How Alkynes Form Other Functional Groups
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.
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.
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.