Organic Chemistry: Concepts and Applications - Headley Allan D. 2020
An Overview of the Reactions of Organic Chemistry
6.4 Reduction Reactions
Reduction is typically defined as the supply of electrons to a molecule and also the loss of oxygen from a molecule. The former definition is most often seen in organic chemistry. Shown below in Reaction (6-12) is a typical reduction reaction from general chemistry in which Zn metal is the reducing agent and supplies two electrons to the Cu2+ ions, the species that is being reduced.
(6-12)
Most of the reducing agents that we will encounter in organic chemistry will not be Zn, but compounds that contain hydrogen or even carbon. As you can imagine, for hydrogen to be a reducing agent, it must be able to supply electrons to the species to be reduced. Organic reducing reagents that have at least one electron pair to donate to the compound being reduced typically include LiAlH4 and NaBH4, these reagents will supply the hydride anion (H:−) to the species to be reduced. The structures of these reducing agents are shown below.
Note that the species being reduced has to be able to accept the electrons. In the case of Reaction (6-12), the copper ion has a positive charge, which means that it can readily accept electrons from the reducing agent. For organic reactions, the species that will be reduced are typically not copper or other metal ions, but organic compounds that typically have polar functional groups, such as carbonyls and nitriles. Such groups are readily reduced since the reducing agent can supply the electrons to the partially positive region of the polar covalent bond; an example of a reduction reaction, in which LiAlH4 is used as the reducing reagent for the reduction of a ketone is shown in Reaction (6-13).
(6-13)
For this reaction, the reducing agent introduces a hydride ion (H:−) to the electropositive carbon of the carbon—oxygen polar double bond to produce the organic salt (lithium alkoxide). Since organic salts, such as the product of the reaction shown above, are rarely used in organic chemistry, such salts are neutralized in an acidic solution to form another organic chemistry functional group, in this case, an alcohol. The neutralization, which is essentially an acid—base reaction, is shown in Reaction (6-14). This type of neutralization reaction is sometimes referred to as acidic workup.
(6-14)
For most organic reactions where there is a sequence of reactions to produce a specific organic product, such as the ones in Reactions (6-13) and (6-14) to produce an alcohol from a ketone using a reducing agent, the reactions are combined in a specific manner. The main organic reactant is written on the left of the reaction, and other reagents, catalysts, solvents, and reaction conditions, such as temperature, are written on the arrow. Also, different steps of the reaction sequence are shown using numbers on the arrow. Thus, an alternate way of writing Reactions (6-13) and (6-14) is to use the format shown in Reaction (6-15).
(6-15)
Note that by using this format, the reaction is not balanced as is typically done in general chemistry and that only the main organic product is typically shown and other reagents, such as the inorganic lithium chloride salt, are not included. Another method that is often used to show the different reactions in a sequence of reactions is shown below for the reactions of Reactions (6-13) and (6-14), in which the two steps are shown adjacent to each other as shown in Reaction (6-16).
(6-16)
Based on the concepts discussed above, it is possible to predict the reduction products if given the reactant and reducing agent, as given in Reaction (6-17).
(6-17)
This reaction involves an imine organic reactant reacting with the reducing agent LiAlH4, which delivers a hydride ion (H:−) to the electropositive carbon of the polar carbon—nitrogen double bond to form a salt (lithium amide). The second reaction is an acidic neutralization reaction of the salt to form, in this case, an amine.
At this point, just about any reduction reaction involving LiAlH4 or NaBH4 can be examined and a prediction made about possible organic products. Problem 6.5 is designed to apply this concept.
Problem 6.5
Give the reduction organic products for the following reactions.
For functional groups that do not have polar covalent bonds as discussed above, such as the alkene and alkyne, reduction is much more difficult, and as a result, a catalyst is typically required. For the reduction (or addition of hydrogen molecule) across a carbon—carbon double bond that is shown in Reaction (6-18), a catalyst is required.
(6-18)
Reduction of alkenes using hydrogen and a catalyst is very important in organic chemistry since the products of these reactions are saturated compounds. Another term that is often used for this specific type of reduction reaction is hydrogenation since a hydrogen molecule is added across a carbon—carbon double bond. Based on the outcome of the reduction of alkenes with hydrogen in the presence of a catalyst, it should not be difficult to predict the outcome of the reduction of ketones under the same reaction conditions, as shown in Reaction (6-19).
(6-19)
Note that even though the reactant has a polar covalent carbon—carbon double bond, reduction is possible using hydrogen in the presence of a catalyst. In this case, however, the product is an alcohol. Note that the product of this reaction is the same as if LiAlH4 were used as the reducing agent, followed by a neutralization reaction. It is not surprising that the same organic product should result since for both types of reactions, H—H is added across the double bond. These examples give an idea of the versatility of the reactions of organic chemistry and especially the choice of different reagents available to accomplish a particular transformation.
Problem 6.6
Give the reduction products for the following reactions (note the format of writing the second reaction!).
