Question #470b6

1 Answer
Dec 9, 2017

It depends on the nature of the alkyne.


The usual procedure doesn't work with alkynes, but you can modify the conditions.

The modified conditions

You use catalytic amounts of #"OsO"_4# and a co-oxidant like #"KClO"_3# in a tert-butyl alcohol-acetone-water solvent system.

Reaction with internal alkynes

Internal alkynes give α-diketones.

#underbrace("C"_6"H"_5"-C≡C-C"_6"H"_5)_color(red)("diphenylacetylene") stackrelcolor(blue)("OsO"_4, "KClO"_3 color(white)(mm))(→) underbrace("C"_6"H"_5"-C(=O)C(=O)-C"_6"H"_5)_color(red)("benzil")#

Reaction with terminal alkynes

Terminal alkynes give a carboxylic acid with one less carbon atom.

#underbrace("C"_6"H"_5"-C≡C-H")_color(red)("phenylacetylene") stackrelcolor(blue)("OsO"_4, "KClO"_3 color(white)(mm))(→) underbrace("C"_6"H"_5"-COOH")_color(red)("benzoic acid") + "CO"_2#

In each case, the reaction is behaving as if it forms a 1,1,2,2-tetraol (the diketone dihydrate), which then loses water in the usual way.