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

Reduction Reactions in Organic Chemistry
10.6 Reduction of Aromatic Compounds, Alkynes, and Alkenes

10.6.1 Reduction Using Dissolving Metals

We will be studying the chemistry of some very stable molecules in later chapters, these molecules are known as aromatic compounds, of which benzene is the most common, its structure is shown below. As you will see, there are six pi (π) electrons in conjugation, which makes it very difficult to add more electrons through a reduction process. These are not impossible reactions, but can be accomplished in the presence of very strong reducing agents, such as reducing metals, as shown in Reaction (10-55). For this reaction, ammonia is the solvent.

(10-55)Image

This type of reaction is often referred to as the Birch reduction. In the first step of the mechanism, the very strong reducing agent, sodium metal, supplies one electron to the conjugated system to form a radical anion, which then abstracts a proton from the solvent, ammonia as shown in Reaction (10-56) to form a radical.

(10-56)Image

Sodium then delivers another electron to form an anion, which abstracts another proton from the solvent, ammonia to form the final reduced product as shown in Reaction (10-57).

(10-57)Image

The same reaction condition as given above can be used to reduce alkynes to alkenes, specifically trans-alkenes, which is given in Reaction (10-58).

(10-58)Image

The general reaction mechanism for Reaction (10-56) is similar to the reduction of benzene above, in that the reducing metal supplies an electron to the pi (π) system of the triple bond in the initial step to form a trans radical anion, which abstracts a proton from the solvent, ammonia, as shown in Reaction (10-59). As you will recall from the valence shell electron pair repulsion(VSEPR) theory, electrons repel each other, and as a result, the three electrons of the radical anion will be in a trans arrangement as shown in Reaction (10-59). This concept is the key to the formation of the trans-product.

(10-59)Image

In the second step of the mechanism, another electron is delivered to the radical to form an anion, which then abstracts a proton from the solvent, as shown in Reaction (10-60).

(10-60)Image

10.6.2 Reduction Using Catalytic Hydrogenation

Another important reduction reaction involving the carbon—carbon multiple bonds is one where a hydrogen molecule adds across the double bond of alkenes in the presence of a catalyst. As we have demonstrated from the reductions of carbonyl compounds and imines with hydrogen, hydrogen is a very stable and unreactive molecule, and as a result, a catalyst and high temperatures and pressures are necessary since the activation barrier for such reactions is very high. These reduction reactions transform unsaturated compounds into saturated compounds and are widely used in industry. Two examples are shown in Reactions (10-61) and (10-62).

(10-61)Image

(10-62)Image

An important observation that should be made for these reactions is that the hydrogen adds to the same side of the double bonds, cis-addition. Figure 10.1 shows how a catalyst is used to reduce the energy of activation required to break the hydrogen—hydrogen bond, which has to take place in order for the addition reaction to occur.

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Figure 10.1 Steps for the cis hydrogenation of an alkene in the presence of a catalyst.

Based on the mechanism shown in Figure 10.1, the addition of the hydrogen atoms across the carbon—carbon double is a cis-addition, which means that both hydrogen atoms add to the same side of the double bond. These reactions are described as stereospecific reactions, and Reaction (10-63) is another example of the reduction of an unsymmetric alkene showing the stereochemistry of the products.

(10-63)Image

For the hydrogenation of alkenes, rearrangement is not observed since as shown in Figure 10.1, the reactants are attached to the surface of the catalyst, and as a result, rearrangement does take place.

Problem 10.12

Give the major product that results from hydrogenation of the following alkenes, show stereochemistry where appropriate.

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DID YOU KNOW?

In today’s health conscious society, we often hear of saturated and unsaturated fats and that unsaturated fats are better for us. Unsaturated fats are classified as unsaturated due to the presence of double bonds. Unsaturated fats can be converted to saturated fats by a hydrogenation process. The example of hydrogenation of oleic acid is shown below.

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Fats can be categorized into the following categories: saturated (e.g. butter, lard, coconut oil); monounsaturated (e.g. olive or canola oils); and polyunsaturated (e.g. omega-6 oils, including sunflower and safflower oil, and omega-3 oils, including fish and flaxseed oils). Hydrogenated fats are unnatural fats, and they have been considered to be unhealthy fats. Hydrogenation and partial hydrogenation of unsaturated fats are carried out on unsaturated fats typically to make them solids at room temperature and they typically have a longer shelf life, compared to unsaturated fats. They are marketed as butters and margarines. Any polyunsaturated oil can be hydrogenated, but the more common ones that are hydrogenated include cottonseed, palm, soy, and corn oils.

ImageDifferent oils, butter, and margarines

Now that we have seen that it is possible to add hydrogen in a cis-manner across a double bond, the question that comes to mind is if it is possible to add just one mole of hydrogen across a triple bond to get a cis-alkene? In the presence of a catalyst, such as Pd, hydrogen will add across the triple bond to first give the alkene and eventually the alkane in the presence of excess hydrogen, as shown in Reaction (10-64).

(10-64)Image

By using specialized catalysts, it is possible to add just one mole of hydrogen across the triple bond of alkynes to form the corresponding alkene. For these reactions, the addition of hydrogen takes place on the same side of the double bond, cis-addition.

(10-65)Image

The mechanism is similar to that shown for the addition of hydrogen to alkenes, which was shown earlier, except the metal surface in this case is deactivated allowing only one mole of hydrogen to add.

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One deactivated catalyst that is used to accomplish this task is called the Lindlar’s catalyst, which is Pd/BaSO4 in the presence of quinone.

Problem 10.13

Complete the following reactions by supplying either the major organic product or the reaction conditions (appropriate catalyst) to give the products shown.

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In summary, the addition of hydrogen across a triple bond involving Lindlar’s catalyst is a stereospecific reaction in that it gives the syn-product (syn-addition) to form (Z) alkenes. The reaction using dissolving metal is also stereospecific in that it gives the trans product (anti-addition) to form (E) alkenes.