Alkyne

The unsaturated hydrocarbons that contain at least one carbon-carbon triple bond are alkyne. Its general chemical formula is C_nH_2n-2.

General methods of preparation of alkyne

1. From carbon and hydrogen:

2. From vicinal dihalide (dehydrohalogenation):

3. From choloroform/Iodoform:

Physical properties:

1. Ethyne is a colourless gas with an ethereal smell.
2. It is insoluble in water.
3. It burns with a luminous flame.

Chemical properties

1. Addition of hydrogen:

2. Addition of hydrogen halide:

3. Addition of water:

Acidic nature of ethyne

The acidic nature of ethyne can be explained on the basis of the hybridization of carbon. The carbon atom in ethyne is sp, in ethene is sp² and in ethane is sp³. The percentage of s character is greater in ethyne. A hybrid orbital having a greater percent of s character will pull the shared electrons to the nucleus. In other words, the electronegativity of carbon atom increases with the increase in percent of s character. Hence ethyne is more acidic.

Compound	Ethyne	Ethene	Ethane
Hybridization	sp	sp2	sp3
% of s character	50	33.33	25
Acidic nature	decreases →

Substitution reaction(reaction with metals)

1. Action with sodium:

2. Action with ammoniacal AgNO₃:

3. Action with ammoniacal Cu₂Cl₂:

Test of unsaturation

Alkene and alkyne are unsaturated compounds. There are some chemical tests that give evidence for the presence of unsaturation. Bromine water test and Baeyer’s test are two common tests.

1. Addition of bromine water (Bromine water test)

Alkene and alkyne react with bromine solution dissolved in carbon tetrachloride as a solvent to give dihalide and tetrahalide respectively. Here the reddish-brown colour of bromine is decolourised.

2. Addition with alkaline potassium permanganate solution (Baeyer’s test):

Ethene is oxidized to Baeyer’s reagent (alkaline solution of KMnO₄) to give glycol. This reaction is called Baeyer’s test. Here, the pink colour of KMnO₄ is discharged.

Ethyne is reacted with Baeyer’s reagent to give oxalic acid. Here, pink colour of KMnO₄ is discharged.

Kolbe’s electrolysis method for preparation of alkane, alkene, and alkyne

1. Electrolysis of concentrated solution of sodium carboxylate gives alkane at anode.

At anode (oxidation):

R-COO^– → R-COO + e^–
R-COO → R + CO₂
R + R → R-R (alkane)

At cathode (Reduction):
Na⁺ + e^– → Na
2Na + H₂O → 2NaOH + H₂

2. Electrolysis of concentrated solution of sodium or potassium salt of succinic acid gives ethene at anode.

3. Electrolysis of concentrated solution of sodium or potassium salt of maleic or fumaric acid gives ethene at the anode.

Some important Questions

1. What happens when ethyne is treated with dil. H₂SO₄ in presence of HgSO₄.
2. Suggest two chemical reactions to show the acidic nature of acetylene.
3. A compound A (C₃H₇Br) reacts with alcoholic KOH to form a compound B. When B is reacted with HBr, two isomeric compounds (C and D) are obtained. Both C and D on reduction with LiAlH₄ give E. Identify A, B, C, D and E with IUPAC name and reaction.
4. An unsaturated compound A (C₂H₄) is reacted with HBr to form B. When B undergoes the Wurtz reaction, C is formed. Identify A, B and C with IUPAC name and suitable reactions.
5. Write short notes on:
   a. Baeyer’s test b. Bromine water test