Catalytic Hydrogenation of Alkenes II (2024)

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Alkene hydrogenation is the syn-addition of hydrogen to an alkene, saturating the bond. The alkene reacts with hydrogen gas in the presence of a metal catalyst which allows the reaction to occur quickly. The energy released in this process, called the heat of hydrogenation, indicates the relative stabily of the double bond in the molecule (see Catalytic Hydrogenation).

Introduction

The reaction begins with H₂ gas and an alkene (a carbon-carbon double bond). The pi bond in the alkene acts as a nucleophile; the two electrons in it form a sigma bond with one of the hydrogen atoms in H₂. With the pi bond broken, the other carbon (the one that did not newly receive a hydrogen) is left with a positive formal charge. This is the carbocation intermediate. The remaining (unreacted) hydrogen is now a hydride anion, as it was left with two electrons previously in the H-H sigma bond. Next, the electrons of the negatively charged hydride ion form a bond with the positively charged carbon. This reaction is exothermic. It will occur, but it is very slow without a catalyst.

The Catalyst

A catalyst increases the reaction rate by lowering the activation energy of the reaction. Although the catalyst is not consumed in the reaction, it is required to accelerate the reaction sufficiently to be observed in a reasonable amount of time. Catalysts commonly used in alkene hydrogenation are: platinum, palladium, and nickel. The metal catalyst acts as a surface on which the reaction takes place. This increases the rate by putting the reactants in close proximity to each other, facilitating interactions between them. With this catalyst present, the sigma bond of H₂ breaks, and the two hydrogen atoms instead bind to the metal (see #2 in the figure below). The \(\pi\) bond of the alkene weakens as it also interacts with the metal (see #3 below).

Since both the reactants are bound to the metal catalyst, the hydrogen atoms can easily add, one at a time, to the previously double-bonded carbons (see #4 and #5 below). The position of both of the reactants bound to the catalyst makes it so the hydrogen atoms are only exposed to one side of the alkene. This explains why the hydrogen atoms add to same side of the molecule, called syn-addition.

Catalytic Hydrogenation of Alkenes II (1)

Hydrogenation takes place in the presence of a metal catalyst.

Note: The catalyst remains intact and unchanged throughout the reaction.

Heats of Hydrogenation

The stability of alkene can be determined by measuring the amount of energy associated with the hydrogenation of the molecule. Since the double bond is breaking in this reaction, the energy released in hydrogenation is proportional to the energy in the double bond of the molecule. This is a useful tool because heats of hydrogenation can be measured very accurately. The \(\Delta H^o\) is usually around -30 kcal/mol for alkenes. Stability is simply a measure of energy. Lower energy molecules are more stable than higher energy molecules. More substituted alkenes are more stable than less substituted ones due to hyperconjugation. They have a lower heat of hydrogenation. The following illustrates stability of alkenes with various substituents:

In disubstituted alkenes, trans isomers are more stable than cis isomers due to steric hindrance. Also, internal alkenes are more stable than terminal ones. See the following isomers of butene:

Catalytic Hydrogenation of Alkenes II (4)

Trans-2-butene is the most stable because it has the lowest heat of hydrogenation.

Note: In cycloalkenes smaller than cyclooctene, the cis isomers are more stable than the trans as a result of ring strain.

Outside Links

Helpful Information: http://www.wou.edu/las/physci/ch334/...ure/lect16.htm
Hydrogenation Wikipedia page: http://en.Wikipedia.org/wiki/Hydrogenation
More professional animation: http://www.jbpub.com/organic-online/movies/cathyd.htm

References

Fox, Marye Anne, and James K. Whitesell. Organic Chemistry. 3rd ed. Sudbury, MA: Janes and Bartlett Publishers, 2004.
Hanson, James R. Functional Group Chemistry. Bristol, UK: The Royal Society of Chemistry, 2001.
Streitwieser, Andrew Jr., and Clayton H. Heathco*ck. Introduction to Organic Chemistry. 2nd ed. New York, NY: Macmillan Publishing Co., Inc., 1981.
Vollhardt, Peter C., and Neil E. Schore. Organic Chemistry: Structure and Function. 5th ed. New York, NY: W.H. Freeman and Company, 2007.
Zlatkis, Albert, Eberhard Breitmaier, and Gunther Jung. A Concise Introduction to Organic Chemistry. New York: McGraw-Hill Book Company, 1973.

Problems and Review Questions

1. Bromobutene reacts with hydrogen gas in the presence of a platinum catalyst. What is the name of the product?
2. Cyclohexene reacts with hydrogen gas in the presence of a palladium catalyst. What is the name of the product?
3. What is the stereochemistry of an alkene hydrogenation reaction?
4. When looking at their heats of hydrogenation, is the cis or the trans isomer generally more stable?
5. 2-chloro-4-ethyl-3methylcyclohexene reacts with hydrogen gas in the presence of a platinum catalyst. What is the name of the product?

Answers

1. Bromobutane
2. Cyclohexane
3. Syn-addition
4. Trans
5. 2-chloro-4-ethyl-3methylcyclohexane

Contributors

Anna Manis (2009)

Alkene hydrogenation is a fascinating chemical process that involves adding hydrogen to an alkene, effectively saturating the double bond. This reaction occurs rapidly when an alkene interacts with hydrogen gas in the presence of a specific metal catalyst, a crucial detail that enables the reaction to take place efficiently.

The energy released during this process, termed the heat of hydrogenation, serves as a marker for gauging the relative stability of the double bond within the molecule. This stability assessment relies on intricate chemical mechanisms, such as the formation of carbocation intermediates and the role of catalysts in accelerating reactions without being consumed themselves.

I can delve into the process's nitty-gritty, starting with the pi bond within the alkene behaving as a nucleophile, allowing two electrons to form a sigma bond with one of the hydrogen atoms from H2. This initiates the breaking of the pi bond, creating a positive formal charge on the carbon that remains devoid of a newly added hydrogen. This intermediate stage, known as the carbocation, sets the stage for subsequent reactions.

The interaction of the remaining hydrogen, now a hydride anion, with the positively charged carbon results in an exothermic reaction. However, without a catalyst, this reaction would proceed at an excruciatingly slow pace. Catalysts like platinum, palladium, or nickel play a pivotal role in lowering the activation energy, thereby accelerating the reaction rate significantly. These metals act as surfaces upon which the reaction occurs, promoting close proximity between reactants and facilitating their interaction.

With the catalyst in play, the sigma bond of H2 breaks, and the hydrogen atoms bind to the metal, while the alkene's pi bond weakens as it interacts with the catalyst. This binding arrangement dictates the addition of hydrogen atoms to the carbons previously forming the double bond, allowing for syn-addition, wherein hydrogen atoms add to the same side of the molecule.

The concept of heats of hydrogenation proves indispensable in determining an alkene's stability. By measuring the energy released during hydrogenation, one can gauge the relative stability of different alkenes. Notably, more substituted alkenes tend to be more stable than less substituted ones due to factors like hyperconjugation.

Additionally, stereochemistry comes into play, distinguishing between trans and cis isomers in disubstituted alkenes. The trans isomers often exhibit greater stability than cis counterparts, primarily due to steric hindrance.

For cycloalkenes smaller than cyclooctene, the cis isomers tend to be more stable than the trans due to ring strain. These complex interactions and stability considerations illuminate the intricate nature of alkene hydrogenation.

The information provided in the article encompasses various facets of organic chemistry, ranging from fundamental chemical reactions to stereochemistry and stability assessments of different alkene structures. If there are specific areas you'd like to explore further or if you have any questions about these concepts, feel free to ask!