Question

What are the products when but-2-enoic acid reacts with aqueous `"KMnO"_4`?

Answer 1

It depends on the conditions.

Explanation:

Reaction with cold, dilute, basic `"KMnO"_4`

The reaction forms 2,3-dihydroxybutenoic acid and a dark brown precipitate of manganese dioxide.

`3underbrace("CH"_3"CH=CHCOOH")_color(red)("but-2-enoic acid") + "2MnO"_4^"-" + "4H"_2"O"`

`→ 3underbrace("CH"_3"CH(OH)CH(OH)COOH")_color(red)("2,3-dihydroxybutanoic acid") + 2underbrace("MnO"_2)_color(red)("dark brown ppt.") +2"OH"^"-"`

The disappearance of the purple colour with the formation of a dark brown precipitate is often used as a test for the presence of `"C=C"` double bonds.

Reaction with hot, acidic `"KMnO"_4`

1,2-Diols are easily oxidized by hot, acidic `"KMnO"_4`.

In hot, acidic solution, but-2-enoic acid is oxidized to acetic acid and oxalic acid.

`"CH"_3"CH=CHCOOH" stackrelcolor(blue)("KMnO"_4, "H"^"+", Δcolor(white)(mm))(→) underbrace("CH"_3"COOH")_color(red)("acetic acid") + underbrace("HOCOCOOH")_color(red)("oxalic acid")`