Organic Chemistry: Concepts and Applications - Headley Allan D. 2020
Reduction Reactions in Organic Chemistry
10.1 Introduction
As mentioned in Chapter 6 in the overview of the reactions of organic chemistry, reduction is the supply of electrons and or the removal of oxygen, and this process typically is accompanied by the supply of hydrogen. In organic chemistry, this type of reaction is very common in order to synthesize many useful compounds. First, it is important to recognize compounds that can be reduced. Such compounds are typically compounds that are electrophilic and can accept electrons from reducing agents. We have seen from previous chapters, various types of electrophiles and some are shown below, in which the electrophilic atom is indicated with a partial positive (δ+).
It is also important to be able to recognize possible reducing agents. As mentioned above, reducing agents are reagents that have the ability to supply electrons to another molecule. You will recall that this definition is also the same definition as a Lewis base and also a nucleophile. For the reactions of this chapter, however, the difference is that the reducing agents have much stronger potential to supply electrons, compared to the nucleophiles that we have discussed thus for, such as water, alcohols, and amines. The reduction potentials of reducing agents are much greater than typical Lewis bases or nucleophiles. Owing to the similarity of the structures of Lewis bases, nucleophiles and reducing agents, the symbol Nu:−, along with its counter cation, is typically used to represent a reducing agent. Reaction (10-1) is a hypothetical reduction reaction of a polar covalent double bond with a reducing agent.
(10-1)
Note that reduction takes place because the reducing agent, Nu:−, uses its unshared pair of electrons to form a new sigma bond in the product. Also, notice that this reaction is a regiospecific addition reaction in that the reducing agent bonds to the electrophilic carbon of the molecule that is being reduced. As you can imagine, the reduction of compounds where there are no polar bonds, such as alkenes and alkynes, is a bit more difficult and requires extremely strong reducing agents and severe reaction conditions, and often a catalyst is used for the reduction of the molecules as shown below.
Appropriate reducing agents and reaction conditions that are suited for the reduction of these types of molecules will be covered later in this chapter. In first sections of this chapter, we will examine the various reducing agents that are commonly used in organic chemistry and also discuss the mechanism for the reduction of different functional groups.