Alkenes II

Overview:

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.