Alkyne Reactions
This is part of HSC Chemistry course under the topic of Products of Reactions Involving Hydrocarbons
HSC Chemistry Syllabus
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investigate, write equations and construct models to represent the reactions of unsaturated hydrocarbons when added to a range of chemicals, including but not limited to:
Addition Reactions with Alkynes
Alkynes are unsaturated hydrocarbons which undergo addition reactions. In this video, we will discuss the following reactions with alkynes:
- hydrogenation
- halogenation
- hydrohalogenation
- hydration
Like alkenes, alkynes can also undergo addition reactions because they are unsaturated. This means the carbon atoms adjacent to the triple carbon-carbon bond can form bonds with more atoms.
Alkynes are considered to be more reactive than alkenes due to the presence of two π-bonds. Each π-bond can 'open up' to allow for the addition of two atoms or groups of atoms. As such, an alkyne molecule can undergo two rounds of addition reactions whereas an alkene molecule can only undergo one round of addition reaction.
Reaction of Alkynes with Hydrogen Halides
The reaction of alkynes with hydrogen halides is known as hydrohalogenation.
This reaction consists of two steps. The first step involves the addition of a hydrogen and a halogen atom to the two carbon atoms adjacent to the triple carbon-carbon bond. This results in a halogenated alkene.
Since alkenes are still unsaturated, the second step of addition takes place when another hydrogen and another halogen atom are added to the carbon atoms adjacent to the double carbon-carbon bond.
This results in the final product of the reaction which is a double halogenated alkane (an alkane molecule with two halogens attached).
Markovnikov's rule also applies to addition reactions involving alkynes. In hydrohalogenation, the major product is the one where the halogen is bonded to the carbon atom with more carbon neighbours. In the example above, this is the middle carbon atom.
In hydrohalogenation of alkynes, the two halogen atoms are both added to the carbon atom with more carbon neighbours in the major product.
Reaction of Alkynes with Halogens
The reaction of alkynes with halogens is known as halogenation.
Halogenation of an alkyne produces an alkanes with tetra-substituted halogens (four halogens) on the carbon atoms which were originally adjacent to the triple carbon-carbon bond.
It is useful to compare the products of hydrohalogenation and halogenation of alkynes. Hydrohalogenation produces di-substituted halogens where the major product is one where both halogen atoms are attached to the same carbon atom (Markovnikov’s rule). In contrast, halogenation produces tetra-substituted halogens.
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Hydrochlorination |
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Hydrobromination |
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Bromination |
Reaction of Alkynes with Hydrogen
The reaction of alkynes with hydrogen is known as hydrogenation.
Hydrogenation of an alkyne produces an alkene which subsequently forms an alkane.
Reagent: hydrogen gas (H2) with Pd/C as catalyst
Reaction of Alkynes with Water
The reaction of alkynes with water is known as hydration.
It is important to know that hydration of alkynes does not produce an alcohol. Instead, either an aldehyde or a ketone is formed. Although hydration of alkynes is considered an addition reaction, it involves mechanisms beyond the HSC Chemistry syllabus and therefore, knowledge is not required.
Summary of Addition Reactions of Alkenes and Alkynes
Table: products of different reactions involving ethene and ethyne. Note that only one product is formed for each reaction because ethene and ethyne both only have two carbon atoms. For other alkenes and alkynes, major and minor products may be formed.
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Reaction |
Alkene |
Alkyne |
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Hydrohalogenation |
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bromoethane
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1,1-dibromoethane
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Halogenation |
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1,2-dibromoethane
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1,1,2,2-tetrabromoethane
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Hydrogenation |
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ethane |
ethane |
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Hydration |
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ethanol |
ethanal (acetaldehyde) |