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Summary of Alkyne Reactions

Instructor: Kristen Procko

Kristen has taught chemistry and biochemistry at the undergraduate and Master's level and has a PhD in chemistry.

In this lesson, you will learn the types of alkynes and related functional groups. You will then learn the major reactions that alkynes undergo: alkylation, reduction, addition, and oxidation.

Types of Alkynes and Related Functional Groups

Functional groups are atoms arranged and bonded in a specific way. They determine the chemistry (and reaction) of every organic compound. An alkyne is a functional group that refers to compounds containing a carbon-carbon triple bond.

Notation for a carbon-carbon triple bond
carbon-carbon triple bond

Alkynes can be internal alkynes or terminal alkynes. Internal alkynes, just like they sound, are in the center of a molecule. Terminal alkynes are at the end, and contain an H atom on at least one of the triply bonded carbon atoms.


internal and external alkynes


Alkanes are similar to alkynes, in that they are made up of carbon and hydrogen atoms, but alkanes contain only single bonds.


butane


Alkenes are functional groups that contain carbon-carbon double bonds. There are two types of alkenes, Z and E.

  • Z-alkenes have the higher molecular weight groups on the same side of the molecule
  • E-alkenes have the higher molecular weight groups on opposite sides


z and e


All of the compounds shown so far are hydrocarbons, or molecules made up of carbon and hydrogen atoms. Alkanes, alkenes, and alkynes do not have to be hydrocarbons; they can be substituted with other elements; for example, if there is a halogen--a chlorine, bromine, or iodine atom--on an alkane, we call it a haloalkane.

Reactions of Alkynes

Alkylation of Terminal Alkynes

Alkylation is the addition of an alkyl group to a molecule, which can be an alkane. Terminal alkynes can undergo alkylation reactions because the H atom on them is somewhat acidic, and therefore willing to donate its proton. The H atom of a terminal alkyne is removed by a relatively strong base.

Alkylation reaction of a terminal alkyne. Sodium amide is used (1), then the deprotonated alkyne is reacted with a haloalkane (2).
Alkylation reaction of a terminal alkyne

Reduction of Alkynes

Both terminal and internal alkynes may undergo reduction reactions, which add molecules of hydrogen across double and triple bonds. You better be wearing your safety glasses when you run this reaction, because hydrogen gas is very flammable!

A hydrogen gas explosion was responsible for the crash of the Hindenburg airship in 1937, one of the first disasters of its type caught on film. Reduction reactions are carried out with an inert, or unreactive, gas in the flask, such as nitrogen or argon. Open it up to air, and you may have a mini Hindenburg incident in your lab!

We can form both alkanes and alkenes from reduction reactions. Let take a look at three reactions.

In reaction (a), hydrogen gas is reacted in the presence of a metal catalyst, and an alkane is produced. An alkene is formed during this reaction, but is not isolated.

Hydrogen gas is reacted with a metal catalyst, producing an alkane
reaction one

A stereoselective reaction occurs when there can be more than one geometry at a carbon atom, but one of the possible products forms preferentially. Stereoselective reduction of alkynes is shown in reactions (b) and (c). Notice in reaction (b) when 2-butyne is reacted with hydrogen gas and a metal catalyst called Lindlar's catalyst, the Z-alkene is formed. Reaction (c) is called a dissolving metal reduction, and produces the E-alkene in sodium and liquid ammonia.

Reduction reactions of alkynes
reactions 2 and 3

Halogenation Reactions

Haloalkanes can be formed from alkynes in two types of reactions. In a halogenation reaction, where one or more hydrogen atoms are replaced by a halogen, molecular bromine is added to the alkene. This adds a bromine atom to each side of the triple bond, forming a haloalkene, which reacts again to form the compound shown, containing four bromine atoms.

A halogenation reaction
reaction a

Reaction with an acid that contains a halogen adds H and the halogen, in this case chlorine, across the bond twice, in a similar fashion to reaction (a). Reaction (b) is regioselective, which means that there are two possible positions that the H and Cl atoms can add to. However, in this case, the two chlorine atoms that add to the molecule add onto the same carbon atom, and the two hydrogen atoms add to the other carbon atom of the alkyne.

Regioselective halogenation with HCl
reaction b

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