Introduction to Alkynes
Alkynes are unsaturated hydrocarbons with the general formula (CnHn-2). Hydrocarbons are compounds containing C and H atoms. Unsaturated hydrocarbon means there is a presence of double or triple bonds between the atoms. Alkynes have a triple bond between C and H atoms. They are hard to find in their pure form, They are sp hybridised and the bond angle between them is 180°. Synthesis of alkynes is useful because of its antibacterial, antifungal, and antiparasitic properties. Here, we will discuss the various methods of preparation of alkynes.
Methods of Preparation of Alkynes
Dehydrohalogenation
The loss of a hydrogen and halogen atom from adjacent alkane carbon atoms leads to the formation of an alkene. Further, the loss of additional hydrogen and halogen atoms from the double‐bonded carbon atoms leads to the formation of alkyne. The halogen atoms may be located on the same carbon or the adjacent carbon atoms.
During the second dehydrohalogenation process, in the presence of a strongly basic medium and high temperature, Vicinal tetra haloalkanes can be dehalogenation with zinc metal to form alkynes. This process is called dehydrohalogenation because hydrogen is eliminated along with a halogen in order to obtain an alkyne.
Preparation of Alkynes from Vicinal Dihalides
Alkynes are prepared from vicinal dihalides by the process of dehydrohalogenation. We know the group 17 elements are known as halogens. So, dehydrohalogenation means the removal of Hydrogen and Halogen atoms. The vicinal term is used when two similar atoms are attached at adjacent positions. Dihalides simply mean two halogen atoms. laboratory preparation of alkynes is done by this method.
The first step involves the preparation of unsaturated halides. These are vinylic halides and are not reactive in nature. These halides are reacted with a strong base which results in the formation of alkynes. By using Metal acetylides small alkynes are converted into large ones.
Preparation of Alkynes from Calcium Carbide
At the industrial level, the synthesis of alkynes is done using calcium carbide. Calcium Carbide is prepared by heating quicklime (CaO) in the presence of coke (C). When calcium carbide is made to react with water, It results in the formation of calcium hydroxide and acetylene.
CaCO3 → CaO + CO2
CaO + 3C → CaC2 + CO
CaC2 + 2H2O → Ca(OH)2 + C2H2
This method is now replaced by another method called pyrolysis of methane, In which methane is heated at a temperature of 1500oC in an airless chamber. It forms the product within a fraction of a second with the liberation of hydrogen. (Air must be excluded from the reaction or oxidation process will occur).
The reaction is endothermic at ordinary temperatures and is thermodynamically favoured at high temperatures.
FAQs on Preparation of Alkynes
1. What are the main methods used for the preparation of alkynes as per the Class 11 syllabus?
According to the Class 11 Chemistry syllabus, there are two primary methods for preparing alkynes:
- From Calcium Carbide: This is a common industrial method where ethyne (acetylene) is produced by reacting calcium carbide with water.
- From Vicinal Dihalides: This laboratory method involves a double dehydrohalogenation reaction. An alkyl dihalide with halogens on adjacent carbon atoms is treated with a strong base, like alcoholic potassium hydroxide (KOH), followed by an even stronger base like sodamide (NaNH₂), to form an alkyne.
2. How is ethyne (acetylene) prepared from calcium carbide?
The preparation of ethyne from calcium carbide is a straightforward, single-step reaction. When water is added to solid calcium carbide (CaC₂), a vigorous reaction occurs, producing calcium hydroxide (Ca(OH)₂) and ethyne gas (C₂H₂). This method is often used for the large-scale industrial production of acetylene, which is widely used in welding.
3. What is the difference between preparing an alkyne and an alkene through an elimination reaction?
The key difference lies in the number of elimination steps. To prepare an alkene from an alkyl halide, you perform a single dehydrohalogenation, removing one hydrogen atom and one halogen atom to form a double bond. To prepare an alkyne from a vicinal or geminal dihalide, you must perform two successive elimination reactions (a double dehydrohalogenation) to remove two hydrogen atoms and two halogen atoms, resulting in the formation of a triple bond.
4. Why is a very strong base like sodamide (NaNH₂) needed to prepare an alkyne from a vicinal dihalide?
The first elimination reaction using a base like alcoholic KOH is relatively easy, forming a vinylic halide. However, removing the second hydrogen and halogen from this vinylic halide is much more difficult. A vinylic halide is less reactive. Therefore, a much stronger base like sodamide (NaNH₂) is required to force the second elimination reaction and successfully form the triple bond of the alkyne.
5. How can a smaller alkyne be used to create a larger, more complex alkyne?
This is done by taking advantage of the acidic nature of terminal alkynes (alkynes with a triple bond at the end of the chain). First, the terminal alkyne reacts with a strong base like NaNH₂ to form an acetylide ion. This acetylide ion then acts as a strong nucleophile and can attack a primary alkyl halide in a nucleophilic substitution reaction. This process attaches the alkyl group from the halide to the alkyne, effectively extending the carbon chain and creating a larger alkyne.
6. What is the main difference between a vicinal dihalide and a geminal dihalide in the context of alkyne preparation?
The difference is the location of the two halogen atoms. In a vicinal dihalide, the two halogen atoms are on adjacent (neighbouring) carbon atoms. In a geminal dihalide, both halogen atoms are attached to the same carbon atom. Both types can be used to prepare alkynes through double dehydrohalogenation, as both provide the necessary atoms to be eliminated to form a triple bond.
7. What are some important industrial and commercial uses of alkynes?
Alkynes, especially ethyne (acetylene), have several important uses:
- Welding: The oxy-acetylene flame produces a very high temperature, making it ideal for cutting and welding metals.
- Chemical Synthesis: They are starting materials for making many other organic compounds, such as acetic acid, acetaldehyde, and polymers like PVC (polyvinyl chloride).
- Artificial Ripening: Ethyne gas is used to artificially ripen fruits like bananas and mangoes.
