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

Reduction Reactions in Organic Chemistry
10.2 Reducing Agents of Organic Chemistry

Our next task is to identify compounds that are potential reducing agents; that is, agents that can readily supply electrons to another compound. In organic chemistry, electrons are typically delivered via a specific atom of the reducing agent, and as you can imagine, such an atom is not very electronegative. In fact, some of the best reducing agents contain very electropositive atoms, such as hydrogen or carbon atoms. In a reduction reaction, electrons are delivered from these very electropositive atoms to the molecule being reduced. If a hydrogen atom has an unshared pair of electrons, it is called a hydride anion, and the counter cation for hydride anions are typically metal cations, such as lithium, sodium, or potassium. The same is true for carbon atom with a pair of electrons, these carbon anions are typically bonded to metal cations, and as a result, they are referred to as organometallic compounds. It is not surprising that since reducing agents contain at least a pair of electrons that they are also bases. In fact, the reducing agents that will be discussed in this chapter are some of the strongest bases of organic chemistry. In this section, we will discuss each type of hydrogen and carbon reducing agents that will be encountered throughout our course of organic chemistry.

10.2.1 Metal Hydrides

Metal hydrides are extremely strong reducing agents, and they deliver the electrons via the hydride anion to form a new covalent bond to the molecule being reduced. One of the strongest metal hydrides reducing agents as mentioned above is a hydrogen atom that has a pair of electrons and has a counter cation, such as sodium or potassium. Another type contains the hydride anion bonded to aluminum or boron. Examples of metal hydrides that are typically used in organic chemistry as reducing agents are shown below.

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Notice that these compounds are ionic, and the actual reducing hydride anion is bonded to an electropositive atom: sodium, potassium, aluminum, and boron, respectively. For lithium aluminum hydride and sodium borohydride, the central atoms in the neutral molecules typically have three bonds instead of four bonds as shown for the compounds above. As a result, lithium aluminum hydride and sodium borohydride react readily delivering a hydride anion (H:) to molecules to be reduced resulting in the formation of a trivalent aluminum and boron products, along with the reduced organic product. Reactions in which these reducing agents are typically used are carried out in non-protic solvents, such diethyl ether or tetrahydrofuran (THF) and not in protic solvents, such as water or alcohols. You will recall that protic solvents have an electrophile in the form of a potential proton (H+), which will react violently with the hydride anion (an extremely strong base) to produce hydrogen (H2) gas. We will use these reducing agents frequently throughout our course in organic chemistry.

10.2.2 Organometallic Compounds

The next set of reducing agents that we will examine are those that contain carbon, and some common organometallic compounds are shown below.

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The first is a carboanion, which is bonded to the electropositive lithium atom. Since lithium is very electropositive, you can imagine that the carbon—lithium bond is very polar, in fact almost ionic, and as a result, the carboanion is very reactive making organolithium compounds strong reducing agents. Organolithium reducing agents are readily made by the reaction of an alkyl halide with lithium, as shown in Reaction (10-2).

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The other reducing agent is a carboanion bonded to magnesium, which is not as electropositive as lithium, but these compounds are still very reactive in delivering electrons, making organomagnesium compounds very reactive reducing agents. François Auguste Victor Grignard, who was born in Cherbourg, France, in 1871, discovered these types of compounds and in 1912 shared the Nobel Prize in Chemistry for this discovery. As a result, these types of reducing agents are often referred to as Grignard reagents. They are readily synthesized by the reaction of an alkyl halide with magnesium metal in a non-protic solvent, typically diethyl ether, an example for the synthesis of a Grignard reagent is shown in Reaction (10-3).

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The last type of organometallic reagent shown is the organocuperate (R2CuLi). As you will see from the periodic table, copper is not as electropositive as either lithium or magnesium, and as a result, the carboanion bond to the copper is polar, but not as polar as that of the other organometallic compounds discussed thus far. As a result, organocuperates are reducing agents, but not as strong as organolithium or the Grignard reagent. Organocuperates are readily made from organolithium compounds, as shown in Reaction (10-4).

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As you can imagine, over the years, a wide variety of organometallic compounds, such as organocadmium (R2Cd), have been developed and used as effective reducing reagents in organic chemistry.

Problem 10.1

Starting with 1-bromopentane, show how to synthesize the following organometallic reagents, include appropriate solvents in your synthesis.

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Another carboanion that we will encounter is the acetylide anion, and it can be synthesized as shown in Reaction (10-5)

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You will recall from Table 7.1 that the hydrogen that is bonded to a carbon of a triple bond is acidic, pKa ~ 25. As a result, a strong base, such as NaNH2, can abstract that proton creating the conjugate acetylide anion base. Note that NH2 anion is a strong base since its conjugate acid is a very weak acid, pKa = 38.

10.2.3 Dissolving Metals

You will recall from your general chemistry course that the elements to the extreme left of the periodic table are very reactive primarily because they want to achieve the electronic configuration of the nearest noble gas, which can be accomplished by these elements giving up electrons. As a result, these elements are extremely good reducing agents. The most reactive of these elements are the ones at the top left of the periodic table, K and Na. The loss of an electron by sodium atom to produce sodium cation is shown in Reaction (10-6).

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These elements in an appropriate solvent will act as very strong reducing agents as we will see in the reactions to be discussed in the next sections.

10.2.4 Hydrogen in the Presence of a Catalyst

Hydrogen is fairly unreactive and is typically not considered to be a strong reducing agent, but in the presence of a catalyst, such as platinum or palladium and under extreme conditions such as elevated temperatures and pressures, reduction using hydrogen gas can take place. An example showing the use of hydrogen to reduce a molecule that has a carbon—oxygen double bond is shown in Reaction (10-7). A wider range of these types of reactions will be covered in the later part of the chapter.

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