Pictures of the Day CH320M328M

The key to understanding alkene chemistry is keeping in mind the special properties of the pi bond (the pi bonding orbital is shown on the left). The electrons in the pi bond are above and below the plane of the bond, so rotating the bond would break it. Thus, as opposed to sigma bonds, pi bonds cannot rotate. This makes possible cis and trans (E and Z) stereoisomers. In addition, the electron density of the pi bond carries with it negative charge, meaning that there is a partial negative charge (red color) above and below the bond. This partial negative charge can interact with molecules having areas of partial or full positive charge. This scenario, in fact, describes almost all of the reactions of pi bonds we will see next chapter.

Most reaction mechanisms we will encounter involve the four common mechanistic elements introduced in class. When drawing mechanisms, always use arrows to indicate the flow of electron density from the electron source, to the electron sink atom. Try to predict which of the mechanistic elements are going to occur under the reaction conditions. In this way you will be learning and understanding the mechanisms (this is good) not simply trying to memorize them (this is bad). If you understand them, you will see how most mechanisms are related, and you will only have to learn a few things before you can successfully write down correct mechanisms (this is good).

For example, the addition of H-Cl can best be understood as "add a proton" mechanistic element followed by "make a bond" between a nucleophile (Cl-) and an electrophile (the carbocation intermediate) . In the first step, electron density of the pi bond attacks the electrophilic hydrogen atom of H-Cl to give a carbocation intermediate and the chloride anion. Note how in the above reaction the more stable of the possible carbocations is produced predominantly, namely the secondary (2°) carbocation (thus explaining Markovinikov’s rule). In the next step, the nucleophilic chloride anion attacks the carbocation electrophile to give the final product. The nucleophile attack can come from the top or the bottom of the trigonal planar (sp2 hybridized) carbocation with equal probability, no matter which face of the alkene the H atom added to, so there is no stereochemical preference of product produced. Click here to see a movie of a related reaction, the addition reaction of H-Br with an alkene.