Alkyne Reactions

This is part of HSC Chemistry course under the topic of Products of Reactions Involving Hydrocarbons

HSC Chemistry Syllabus

  • 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: – hydrogen (H2) – halogens (X2) – hydrogen halides (HX) – water (H2O) (ACSCH136)

  • investigate, write equations and construct models to represent the reactions of saturated hydrocarbons when substituted with halogens

How Can We Conduct Addition Reactions With Alkynes?

  • Alkynes are more reactive than alkenes due to the presence of two pi-bonds, both of which are strong nucleophiles.

 

Hydrohalogenation & Halogenation of Alkynes

  • Hydrohalogenation: addition of hydrogen halide and halogen works via a similar mechanism (not identical) as it would with alkenes. After first addition reaction, an alkyne would form an alkene, which can further react (if reagents are supplied in sufficient quantities) to form an alkane.

 

 

  • Halogenation of alkynes produces alkanes with tetra-substituted halogens across the triple bond

 

 

  • Hydrohalogenation of alkynes produces di-substituted halogens where the major product is one where both halogen atoms are attached to the same carbon atom (Markovnikov’s rule).
  • (extension): This is because a halogen atom can stabilise an adjacent carbocation by donating one of its electron lone pairs to that carbocation. This overcomes the halogen’s electron-withdrawing effect / electronegativity.

 

Hydrochlorination

Hydrobromination

Bromination

 

 

Hydrogenation of Alkynes

  • Hydrogenation of an alkyne produces an alkene which in turn forms an alkane. Therefore, the final product of an alkyne would be the same as that of its corresponding alkene.
  • Reagent: hydrogen gas (H2) with Pd/C as catalyst

 

 

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, so the effect of carbocation rearrangement is negligible.

 

Reaction

Alkene

Alkyne

Hydrohalogenation

 

bromoethane
1,1-dibromoethane

Halogenation

 

1,2-dibromoethane
 
1,1,2,2-tetrabromoethane

 

Hydrogenation

 

ethane

 

ethane

 

Hydration

 

ethanol

ethanal (acetaldehyde)