Preparation of Alkynes

An alkyne is an unsaturated hydrocarbon that possesses one carbon-carbon triple bond and is a part of organic chemistry.

Alkynes are unsaturated hydrocarbons with one triple bond. The general structural formula for alkynes is CnH2n-2, and the triple bond is referred to as the ‘acetylenic bond.’ Alkynes can be found in abundance in nature. With two carbon atoms bonded by a triple bond, ethyne (C2H2) is the first member of the alkyne family.

An alkyne is an unsaturated hydrocarbon with at least one carbon-carbon triple bond in organic chemistry. Alkynes, like other hydrocarbons, are hydrophobic. The word acetylene is most usually used to refer to ethyne. It is the most basic of the alkynes, with two carbon atoms joined by a triple bond and each carbon atom capable of bonding to one hydrogen atom.

Carbon bonding in alkynes

Two sp hybridised orbitals exist on an ethyne carbon atom. The sp hybridised orbitals of the two carbon atoms overlap head-on to form the carbon-carbon sigma bond. Along the internuclear axis, each carbon atom’s remaining sp hybridised orbital overlaps with the 1s orbitals of the two hydrogen atoms, forming two C-H sigma bonds. H-C-C has a 180° bond angle. Each carbon atom has two unhybridised p orbitals that are perpendicular to each other and the C-C sigma bond plane. The 2p orbitals of one carbon atom are parallel to the 2p orbitals of the other carbon atom, which overlap next to each other or sideways to form two pi bonds between them.

One C–C bond, two C–H bonds, and two C–C bonds are found in an ethyne molecule. The C–C bond is stronger than both the C=C (bond enthalpy 681 kJ mol–1) and the C–C (bond enthalpy 348 kJ mol–1) bonds. C–C bonds are shorter (120 pm) than C=C and C–C bonds (133 pm) and (154 pm) respectively. Around the internuclear axis, the electron cloud created by two carbon atoms is cylindrically symmetric making ethyne a linear molecule.

Preparation of Alkynes

By Kolbe’s Electrolysis-

The production of an alkyne can also be carried out by electrolysis of sodium or potassium salts of unsaturated carboxylic acids containing carbon carbon (C-C) triple bond in it. Both the terminal and non-terminal alkynes can be prepared by this method.

By Dehydrohalogenation

The production of an alkene is caused by the loss of a hydrogen and halogen atom from adjacent alkane carbon atoms. Alkyne is formed as a result of the loss of extra hydrogen and halogen atoms from the double-bonded carbon atoms. Halogen atoms can be found on the same carbon atom or neighbouring carbon atoms.

Vicinal tetra haloalkanes can be dehalogenation with zinc metal to generate alkynes during the second dehydrohalogenation phase, which takes place in the presence of a strongly basic solution and a high temperature. Because hydrogen is removed together with a halogen to obtain an alkyne, this process is known as dehydrohalogenation.

Preparation of alkynes from vicinal dihalides:

Dehydrohalogenation is used to produce alkynes from vicinal dihalides. Halogens are the elements of Group 17 that we are familiar with. Dehydrohalogenation is the process of removing the hydrogen and halogen atoms from a substance. When two comparable atoms are bonded at nearby places, the word vicinal is employed. Dihalides are compounds that contain two halogen atoms. This approach is used to prepare alkynes in the laboratory.

The preparation of unsaturated halides is the initial stage. These are vinylic halides, which are non-reactive. When these halides come into contact with a strong base, alkynes are formed. Small alkynes are transformed into large alkynes utilising metal acetylides.

Preparation of alkynes from calcium carbide:

Calcium carbide is used to synthesise alkynes at the industrial level. Quicklime (CaO) is heated in the presence of coke to produce calcium carbide (C). When calcium carbide reacts with water, calcium hydroxide and acetylene are formed.

CaCO3 + CO2 → CaO

CaC2 + CO → CaO + 3C

Ca(OH)2 + C2H2 → CaC2 + 2H2O

This approach has now been supplanted by pyrolysis of methane, which involves heating methane to a temperature of 1500°C in an airless chamber. With the liberation of hydrogen, it creates the product in a fraction of a second. (If air is present in the reaction, the oxidation process will occur.)

By dehalogenation of halo forms

Chloroform and iodoform dehalogenase and give ethyne when heated with silver powder.

Uses of Alkynes

• Because of its extremely high fire, ethyne is commonly used in oxyacetylene gas welding and oxyacetylene gas cutting. When ethyne is ignited with oxygen, the resulting fire is estimated to be roughly 3600 Kelvin in temperature.

• The component alkyne in acetylene is used as a fuel, and a large amount of it is produced each year by partial oxidation of gaseous gasoline.

• A fraction of these alkynes is used to make substance mixes such as ethanoic corrosive, acrylic corrosive, and ethanol.

• Natural mixes such as ethanol, ethanoic corrosive, and acrylic corrosive are commonly made from ethyne. It’s also employed in the production of polymers and raw ingredients for them.

• The carbon and hydrogen components of acetylene are separated. This reaction generates a lot of heat, which can cause the gas to flare up even if there is no air or oxygen present.

• Alkynes are commonly used as starting materials in the production of a wide range of mechanically important natural mixes, such as chloroprene, vinyl chloride, and so on.

Conclusion

The preparation of alkynes is an important topic and is a part of Hydrocarbons in Chemistry. Alkynes are unsaturated hydrocarbons with at least one triple bond between the carbon-carbon atoms, as defined by organic chemistry. The article discusses several methods to prepare alkynes from different compounds. The antibacterial, antifungal, and antiparasitic characteristics of alkynes make them useful for drug development.