Pictures of the Day CH320M328M

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.

Carbocation hybridization is sp2, with an empty 2p orbital (as shown on the right). Thus, carbocations are trigonal planar, a fact that is important because it means carbocations can be attacked by nucleophiles from the “top” or the “bottom” with equal probability, leading to enantiomer products if a new stereocenter is created. The nucleophiles start reacting with carbocations by placing electron density (i.e. a lone pair of electrons) into the empty 2p orbital. Carbocations are extremely reactive because they possess both an unfilled valence shell and a full positive charge. Thus, you will only encounter them as relatively high energy reaction intermediates during the mechanisms of reactions.

Carbocations are stabilized by adjacent alkyl groups, so more highly substituted carbocations are more stable. Alkyl groups stabilize carbocations by a combination of an inductive effect (positively-charged carbon is electronegative, i.e. it wants electron density, so it withdraws some of the electron density of an alkyl group through sigma bonds) and hyperconjugation in which the empty 2p orbital partially overlaps in space with the C-H sigma bonds of the adjacent alkyl groups. In both cases, some electron density is placed on the positively-charged carbon atom from the alkyl groups, leading to a greater distribution of the positive charge around the molecule. The more alkyl groups, the more highly distributed is the positive charge. The more highly distributed the positive charge, the more stable the carbocation. You can see the dramatic differences in charge distribution quantitatively on the bottom row in which carbocations with more alkyl groups attached have less intense blue color on the positively-charged carbon atom, indicating more delocalization of positive charge and thus greater stability. Keep in mind that even tertiary (3°) carbocations are still carbocations and extremely reactive with even weak nucleophiles, only surviving for exceedingly short amounts of time during reaction mechanisms.