Addition Reactions of Alkynes

A representative example — structures drawn live.

Hydrohalogenation (adding HX)

Hydrogen halides (HCl, HBr, HI) add across a triple bond following Markovnikov's rule: the hydrogen goes to the carbon with more hydrogens, and the halogen goes to the more substituted carbon, which builds the more stable vinyl cation. With one equivalent you get a vinyl halide (a haloalkene).

With two equivalents of HX, addition happens twice. Both halogens end up on the same carbon — the original more-substituted carbon — giving a geminal dihalide (gem-dihalide). For a terminal alkyne like propyne, 2 HBr gives 2,2-dibromopropane.

Acid-catalyzed hydration → ketone

Treating an alkyne with water under acid catalysis with a mercury(II) catalyst (H₂O, H₂SO₄, HgSO₄) adds water across the triple bond with Markovnikov regiochemistry. The OH lands on the more substituted carbon, giving an enol (a C=C bearing an OH).

Enols are unstable and immediately tautomerize — a proton and the double bond shift — to the more stable carbonyl form. For a Markovnikov enol, that carbonyl is a ketone. So terminal alkynes give methyl ketones, and internal alkynes give ketones as well (a symmetrical internal alkyne gives a single ketone; an unsymmetrical one gives a mixture).

Hydroboration–oxidation → aldehyde

The anti-Markovnikov complement uses a borane (often a bulky one such as disiamylborane) followed by H₂O₂/NaOH. Boron adds to the less substituted carbon, so after oxidation the OH ends up there, giving an anti-Markovnikov enol.

That enol tautomerizes to a carbonyl too — but because the oxygen is now on the terminal carbon, a terminal alkyne gives an aldehyde rather than a ketone. This is the standard way to convert a terminal alkyne into an aldehyde. The contrast is worth memorizing: Markovnikov hydration → ketone; hydroboration → aldehyde.

Reduction: controlling cis vs. trans

You can partially or fully reduce a triple bond, and the reagent dictates the stereochemistry of the alkene you get:

• H₂ with Lindlar catalyst (poisoned Pd) stops at the alkene and delivers both hydrogens to the same face — a syn addition giving the cis (Z) alkene.
• Na (or Li) in NH₃ — dissolving-metal reduction — proceeds through a radical/anion pathway and gives the trans (E) alkene.
• H₂ with Pd/C (ordinary catalyst) does not stop; it reduces all the way to the alkane.

Reagent → product cheat sheet

For a terminal alkyne, here is the quick map:

• 1 eq HX → vinyl halide (Markovnikov); 2 eq HX → geminal dihalide.
• H₂O, H₂SO₄, HgSO₄ → methyl ketone (via Markovnikov enol).
• 1) R₂BH; 2) H₂O₂, NaOH → aldehyde (via anti-Markovnikov enol).
• H₂, Lindlar → cis alkene; Na, NH₃ → trans alkene; H₂, Pd/C → alkane.

Keep two themes in mind across all of these: alkynes can add twice, and enol intermediates always tautomerize to the carbonyl. Get those two ideas plus the four regio/stereochemistry controls above, and alkyne addition problems become predictable.