Question

One mole of each of the following alkenes is catalytically hydrogenated. The quantity of heat evolved will be the lowest in the case of
A) 1-butene
B) Trans-2-butene
C) Cis-2-butene
D) 1,3-butadiene

Answer 1

Hint: The hydrogenation reaction defines the reaction of molecular hydrogen with an alkene in presence of catalysts like nickel, palladium, etc. Here, we have to compare the heat released in the hydrogenation of the given alkenes.

Complete Step by Step Answer:
Let's understand what the heat of hydrogenation is. This is the quantity of heat that evolved in a hydrogenation reaction. The stability of the alkenes has an impact on the release of heat. The more the alkene's stability, the less the amount of heat released.

Now, look at the options. Here, we have to find symmetrical molecules because they are highly stable.
1-butene is not a symmetrical alkene. So, it has a high heat of hydrogenation. So, option A is wrong.
1,3-butadiene is also not a symmetrical alkene. So, option D is also wrong.

Now, the remaining alkenes are symmetrical alkenes. They are, trans-2-butene and cis-2-butene.


Image: cis-2-butene and trans-2-butene

As we find that, both these alkenes are symmetrical. Now, we have to compare the heat released in the hydrogenation of these two alkenes. The stability of trans-2-butene is more than cis-2-butene because, in the former, the methyl groups are at the distant positions but in the latter, the methyl groups are in the crowded position that causes steric repulsion. So, the heat of hydrogenation is the least in the case of trans-2-butene.
Hence, option B is right.

Note: It is to be noted that, the direct conversion of an alkene to alkane happens by the addition of hydrogen in presence of catalysts like nickel. The hydrogenation of alkynes can be done by using Lindlar's catalyst.