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 is the dehydrohalogenation reaction?
It is a type of elimination reaction. As the name suggests, it is the process of removing hydrogen halide from the reagent molecule. It is generally used in different laboratory practises, specifically in the preparation of alkynes. The alkyl halides or the vicinal halides are the substrates of the dehydrohalogenation reaction.
The dehydrohalogenation of alkyl halides is usually carried out in ethanol with sodium ethoxide, in methanol with sodium methoxide, or tertbutyl alcohol with potassium tert-butoxide, or dimethyl sulfoxide. A reaction in which a hydrogen atom and a halogen atom in a molecule are separated from neighbouring atoms produces an alkene or an alkyne.
Alkyl halides generally undergo a 1,2 elimination reaction to produce alkenes when heated with solid bases. Typical bases are NaOH or KOH or NaOR or KOR (alcoholate) in alcohol as a solvent, in particular NaOEt or KOtBu. Order of relative reactivity of the alkyl halide: I> Br> Cl> F
Aqueous KOH is alkaline, that is to say, it dissociates to create a hydroxide ion. As a heavy nucleophile, these hydroxide ions replace the halogen atom in an alkyl halide. This results in the formation of alcohol molecules, and the process is called the nucleophilic substitution reaction.
2. What is the dissimilarity between halogenation reaction and dehydrohalogenation reaction?
The halogenation reaction is a type of chemical reaction that involves adding one or more halogens to a compound or material, while the dehydrohalogenation reaction is a type of elimination reaction that removes a halide from the hydrogen of a substrate.
The removal of a hydrogen atom and a halogen atom is known as dehydrohalogenation, when the two outgoing atoms are halogen, the reaction is known as dehalogenation. Likewise, the removal of a water molecule, usually from alcohol, is known as dehydration, when the two outgoing atoms are hydrogen atoms, the reaction is known as dehydrogenation.
3. What are the halogenation reactions?
A halogenation reaction occurs when one or more atoms of fluorine, chlorine, bromine, or iodine replace the hydrogen atoms in an organic compound. The sequence of reactivity is fluorine> chlorine> bromine> iodine.
Fluorine is particularly aggressive and can respond violently with organic materials, it also manages to construct the organohalogen more durable and it is challenging to remove a fluorine particle once added. Thus, the electronegativity of the halogen particle is a driving power for halogenation reactions. The reactions also rely on the qualities of the substrate molecule to be halogenated.
Halogenations occur with different processes depending on the substrate: halogens of saturated hydrocarbons by a radical process; unsaturated organic halogenates via an addition reaction; aromatics halogenated by electrophilic substitution.
4. What are the methods of preparing alkynes, and how are you going to prepare the higher alkynes from the lower alkynes?
Manufacturing contains dehydrohalogenation of alkyl halides, dehydration of alcohols, and dehalogenation of alkanes.
Higher alkynes can be created by reacting an alkyl halide with an acetylide ion, which is generated by a lower alkyne. Since acetylide ions are bases, elimination reactions can occur, leading to the formation of an alkene from the alkyl halide.
5. How are alkynes used?
Alkynes are also used to produce polymers and their raw materials. For example, vinyl chloride is used as the starting material for PVC and chloroprene is used for synthetic rubber neoprene. Ethyne is used to prepare many organic solvents. These are generally used to artificially ripen fruit.
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