How Alkynes Form Other Functional Groups

By Arthur Winter

Alkyne reactions are similar to those of the alkenes, and these reactions use reagents similar to those used in alkene reactions. Alkynes can form a variety of functional groups, including tetrabromides, alkanes, alkenes, ketones, and aldehydes.

Brominate an alkyne to form a tetrabromide

Bromine reacts with the pi bond in alkynes to make the dibromide via the same mechanism as bromine addition to alkenes (think “bromonium ion”). Because there are two pi bonds in an alkyne, two equivalents (units) of bromine can be added to make a tetrabromide, as shown here.

The bromination of an alkyne.
The bromination of an alkyne.

Saturate an alkyne to form an alkane

Alkynes can also be reduced to alkanes by bubbling two equivalents of hydrogen gas (H2) over the alkyne in the presence of a metal catalyst as shown in the next figure. This catalyst is usually palladium on carbon (Pd/C), but platinum (Pt) is also sometimes used.

The saturation of an alkyne with hydrogen.
The saturation of an alkyne with hydrogen.

Add one hydrogen molecule to form an alkene

Stopping the reaction of alkynes with hydrogen at the alkene stage is possible because alkenes are somewhat less reactive than alkynes, but this reaction requires a special catalyst. To reduce an alkyne to the cis alkene, you use Lindlar’s catalyst, which is a cocktail of palladium (Pd) powder made less reactive with added lead (Pb) and quinoline (C9H7N). (In reaction diagrams, instead of writing out all the components of the catalyst, chemists often write “Lindlar’s catalyst” over or under the arrow.) Lindlar’s catalyst is not as reactive as palladium on carbon (Pd/C) and generates the cis alkene, as shown here.

To convert an alkyne to the trans alkene, you use sodium metal (Na) in liquid ammonia (NH3), as shown in the next figure.

Oxymercurate an alkyne to form a ketone

Reacting alkynes with mercury, water, and acid, you may expect the reaction to make the alcohol on the double bond, just as mercuric acetate reacts with alkenes to make the Markovnikov alcohol. Indeed, the Markovnikov enol (an alcohol on a double bond) is formed in which the alcohol group is placed on the more-substituted carbon. However, enols are unstable and rapidly convert into ketones, as shown here.

The oxymercuration of an alkyne.
The oxymercuration of an alkyne.

The reaction that converts the enol into the ketone is called a tautomerization reaction, and both the enol and the ketone are considered tautomers of each other. Tautomers are molecules that differ only in the placement of a double bond and a hydrogen.

Apply a hydroboration reaction to an alkyne to form an aldehyde

The hydroboration reaction of alkynes works in the same way as the hydroboration reaction of alkenes, and forms the anti-Markovnikov product. In the case of the hydroboration of alkynes, the product is an enol with the alcohol group on the least-substituted carbon, as shown in the next figure. As with the oxymercuration reaction, this enol is unstable and tautomerizes to the aldehyde.

The hydroboration of an alkyne.
The hydroboration of an alkyne.

Oxymercuration and hydroboration of carbon-carbon triple bonds are useful primarily with terminal alkynes because with terminal alkynes, you get only a single product (with hydroboration, you get the aldehyde; with oxymercuration, you get the ketone). With internal alkynes, both sides of the alkyne are equally substituted, so water can be added equally well to either side of the triple bond, so these reactions yield mixtures of two products.