Question

What is the mechanism for the acid-catalyzed hydration of but-2-yne?

Answer 1

See below.

Explanation:

The reaction has two parts:

1. Addition of water to the alkyne to form an enol.
2. Tautomerization of the enol to form a ketone.

Part 1. Formation of the enol

Step 1. Protonation of the alkyne to form a carbocation.

`"CH"_3"C≡CCH"_3 +"H"_3stackrel(+)("O") -> "CH"_3stackrel(+)("C")"=CHCH"_3 + "H"_2"O"`

Step2. Water attacks the carbocation and forms an oxonium ion.

`"CH"_3stackrel(+)("C")"=CHCH"_3 +"H"_2"O" -> "CH"_3"C("stackrel(+)("O")"H"_2")-CHCH"_3`

Step 3. Deprotonation of the oxonium to form an enol.

`"CH"_3"C("stackrel(+)("O")"H"_2")-CHCH"_3 + "H"_2"O" -> "CH"_3"C(OH)=CHCH"_3 + "H"_3stackrel(+)("O")`

Part 2. Tautomerization of the enol to a ketone

Step 4. Protonation of the alkene carbon atom to form a carbocation.

`"CH"_3"C(OH)=CHCH"_3 + "H"_3stackrel(+)("O") -> "CH"_3"C"("="stackrel(+)("O")"H")"-CH"_2"CH"_3 +"H"_2"O"`

Step 5. Deprotonation of the oxygen to form the ketone.

`"CH"_3"C"("="stackrel(+)("O")"H")"-CH"_2"CH"_3 +"H"_2"O" -> "CH"_3"C(=O)-CH"_2"CH"_3 + "H"_3stackrel(+)("O")`

If you have trouble following the steps above with condensed structures, here's an image of the same mechanism for the hydration of propyne.

(from www.masterorganicchemistry.com)