition Reactions Involving Alkenes and Alkynes
8.2 The Mechanism for Addition Reactions Involving Alkenes

Reaction (8-1), (8-2), and the reactions covered in Chapter 6 do not just magically proceed from reactants to products. For most reactions, there are different steps before the products are actually formed; the reactants react to form intermediates and go through various steps before forming the products. This step-by-step description of reactions is also known as the reaction mechanism. Most of the addition reactions that will be examined in this course, take place by going through a number of different steps before the final product is finally formed. In this chapter, the addition of different molecules to alkene will be examined. As shown in Chapter 2, alkenes have the carbon—carbon double bond, which has a pair of pi (π) electrons that can be used to react with a Lewis acid as shown in Reaction (8-3).

(8-3)Image

The curved arrow formulism is often used to show electron movements from the Lewis base to the Lewis acid as shown in Reaction (8-3). For these addition types of reactions, there are some specific names that are used to represent the Lewis base and Lewis acids. The carbon—carbon double bond of alkenes is electron rich due to the presence of the pi (π) electrons, and as a result, alkenes will act as a source of electrons in a reaction. This double bond, which is a Lewis base, as shown in the reaction in Reaction (8-3), is also known as a nucleophile. A nucleophilic species has at least one pair of electrons available to form a new covalent bond with a Lewis acid; thus, alkenes are nucleophilic. Another term that is often used for the Lewis acid of these reactions is electrophile. An electrophile, as the name suggests, is an electron-loving species, which will react with the electron pair of a nucleophile to form a new covalent bond as shown in Reaction (8-3). The proton is easy to identify as an electrophile since it has a vacant orbital to accept electrons and form a new covalent bond with a nucleophile. Hence, Lewis bases and acids that were discussed in the previous chapter are also nucleophiles and electrophiles, respectively. Note that in Reaction (8-3), the pi (π) electrons of the carbon—carbon double bond can form a bond to either side of the initial double bond giving rise to two different intermediate carbocations.

Whenever these reactions are carried out in the lab, the electrophile is a neutral molecule, and not just a cation as shown in Reaction (8-3). The counter ion for an electrophile is an anion, which is also a nucleophile. Thus, the reaction of a neutral electrophile with an alkene will take place in two steps. First is the addition of the electrophile to the nucleophilic double bond to form an electrophile carbocation, and the second is the addition of the nucleophilic counter anion of the electrophile to the electrophilic carbocation intermediate to form a neutral product. The first step of the reaction in this type of reaction mechanism is shown in the reaction in Reaction (8-4).

(8-4)Image

Note that since the alkene is unsymmetrical, there are two possible carbocations formed, and as a result, there will be two possible products, as shown in the reactions in Reactions (8-5) and (8-6).

(8-5)Image

(8-6)Image

ΔImage

Figure 8.1 Energy profile for the hypothetical electrophilic addition reaction.

This type of reaction in which electrophiles (typically along with its counter anion) are added to the carbon—carbon double bond of an alkene is called an electrophilic addition reaction. The overall general reaction is shown in Reaction (8-7).

(8-7)Image

The two steps of the reaction mechanism shown in Reactions (8-5) and (8-6) are called the elementary steps of the reaction mechanism, and these steps are shown in the energy profile diagram given in Figure 8.1. Note that in the energy profile diagram, the relative energies of the reactants, intermediates, and products are shown for each of the elementary steps and that the intermediates are always shown in the middle of the reaction profile diagram.

Energy profile diagrams will be used frequently throughout this course to analyze various types of reactions.