Reactions of Alkynes

Alkynes have one or more triple bonds. A triple bond consists of two pi bonds and one sigma bond. The unsaturation of alkynes is higher than that of alkenes. Alkynes can undergo reactions through the cleavage of their pi bonds. These two pi bonds react with electrophiles independently.

Also, it follows Markovnikov’s rule. The electron density around the triple bond is high. Therefore, electrophiles can be attracted by the triple bond. Thus, alkynes undergo electrophilic substitution reactions.

When compared to alkenes, the reactivity of alkynes is lower than that of alkenes. Because alkynes have triple bonds. And the triple bonds have much higher bond energy. Therefore, it needs higher energy for the cleavage of bonds.

01. Catalytic addition of hydrogen to alkynes

In the presence of Ni (nickel), Pt (platinum), or Pd (palladium) catalysts, alkyne is heated with an H₂ air current to get alkane. Here, two pi bonds are broken down, and Hydrogen is added. For 1 mol of triple bonds, 2 mol of H₂ are added. The hybridization of carbon changes from sp to sp³.

In this reaction, first, the alkyne is reduced into an alkene and further reduced into an alkane. If this reaction should be stopped at the alkene stage, it uses a Pd catalyst deposited on BaSO₄ and deactivated by quinoline. It reduces the catalytic activity, and the reaction is stopped at the alkene stage.

02. Addition of bromine to alkynes

Bromine is added to the alkyne in a medium of CCl₄. Water is not used as a reaction medium here. Because, if water is used, the -OH group is also added to the alkyne. Also, CCl₄ is a non-polar solvent. Therefore, Br₂ can be easily dissolved in CCl₄ since Br₂ is also a non-polar molecule.

In this reaction, it can be observed that the red color of the Br₂ liquid turns colorless. Therefore, this reaction can be used to identify unsaturation in organic compounds.

03. Reactions of alkynes with hydrogen halides

Alkynes react with hydrogen halides such as HCl, HBr, and HI and result in the respective alkyl halide. This reaction also follows Markovnikov’s rule. The carbon with more hydrogen gets the hydrogen atom, and the other carbon gets the halide.

For 1 mol of triple bonds, 2 mol of HX (X = Cl, Br, I) are added. Both halogens are attached to the same carbon atom.

04. Addition of water to an alkyne

When an alkyne is heated with Hg²⁺ catalysts in the presence of dilute sulfuric acid, one mole of water molecules is added per one mole of triple bonds in the alkyne. It forms an enol that has a C=C double bond and an -OH group attached to one carbon atom in the double bond. However, enols are unstable, so they will rearrange into a stable keto compound (aldehyde or ketone).

An aldehyde results from only ethyne. All the other alkynes will result in ketones.

05. The acidic nature of alkynes with terminal hydrogen

The two carbon atoms that are included in the triple bond are sp-hybridized. Sp hybridized orbitals have higher s characters (50%), and they are smaller when compared to sp² and sp³ hybridized orbitals. So, the bond length of a C-C triple bond is less than a C-C double bond or a single bond.

Since the bond length is low, the electrons in the bond are closer to the nucleus. That means the electrons (negatively charged) on the carbon atom are closer to the positively charged nucleus. Therefore, the electronegativity of a sp-hybridized carbon is high. It can bear a negative charge easily.

When there is a triple bond at the terminal carbon atoms, the Hydrogen attached to the terminal carbon can emit as an H⁺ ion, leaving a negative charge on the terminal carbon. Therefore, an alkyne with a terminal hydrogen has acidic properties. That means alkynes could react with metals and bases.

However, the acidity of terminal alkynes is very low when compared to the inorganic acids. Also, the acidity of terminal alkynes is even lower than water and alcohol. They could only react with highly reactive metals like sodium (Na) and strong bases like NaNH₂.

Terminal alkynes result in the salt of the acetylide ion and H₂ as products, from the reactions with the above reactants. In these reactions, the terminal hydrogen is substituted by the Na⁺ ion. Therefore, this reaction is an electrophilic substitution reaction.

Tests to identify terminal alkynes

In some instances, terminal alkynes form insoluble metal acetylides. Those reactions can be used to identify terminal alkynes.

Reaction with ammoniated silver nitrate

Terminal alkynes react with ammoniated silver nitrate (NH₄OH + AgNO₃) and result in a white precipitate of the metal acetylide. The acidic hydrogen is substituted by the Ag⁺ ion.

Reaction with ammoniated copper chloride

Terminal alkynes react with ammoniated copper chloride (NH₄OH + CuCl) and result in a red/brown color precipitate of the metal acetylide. The acidic hydrogen is substituted by a Cu⁺ ion.