Alkynes have a general formula of CnH2n-2, and that is 4 H atoms less than alkanes, which have the general formula CnH2n+2. In between, we have the alkenes with the CnH2n formula. This means that alkynes can be reduced by the addition of one or two equivalents of H2, to alkenes and alkanes, respectively:
Reduction of an Alkyne to an Alkane
In the presence of a metal catalyst such as Pd and Pt, a two-equivalent addition of H2 occurs, reducing the alkyne into the corresponding alkane. It is not possible to stop the reduction at equivalent addition and isolate the intermediate alkene:
Overall, four new C-H bonds are formed via a syn addition to the alkene.
Reduction of an Alkyne to a Cis Alkene
In order to stop the reduction of alkenes, the reducing power of the H2 should be decreased, and for this, a less active Pd catalyst is used in the form of a Pd-adsorbed onto CaCO3 mixture with Pd(II) acetate and quinoline. This catalyst is called the Lindlar catalyst after its inventor, Herbert Lindlar.
Because it is deactivated, we also called it a “poisoned” catalyst. With Lindlar catalyst, one equivalent of H2 reduces the alkyne to a cis alkene by a syn addition:
Nickel boride, called the P-2 catalyst, is another catalyst that can be used for the hydrogenation of an alkyne to a cis alkene:
Reduction of an Alkyne to a Trans Alkene
The catalytic reduction of alkynes does not allow for preparing (E )- or trans-alkenes as the hydrogen adds in syn geometry. Trans alkenes are prepared by reducing alkynes by dissolving Na or Li in NH3. The reaction goes through a radical mechanism, and the hydrogen is added in an anti-fashion:
The reaction starts with an electron transfer from lithium (or sodium) atoms to the triple bond of the alkyne, forming a radical anion which deprotonates ammonia. The resulting radical picks up another electron from the metal atom, turning into a carbanion, which is again protonated by ammonia.
Check Also
- Introduction to Alkynes
- Naming Alkynes by IUPAC Nomenclature Rules – Practice Problems
- Preparation of Alkynes by Elimination Reactions
- Hydrohalogenation of Alkynes
- Addition of Water to Alkynes
- Acid-Catalyzed Hydration of Alkynes with Practice Problems
- Reduction of Alkynes
- Halogenation of Alkynes
- Hydroboration-Oxidation of Alkynes with Practice Problems
- Ozonolysis of Alkynes with Practice Problems
- Alkylation of Terminal Alkynes in Organic Synthesis with Practice Problems
- Reactions of Acetylide Ions
- Alkyne reactions summary practice problems
- Alkyne Synthesis Reactions Practice Problems
- Alkyne Naming and Reactions Practice Quiz
- Reactions Map of Alkynes
