Question #9fa33

This reaction is the hydrogenation of an alkyne.

Hydrogenation is an addition reaction, in which one of the `"H"` atoms in `"H"_2` adds to one end of an unsaturated carbon atom, and the second `"H"` atom adds to the other end of the π bond.

The addition requires a `"Pt"`, `"Pd"`, or `"Ni"` catalyst.

With an alkyne such as propyne, we can think of the reaction as occurring in two steps.

(a) 1 mol of `"H"_2` adds across one of the π bonds in propyne.

`underbrace("CH"_3"C≡CH")_color(red)("propyne") + "H"_2 stackrelcolor(blue)("Pt"color(white)(X))(→) underbrace("CH"_3"CH=CH"_2)_color(red)("propene")`

(b) Another mole of `"H"_2` adds across the π bond in the newly-formed propene.

`underbrace("CH"_3"CH=CH"_2)_color(red)("propene") + "H"_2 stackrelcolor(blue)("Pt"color(white)(X))(→) underbrace("CH"_3"CH"_2"CH"_3)_color(red)("propane")`

The overall reaction is

`underbrace("CH"_3"C≡CH")_color(red)("propyne") + "2H"_2 stackrelcolor(blue)("Pt"color(white)(X))(→) underbrace("CH"_3"CH"_2"CH"_3)_color(red)("propane")`

I think the questioner could have given more information, as the hydrogenation requires a catalyst.

From the question, with no specification of the catalyst, I can see two different reactions, both giving syn addition of `"H"_2`, but one of them stops at the alkene.

In general:

Lindlar's Catalyst is a "poisoned" variant on a palladium catalyst. It reduces alkynes to cis-alkenes, but doesn't reduce alkenes.