# Rate of catalytic hydrogenation of alkenes

What does the rate of catalytic hydrogenation of alkenes depend upon? What'd be the increasing order of rate towards catalytic hydrogenation of the following alkenes, A (2-methylpropene), B (cis-but-2-ene), and C (trans-but-2-ene)?

I suppose this has something to do with sterics and orientation - as catalytic hydrogenation is a surface phenomenon (occurs on surface of metal catalyst with adsorbed hydrogen), but I'm not sure about the above order in particular (I don't think a clear comparison can be made without looking at data, as the alkenes are quite similar), and the factors on which rate of hydrogenation of hydrocarbons depends in general.

• – Gaurang Tandon Jul 12 '18 at 14:09
• Stability (thermodynamic) does not dictate kinetics of a reaction. – strawberry-sunshine Jul 14 '18 at 7:03

Catalytic hydrogenation is more or less a surface phenomenon so sterics play the deciding role in the rate of hydrogenation.

From Wikipedia:

Here's a newman projection of the three Alkenes.

A = cis-dimethylbutene, B = trans-dimethylbutene, C = 2-methylpropene

From here it is easy to see that the cis-dimethylbutene will have the least hindered approach towards the surface of the catalyst as both the methyl groups can be made to face away from the surface.

The rates of hydrogenation in the case of trans-dimethylbutene and 2-methylpropene will be decided by which one of them is more strained.

2-methylpropene is more strained than trans-dimethylbutene.

So, we can now establish the order.

$$\text{A > C > B}$$

References

• This rationale doesn't seem entirely correct.. for proper syn delivery of both the hydrogens,one entire face of the alkene has to be easily adsorbed on the catalyst surface(like in B) .. even if it is easier for one side of the alkene to diffuse towards the catalyst surface(like in A)... The other side will have to come to the surface as well to receive the other H. So it seems that B will be more susceptible to complete hydrogenation than A or C – Yusuf Hasan Jun 5 '19 at 2:38
• Yep,I was right. This answer is not completely correct. See this – Yusuf Hasan Jun 5 '19 at 2:49

The predominantly accepted Horiuti-Polanyi mechanism states that the rate depends on the three steps: (1) alkene adsorption on the surface (2) hydrogen migration to the β-carbon of the alkene with formation of a M σ-bond and (3) reductive elimination. There is a good discussion in J. Chem. Educ., 2013, 90 (5), pp 613–619.

You can infer the rate differences in reactivity (step 2 and 3) from first principles and experimental data on homogeneous systems. In hydrogenation experiments reported in Faraday Discuss. Chem. Soc. 1968, (46), p. 60 with homogeneous Wilkinson’s catalyst, the relative rates of hydrogenation normalized against 1-octene are 0.17 for trans-4-octene; 0.54 for cis-4-octene; and 0.69 for 2-methyl-1-butene. For heterogeneous hydrogenation, The transport phenomena associated with step 1 limit predictive ability for overall rates.