# Rate of hydrogenation

The rate of hydrogenation is faster for which alkene : cis or trans. I know it depends on steric hindrance on the carbon, but both have steric hindrance. Can someone please help

Are you asking about catalytic hydrogenation or non-catalytic hydrogenation (e.g. diimide)? For the time being, let's assume that your question is about the catalytic process. All the catalyst does is lower the activation energy required to reach the transition state (TS) and the TS will look about the same independent of which isomer you start with. Therefor, ground state stability differences between the two olefins will determine which isomer reacts faster. Since trans-olefins are generally more stable than their corresponding cis isomers, it will take more energy (the difference in stability between the cis and trans isomers) for the more stable (lower energy) trans isomer to reach the TS, and less energy for the cis isomer to reach the TS. Consequently cis isomers react somewhat faster than their trans-counterparts during catalytic hydrogenation. I suspect that the same considerations would apply to non-catalytic processes.

As an aside, remember that in the catalytic process, depending upon catalyst, temperature, solvent, pressure, etc.; many other things can be going on, such as cis-trans isomerization of the starting material, positional isomerization, etc. So if the reaction conditions are not chosen carefully and 1) if cis-trans isomerization is faster than hydrogenation and 2) let's say the cis isomer reacts 100 times faster than the trans; then both isomers could appear to give the same rate of hydrogenation because all hydrogenation would proceed through the cis isomer.

Imagine that the mechanism for the addition of $\ce{H-Hal}$, $\ce{Hal-Hal}$, and $\ce{H-H}$ to an alkene are fundamentally different.

In the first case, subsequent addition of the proton and (then) the halogen is assumed. In the second case, formation of a halonium ion is postulated as the first step, followed by addition of the halide (sort of backside attack to the existing $\ce{C-Hal}$ bond).

In the latter case, the simultaneous addition of both hydrogen atoms to the same face of the alkene is assumed. Isn't it likely under these conditions that a cis alkene, which is less shielded on one side, is faster hydrogenated then?

• But, hydrogens are added from above or below the plane of the double bond, and not from the side of the double bond. So what do you mean by shielded on one side? Why should this make a difference? May 14 '14 at 8:05
• Hydrogenation is generally a catalytic process, so somewhere the explanation also has to involve the binding to the catalyser. It might be different for heterogeneous catalysis than for solution phase catalysis. An other issue might be related to thermodynamics: cis- isomers tend to be less stable than trans- isomers due to steric repulsion. If the transition state is close to the product, the product is the same and thus the cis- isomer might hydrogenate faster.
– PLD
May 14 '14 at 15:31