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Hydrocarbons” are natural chemicals that contain both carbon and hydrogen. Aliphatic hydrocarbons are compounds with saturated single bonds (alkanes) or unsaturated double or triple bonds in their molecules. Alkenes have the structural formula CnH2n and include carbon–carbon double bonds. Alkynes are unsaturated hydrocarbons with carbon–carbon triple bonds that are structurally represented by the formula CnH2n-2.
The most basic alkyne is ethyne, often known as acetylene, a colourless gas that burns at high temperatures and is used as a welding fuel. Alkynes can be found in a wide range of natural and manufactured chemicals. Natural alkynes, for example, may be found in the venom of South American tree frogs. Synthetic alkyne-containing chemicals, on the other hand, play a major role in medications like ethynylestradiol, an oral contraceptive. Other alkyne-based medicines, such as selegiline, are used to treat Parkinson’s disease in conjunction with synthetic dopamine or L-dopa.
Methods of Preparation of Alkynes
Dehydrohalogenation
The production of an alkene is caused by the loss of a hydrogen and a halogen atom from carbon atoms close to alkane carbon atoms. A further consequence of the loss of additional hydrogen and halogen atoms from the doubled-bonded carbon atoms is the production of alkyne, which is a cyclic compound. The halogen atoms may be found on the same carbon atom or on carbon atoms that are close to it.
The second dehydrohalogenation procedure, which takes place in the presence of a highly basic medium and high temperature, might result in the formation of alkynes from Vicinal tetra haloalkanes, which are then dehalogenated with zinc metal. Due to the fact that hydrogen is removed together with a halogen in order to obtain an alkyne, this process is referred to as dehydrohalogenation.
Preparation of Alkynes from Vicinal Dihalides
The method of dehydrohalogenation is used to produce alkynes from vicinal dihalides. We are all aware that the elements of group 17 are referred to as halogens. As a result, dehydrohalogenation refers to the removal of Hydrogen and Halogen atoms from a substance. When two comparable atoms are bonded at nearby places, the vicinal word is used to describe the situation. Dihalides are simple compounds that contain two halogen atoms. This approach is used for the production of alkynes in the laboratory.
The preparation of unsaturated halides is the initial stage in this process. This group of compounds is known as vinylic halides, and they are not reactive in nature. In the presence of an acidic base, these halides undergo a reaction that culminates in the production of alkynes. Small alkynes are transformed into big alkynes by the use of metal acetylides.
Preparation of Alkynes from Calcium Carbide
The synthesis of alkynes is carried out at the industrial level with the use of calcium carbide. Calcium Carbide is made by heating quicklime (CaO) in the presence of coke until it becomes brittle (C). When calcium carbide is exposed to water, it reacts to generate calcium hydroxide and acetylene, which are both toxic.
CaCO3 → CaO + CO2
CaO + 3C → CaC2 + CO
CaC2 + 2H2O → Ca(OH)2 + C2H2
This approach has now been superseded by another method known as pyrolysis of methane, in which methane is heated at a temperature of 1500°C in an airless chamber for an extended period of time. The product is formed in a fraction of a second as a result of the release of hydrogen from the reaction. It is necessary to keep air out of the reaction or an oxidation process would ensue.
At ordinary temperatures, the reaction is endothermic; however, at high temperatures, the process is thermodynamically favoured.
Conclusion
The H–C≡C–H bond angles in the compound acetylene are 180°. Alkynes have a rod-like shape due to this bond angle. Cyclic alkynes, on the other hand, are uncommon. Benzyne is a very volatile compound. The C–C bond in alkanes is shorter than the C=C bond in alkenes (134 pm), and the C–C bond in alkanes is shorter than the C=C bond in alkenes (134 pm) (153 pm).Alkynes have physical characteristics that are identical to those of their parent alkane or alkene. They are insoluble in water and polar solvents because they are nonpolar compounds with a lower density than water. They do, however, dissolve well in nonpolar organic solvents. At normal temperature, lower-molecular-weight alkynes like ethyne and propyne exist as gases, but higher-molecular-weight alkynes like 1-octyne and 1-decyne exist as liquids.
