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Alkynes are a type of unsaturated hydrocarbon compounds with the general formula (CnHn-2). It also contains C and H atoms. Double or triple bonds between the atoms are present. Alkyne synthesis can be of great benefit due to its antibacterial, antifungal and anti-parasitic characteristics. It has a triple bond between C and H atoms. They are not easy to find in their pure form; since they are sp hybridized and their bond angle in between is 180°. Ethyne which is also known as acetylene is denoted as the first stable member of the alkyne series.
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Key Takeaways: Alkynes, dihalides, electrolysis, dehydrohalogenation, Unsaturated hydrocarbons.
Preparation of Alkynes from Vicinal Dihalides
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Vicinal dihalides prepare Alkynes by undergoing the process of dehydrohalogenation. Dihalides are acquired from respective alkenes by the addition of halogens which are Group 17 elements in the periodic table. The term dehydrohalogenation means the elimination of Hydrogen and Halogen atoms. The term vicinal is used when two identical atoms are connected at conjoining positions. Similarly two halogen atoms are termed as Dihalides.
Preparation of alkynes from vicinal dihalides
The laboratory preparation of alkynes can be explained in two steps; the preparation of unsaturated halides is determined as the first step. These are also known as vinylic halides which are also not reactive in nature. The second step involves the halides to react with a strong base resulting in the formation of alkynes. Small alkynes are converted into large ones with the help of Metal acetylides.
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Preparation of Alkynes from Calcium Carbide
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Calcium carbide helps in the process of alkyne synthesis on a larger scale. It is formulated by heating quicklime (CaO) when coke (C) is also present. The calcium carbide is then reacted with water, resulting in the evolution of calcium hydroxide and acetylene.
Another method called pyrolysis of methane replaces the existing process where methane is heated at a 1500oC temperature in an electric arc. Acetylene is synthesized by clashing an electric arc between carbon electrodes thus releasing hydrogen where air must not be present in the reaction or oxidation process will take place. The chemical reactions involved in the process are as followed:
CaCO3 → CaO + CO2
CaO + 3C → CaC2 + CO
CaC2 + 2H2O → Ca (OH)2 + C2H2
Alkyne Preparation by Kolbe's Electrolytic Method
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The aqueous solution of sodium or potassium fumarate which is also known as Maleate with the help of electrolysis forms acetylene at anode.
Kolbe’s Electrolysis Method
Dehydrohalogenation
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Alkenes react with a halogen which leads in formation of a substituted alkane from its reaction. Alkanes formed thereby are passed through alcoholic KOH to form substituted alkenes. It then reacts with sodium amide that leads to form alkynes. This whole process is called dehydrohalogenation since a hydrogen atom is replaced along with a halogen to get an alkyne in result.
Dehydrohalogenation
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Things to Remember
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- Alkynes are a type of unsaturated hydrocarbon compounds having the general formula (CnHn-2). It has a triple bond between C and H atoms.
- Alkynes are not easy to be found in their pure form; as they are sp hybridized and their bond angle in between is 180
- Ethyne or acetylene is known as the first stable member of the alkyne series.
- Dehydrohalogenation of alkenes in the presence of alcoholic KOH replacing the hydrogen atom along with a halogen to get an alkyne in result.
- Preparing alkynes from vicinal dihalides in two steps; first by the process of dehydrohalogenation resulting in formation of vinylic halides and second the halides are reacted with strong bases to form alkynes.
- Calcium carbide is made to react with water resulting in the formation of calcium hydroxide and acetylene. And later with the help of pyrolysis of methane where hydrogen is liberated and alkyne is also formed.
- Kolbe electrolysis of maleate also helps in the formation of acetylene, a form of alkyne substance.
Sample Questions
Ques: Will pH of solution change in Kolbe’s electrolysis? [1 mark]
Ans: The concentration of NaOH solution increases hence pH of solution changes with time.
Ques: How are trans-alkenes formed by alkynes? [1 mark]
Ans: Trans-alkenes are formed when alkynes are reduced with sodium in the presence of liquid ammonia.
Ques: How is cis – alkene formed by alkyne? [2 marks]
Ans: Cis-alkene is formed when alkyne are partially reduced with a distinct amount of di-hydrogen in the presence of palladised charcoal and then partially deactivated with the help of poisons like sulphur compounds or quinoline.
Ques: Explain briefly how is alkyne prepared from calcium carbide? [2 marks]
Ans: Ethyne also known as acetylene is produced by treating calcium carbide with water. The reaction is given below:
CaC2 + 2H2O → Ca (OH)2 + C2H2
Ques: Write the chemical reaction for conversion of ethene to ethyne? [1 mark]
Ans: The reaction is given below:
Ques: What do you mean by dehydrohalogenation? [1 mark]
Ans: The term dehydrohalogenation means the elimination of Hydrogen and Halogen atoms.
Ques: What are the uses of acetylene? [2 marks]
Ans: Uses of acetylene are-
- Oxyacetylene flame is used in welding and cutting.
- Acetylene can also be used as an illumining agent.
- Acetylene is also used for artificial ripening of fruits.
- It is also used as a general anesthetic.
Ques: Which is the first stable member of the alkylene series? [1 mark]
Ans: The first stable member of the alkylene series is ethyne or acetylene.
Ques: How is alkyne produced from vinyl dihalides? [3 marks]
Ans: The preparation of alkynes from dihalides can be explained in two steps:
- The process of dehydrohalogenation removes hydrogen and a halogen atom resulting in formation of vinylic halides.
- The second step involves the halides to react with a strong base resulting in the formation of alkynes. Small alkynes are converted into large ones with the help of Metal acetylides.
Ques. Why aren't alkynes geometrically isomeric? (2 marks)
Ans: The relative orientation of functional groups within a molecule causes geometrical isomerism. There are four separate places where different molecules or groups can be found in alkyne. A geometric isomer is formed by switching the positions of the two groups, whether cis (opposite sides) or trans (across).
Ques. Why does hydrogenation necessitate the use of a catalyst? (3 marks)
Ans: The reaction in the gaseous phase is kinetically unfavourable. This is due to the lack of a mechanism that allows molecules to interact. There are other ways to perform the reaction, such as photochemically, but typically hydrogenation takes place on transition metal catalyst surfaces, where double bonds and hydrogen atoms can bind, traverse the 2D plane, and interact one-by-one on the surface due to greatly changed orbital configurations.
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