10.1: Hydrogenation

Let’s take a step back again and talk about the electrophilic addition reaction. Alkenes can undergo electrophilic addition reactions to give substituted alkanes such as alcohols, alkyl halides. In the mechanism of this reaction, the $π_{C-C}$ bond is the HOMO, which reacts with the $σ^{*}_{H-X}$ to become protonated and generate a carbocationic intermediate that is the most stable. We’ve said that this happens because unsymmetrical alkenes have $π_{C-C}$ wavefunctions that are polarized due to imbalanced $σ$-donation. This makes the carbon that is less highly substituted more negatively charged ($δ^{–}$) and the more highly substituted alkene carbon more positively charged ($δ^{+}$). Thus, the proton becomes attached to the less highly substituted carbon, generating the more stable carbocation. We call this Markovnikov regioselectivity. However, if the less stable carbocation is formed, we call it anti-Markovnikov regioselectivity. This will be important moving forward, especially when trying to synthesize complex molecules.

You might imagine that there are a variety of other LUMOs that the alkene can react with (not just $σ^{*}_{H-X}$). In this section, we will discuss other reactions of alkenes that use the $π_{C-C}$ as the HOMO. These reactions are (for the most part) concerted mechanisms, so there is no carbocation formed. But you must still pay special attention to the regiochemical (Markovnikov vs. anti-Markovnikov) and stereochemical (racemization, inversion, retention, but also cis/trans isomerism) outcomes.

Hydrogenation of alkenes results in the formation of an alkane directly from an alkene using hydrogen gas and a catalyst. It is a formal addition of H₂ across the double bond. A catalyst (Pd/C, PtO₂, Ni⁰) is needed to weaken the $σ_{H-H}$ bond, and the reaction is normally heterogeneous (occurs on a surface). The reaction is stereospecific since hydrogen is added in a concerted fashion from the same face of the alkene, without any opportunity for bond rotation. This is known as a syn addition.

In the example above, we also get a stereoselective reaction since the bridgehead carbon is hindering attack of one face of the $π_{C-C}$ bond. The other side is not blocked, so it can interact with the activated catalyst.