Grade 11 → Hydrocarbons → Alkynes ↓
Preparation and Properties of Alkynes
Alkynes are an important group of organic compounds characterized by the presence of at least one carbon-carbon triple bond. They have the general formula C n H 2n-2
, where n
is the number of carbon atoms. Alkynes are unsaturated hydrocarbons and are known for their reactivity due to the triple bond. In this lesson, we will explore the preparation methods and properties of alkynes in detail.
Preparation of alkynes
1. Dehydrohalogenation of dihalides
Alkynes can be prepared by dehydrohalogenation of 1,2-dihalides. For example, consider the synthesis of ethene (commonly known as acetylene):
CH 2 CHBr + KOH ⟶ CH≡CH + KBr + H 2 O
In this reaction, the dihalide is reacted with an alcoholic solution of potassium hydroxide to remove one molecule of hydrogen halide, forming an alkyne.
Visual example:
2. Hydrolysis of calcium carbide
Another method of preparing ethyne is by the hydrolysis of calcium carbide:
CaC 2 + 2 H 2 O ⟶ C 2 H 2 + Ca(OH) 2
In this laboratory method, calcium carbide is reacted with water to form ethyne and calcium hydroxide.
Visual example:
3. Alkylation of terminal alkynes
Terminal alkynes can be alkylated using a strong base and an alkyl halide. This process increases the length of the carbon chain:
HC≡CH + NaNH 2 + RX ⟶ HC≡CR + NaX + NH 3
A terminal alkyne reacts with a strong base such as sodium amide to yield an alkyne ion, which can react with an alkyl halide to form a long-chain alkyne.
Properties of alkynes
1. Physical properties
Boiling and melting point
The boiling and melting points of alkynes increase with molecular weight. However, the boiling point of alkynes is slightly higher than that of alkenes and alkenes of the same molecular weight because of better packing due to the linear structure.
Butyne (C 4 H 6 ) boiling point ≈ 9.5°C Octyne (C 8 H 14 ) boiling point ≈ 125°C
Solubility
Alkynes are generally insoluble in water due to their nonpolar nature, but are soluble in nonpolar solvents like benzene, ether, and acetone.
2. Chemical properties
Acidity of terminal alkynes
Terminal alkynes have a slightly acidic hydrogen atom at the terminal carbon, which is due to the s-character of the sp-hybridized carbon:
HC≡CH + NaNH 2 ⟶ HC≡C⁻ Na⁺ + NH 3
Hydrogen can be removed by strong bases such as sodium amide to form acetylide ions.
Addition reactions
Due to the presence of multiple bonds, alkynes undergo addition reactions similar to alkenes:
CH≡CH + H 2 ⟶ CH 2 CH 2 (Ethene) CH≡CH + 2H 2 ⟶ CH 3 CH 3 (Ethane)
In these reactions, hydrogen adds across the triple bond to convert the alkyne to an alkene or alkane.
Halogenation
When treated with halogens, alkynes form dihalides and tetrahalides:
CH≡CH + Cl 2 ⟶ CHCl=CHCl (1,2-Dichloroethene) CH≡CH + 2Cl 2 ⟶ CCl 2 HCCl 2 (Tetrachloroethane)
Visual example:
Alkynes offer a wide variety of reactions and methods of synthesis, highlighting their importance in synthetic organic chemistry. Understanding the properties and preparation of these compounds provides a foundation for delving deeper into more complex chemical reactions and applications.
This detailed information about the preparation and properties of alkynes is important for understanding their role and utility in chemistry and industrial applications. By mastering the basic principles explained in this guide, you lay a strong foundation for exploring advanced organic synthesis and processes involving alkynes.