Series: Organic Chemistry: Alkenes

This two-lesson series on Alkenes is made up of two lessons: Alkenes I and Alkenes II. Alkenes I is the first of a two-part study of the compounds containing a double bond. It addresses the structure of alkenes, the degrees of unsaturation, nomenclature, the physical properties of these compounds, the nature of the carbocation, and alkene preparations.

A study of the structure of alkenes leads to a study of degrees of unsaturation, the way in which those degrees may be determined, and the role played by elements other than carbon and hydrogen. Degrees of unsaturation are important in analysis throughout the course.
Nomenclature of alkenes is addressed using the functional group as the important characteristic. However, the limitations of cis and trans notations are quickly realized and the Z (zusammen) and E (entgegen) terms are introduced, explained, and utilized. The Cahn-Ingold-Prelog rules used to determine the order of groups in stereochemistry are reviewed and used here to determine Z and E nomenclature.

The physical properties of alkenes are considered and compared to alkanes. The impact of the double bond, as well as cis and trans orientations on physical properties is addressed.

The first preparation, dehydration of alcohols, is used to introduce the concept of the carbocation. This reactive structure is studied in depth, and the role it plays in how a reaction occurs is presented clearly and vividly using a series of animations. The manner in which structural geometry shifts in the course of a reaction is readily demonstrated using these animations, a feature virtually impossible to achieve in a textbook. The halide shift and alkyl shift are also clearly described. With these most appropriate visuals, it becomes easy to understand the order of stabilities of the carbocations produced and why the Saytzeff rule works as it does.

The preparations of alkenes addressed are dehydration of alcohols, dehydrohalogenation of alkyl halides, dehalogenation of vicinal dihalides, the Corey-House, and reduction of alkynes. The carbocation mechanism for applicable preparations is discussed and animated where helpful so the student can be thoroughly familiar with the application of this important mechanism. The versatile and useful Corey-House reaction with its Gilman reagent is described at length and the few limitations noted.

Students frequently become confused when trying to learn why hydrogenation of alkynes results in cis with hydrogen and palladium coated barium sulfate, as opposed to trans addition. However, when the student sees the animated reaction, it becomes immediately clear why the cis structure must predominate. It is techniques such as this that makes the Chemistry Professor so successful in teaching.

Alkenes II continues the study of the hydrocarbons containing a double bond. Topics addressed in this unit include structure and bonding, reactions at the double bond and near the double bond, polymers and polymerization, stereochemical aspects of alkenes, and chemical analysis.

The structure and nature of alkene bonding is covered in greater depth here than in Alkenes I in order to introduce addition reactions that occur at the point of unsaturation in a manner that is clear and logical. These addition reactions, reactions that involve attack by a good electrophile followed by a nucleophile, are diagrammed in general using highly effective animations. After the general diagram of the reaction is discussed, specific reactions are introduced in such a manner as to capitalize on the general explanation.

The reactions studied are hydrogenation, hydration, hydrohalogenation, hydrobromination with peroxides, halogenation, hydroxylation, oxidative cleavage, halohydrin formation, carbene insertion, and peroxyacid oxidation. Over four hundred screens are used in explanation of these reactions. For example, the energy path generally accepted in heterogeneous hydrogenation involving an alkene and hydrogen gas in the presence of a solid catalyst is examined using potential energy diagrams. Hydration via oxymercuration followed by demercuration using the cyclic mercurinium ion is clearly diagrammed to explain the lack of rearrangement, as opposed to the rearrangement expected and realized when hydration with sulfuric acid occurs. The anti-Markovnikov hydration using hydroboration/oxidation adds yet another interesting aspect. Hydroxylation with permanganate, carbine insertion using the Simmons-Smith reagent, and oxidative cleavage are a few more of the reactions explained at lengh.

A small section introduces the topic of polymers and polymerization and the methods by which polymers may be formed. Free-radical, cationic, and anionic induced polymerization are addressed, with emphasis placed on free-radical and cationic polymerization. Anion mechanisms are addressed in a separate unit later in the course. The impact of structure on polymer properties is briefly mentioned.

Animations are used to show the stereospecific products formed when anti addition of halogens to alkenes to form both the cis and trans structures occurs. Fisher projection diagrams are used, also.

Simple test tube tests are introduced as a way to quickly suggest the presence of a double bond. The common tests producing a nice color change such as bromination in the dark, the addition of basic permanganate, and the addition of acid permanganate demonstrate the way in which the reactions studied may have elegantly simple applications. It is considered important for the student to realize that reactions play a role in ways other than synthesis.

This series is excerpted from a standard college organic chemistry course. The full course, Organic Chemistry, is available from MindBites or in DVD format from the Chemistry Professor

Sandra Y. Etheridge, Ph.D., is a Professor Emerita. She received her Ph.D. in Educational Leadership with emphasis in curriculum development in Chemistry from Florida State University in 1982, MAT in Teaching Chemistry from Duke University in 1963, and a BS in Chemistry at Limestone College in 1962. In addition to her education, she studied Nuclear Magnetic Resonance and InfraRed Spectroscopy at the University of West Florida.

Professional Experience:

1983-2009 Developed distance education video, etc. courses in chemistry.
1993-2003 Chair Division of Natural Sciences, Gulf Coast State College
1984-2003 Professor of Chemistry, Gulf Coast State College
1967-1983 Assistant Professor of...

• Organic Chemistry Lesson: Alkenes 1
• Organic Chemistry Lesson: Alkenes 2

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