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Organic Chemistry

8.6 Reduction of Alkenes: Hydrogenation

Organic Chemistry8.6 Reduction of Alkenes: Hydrogenation

8.6 • Reduction of Alkenes: Hydrogenation

Alkenes react with H2 in the presence of a metal catalyst such as palladium or platinum to yield the corresponding saturated alkanes. We describe the result by saying that the double bond has been hydrogenated, or reduced. Note that the word reduction is used somewhat differently in organic chemistry from what you might have learned previously. In general chemistry, a reduction is defined as the gain of one or more electrons by an atom. In organic chemistry, however, a reduction is a reaction that results in a gain of electron density for carbon, caused either by bond formation between carbon and a less electronegative atom—usually hydrogen—or by bond-breaking between carbon and a more electronegative atom—usually oxygen, nitrogen, or a halogen. We’ll explore this topic in more detail in Section 10.8.

ReductionIncreases electron density on carbon by: forming this:C−Hor breaking one of these:C−OC−NC−X ReductionIncreases electron density on carbon by: forming this:C−Hor breaking one of these:C−OC−NC−X

A reaction shows the reduction of an alkene with hydrogen in the presence of catalyst to form an alkane.

Platinum and palladium are the most common laboratory catalysts for alkene hydrogenations. Palladium is normally used as a very fine powder “supported” on an inert material such as charcoal (Pd/C) to maximize surface area. Platinum is normally used as PtO2, a reagent known as Adams’ catalyst after its discoverer, Roger Adams at the University of Illinois.

Catalytic hydrogenation, unlike most other organic reactions, is a heterogeneous process rather than a homogeneous one. That is, the hydrogenation reaction does not occur in a homogeneous solution but instead takes place on the surface of solid catalyst particles. Hydrogenation usually occurs with syn stereochemistry: both hydrogens add to the double bond from the same face.

A reaction shows 1,2-dimethyl-cyclohexene reacting with hydrogen, and platinum dioxide in ethanol solvent to form cis-1,2-dimethyl-cyclohexane (82 percent yield).

As shown in Figure 8.7, hydrogenation begins with adsorption of H2 onto the catalyst surface. Complexation between catalyst and alkene then occurs as a vacant orbital on the metal interacts with the filled alkene π orbital on the alkene. In the final steps, hydrogen is inserted into the double bond and the saturated product diffuses away from the catalyst. The stereochemistry of hydrogenation is syn because both hydrogens add to the double bond from the same catalyst surface.

Figure 8.7 MECHANISM
Mechanism of alkene hydrogenation. The reaction takes place with syn stereochemistry on the surface of insoluble catalyst particles.
Mechanism of catalytic alkene hydrogenation, showing hydrogen and alkene bound to the catalyst, followed by syn addition of hydrogens to the alkene,  and release of the reduced alkane and catalyst.

An interesting feature of catalytic hydrogenation is that the reaction is extremely sensitive to the steric environment around the double bond. As a result, the catalyst usually approaches the more accessible face of an alkene, giving rise to a single product. In α-pinene, for example, one of the methyl groups attached to the four-membered ring hangs over the top face of the double bond and blocks approach of the hydrogenation catalyst from that side. Reduction therefore occurs exclusively from the bottom face to yield the product shown.

A reaction shows alpha-pinene reacting with hydrogen in the presence of palladium on carbon to form the endo-hydrogenated alkane. The structure of the not formed  exo-hydrogenated alkane is also depicted.

Alkenes are much more reactive toward catalytic hydrogenation than most other unsaturated functional groups, and the reaction is therefore quite selective. Other functional groups, such as aldehydes, ketones, esters, and nitriles, often survive alkene hydrogenation conditions unchanged, although reaction with these groups does occur under more vigorous conditions. Note that, particularly in the hydrogenation of methyl 3-phenylpropenoate shown below, the aromatic ring is not reduced by hydrogen and palladium even though it contains apparent double bonds.

Catalytic hydrogenation of cyclohex-2-enone, methyl 3-phenylpropenoate, and cyclohexylideneacetonitrile with hydrogen and palladium on carbon in ethanol provided cyclohexanone, methyl 3-phenylpropenoate, and cyclohexylacetonitrile, respectively.

In addition to its usefulness in the laboratory, catalytic hydrogenation is also important in the food industry, where unsaturated vegetable oils are reduced on a large scale to produce the saturated fats used in margarine and cooking products (Figure 8.8). As we’ll see in Section 27.1, vegetable oils are triesters of glycerol, HOCH2CH(OH)CH2OH, with three long-chain carboxylic acids called fatty acids. The fatty acids are generally polyunsaturated, and their double bonds have cis stereochemistry. Complete hydrogenation yields the corresponding saturated fatty acids, but incomplete hydrogenation often results in partial cis–trans isomerization of a remaining double bond. When eaten and digested, the free trans fatty acids are released, raising blood cholesterol levels and potentially contributing to coronary problems.

Catalytic hydrogenation of a polyunsaturated fatty acid in vegetable oil with two molecules of hydrogen in palladium on carbon to form a saturated and trans-fatty acid in margarine.
Figure 8.8 Catalytic hydrogenation of polyunsaturated fats leads primarily to saturated products, along with a small amount of isomerized trans fats.

Double-bond reductions are very common in biological pathways, although the mechanism is completely different from that of laboratory catalytic hydrogenation over palladium. As with biological hydrations (Section 8.4), biological reductions usually occur in two steps and require that the double bond be adjacent to a carbonyl group. In the first step, the biological reducing agent NADPH (reduced nicotinamide adenine dinucleotide phosphate), adds a hydride ion (H:) to the double bond to give an anion. In the second, the anion is protonated by acid HA, leading to overall addition of H2. An example is the reduction of trans-crotonyl ACP to yield butyryl ACP, a step involved in the biosynthesis of fatty acids (Figure 8.9).

A reaction shows trans-crotonyl A C P reacting with N A D P H to form anion intermediate, which reacts with H A to form butyryl A C P.
Figure 8.9 Reduction of the carbon–carbon double bond in trans-crotonyl ACP, a step in the biosynthesis of fatty acids. One hydrogen is delivered from NADPH as a hydride ion, H:; the other hydrogen is delivered by protonation of the anion intermediate with an acid, HA.
Problem 8-12
What product would you obtain from catalytic hydrogenation of the following alkenes?
The condensed structural formula has a 5-carbon chain. C2 is double bonded to C3 and single bonded to a methyl group.
The structure has a cyclopentene ring. C3 is bonded to two methyl groups.
The structure has a cyclohexene ring with a tert-butyl group bonded to the C3 cyclohexyl carbon. To the right, the equivalent 3D structure is depicted.
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