Alkenes I

Overview:

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.