Alkynes: Preparation, Properties, Structure & Examples

Alkynes are unsaturated compounds that have at least one triple bond. The general formula for the compound is C_nH_2n-2. The first stable member of the group is Ethylene (C₂H₂). 

• Alkynes are the base materials used for making organic compounds.
• It consists of no other functional groups and forms a homologous series.
• Alkynes are hydrophobic in nature.
• The compound is traditionally known as acetylenes, though it specifically refers to the compound ethyne.
• Each of the carbon atoms in acetylene is sp hybridisation.
• Alkynes are most commonly used for the production of a large number of organic compounds, such as chloroprene.
• The compound is used as a fuel for oxyacetylene torches in welding.

General Formula: C_nH_2n-2

Key Terms: Alkynes, Ethylene, Homologous Series, Unsaturated Compounds, Functional Groups, Carbon, Atoms, Electron, Alkenes, Orbital, Corrosion, Oxygen, Bonds, Polymers

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What are Alkynes?

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Alkynes are unsaturated compounds that consist of at least one carbon-carbon triple bond. Like alkenes, these compounds also include carbon orbital overlaps, resulting in a weak zone between the carbon atoms. 

• The electron pair is located in the weak zone.
• The bonds created inside these regions are pi bonds.

Since pi bonds are weaker than sigma bonds, alkynes' reactivity ensures that sigma bonds are formed, resulting in a robust and stable structure. Alkynes can undergo both addition and reduction processes, making them reactive. 

• Although these compounds have low acidity, the addition reaction occurs when they come into contact with a metal surface.
• It will initiate the corrosion process. 
• Some alkynes, such as acetylene and ethyne, can burn in oxygen to produce water and carbon monoxide.
• Corrosion inhibitors can be made from certain alkyne polymers (cathodes).

Alkynes

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Structure of Alkynes

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The structure of alkynes is as follows:

• The perpendicular orientation of the sigma bond and two pi bonds originate from the sp hybridization of the carbon-carbon triple bond. 
• The electrons' near closeness in this geometric orientation causes molecules to be less stable.
• The structure of the carbon-carbon triple bond substantially influences the reactivity and the acidity of terminal alkynes. 
• A ten-membered carbon ring is the smallest that may handle this function.
• It occurs without significant strain due to its linear configuration (the bond angle of a sp-hybridized carbon is 180^o).

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Nomenclature of Alkynes

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The nomenclature of alkynes is divided into two categories which are as follows:

Common Name

The members of the alkynes group are named in terms of acetylene derivatives.

IUPAC Name

According to the IUPAC standard, the nomenclature of alkyne compounds is similar to the nomenclature of alkane compounds. As a result, it is quite identical except that the suffix “yne” is replaced with the suffix “ane”.

• The counting of the compound will start with the first carbon triple bond.

Nomenclature of Alkynes 

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Isomerism in Alkynes

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Alkynes is an organic compound that exhibit three types of isomerism which are as follows:

Chain Isomerism

These types of alkynes compounds are formed by different arrangement of carbon atoms that are arranged in straight-chain or branched chain.

Position Isomerism

Position Isomerism is formed due to the difference in the location of the triple bond.

Functional Isomerism

Functional Isomerism involves with alkadienes being represented by the general formula C_nH_2n-2. 

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Properties of Alkynes

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Properties of alkynes are divided into the category of physical and chemical properties which are as follows:

Physical Properties of Alkynes

The physical properties of alkynes are as follows:

• Alkynes are nonpolar, unsaturated hydrocarbons with physical properties similar to alkanes and alkenes.
• They are soluble in organic solvents, polar solvents with a low solubility, and insoluble in water.
• These compounds have slightly higher boiling points than alkanes and alkenes.
• Ethane, for example, has a boiling point of -88.6°C, but ethene has a boiling point of -103.7°C.
• Ethyne has a boiling temperature of -84.0°C.

Chemical properties of Alkynes

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The chemical properties of alkynes are as follows:

Addition of Electrophilic Reagents

Alkynes are involved in the majority of alkene addition reactions, with similar regio- and stereoselectivity. Although these electrophilic additions to alkynes are slow, they do happen and are generally regioselective, according to Markovnikov's Rule. 

• One issue is that the results of these additions are substituted alkenes, and so can be added again.
• Halogen substituents on a double bond lessen the nucleophilicity of the bond, lowering the rate of electrophilic addition processes. 
• As a result, determining the product of an initial addition to an alkyne will be further added to a saturated product is a delicate balancing.

Addition of Hydrogen Halide to an Alkyne

Anti-addition occurs when excess HX is added, resulting in the formation of a geminal dihaloalkane. HI > HB r> HCl > HF is the reactivity order of hydrogen halides according to Markovnikov's rule.

• Hydrogen element adds to the carbon with the most hydrogens.
• The halogen element adds to the carbon with the fewest hydrogen.
• The more stable carbocation is where protonation takes place. 
• Haloalkanes are formed when HX is added.

Addition of HX to an Alkyne

As an example, HBr attacks the double bond in 2-butyne to produce 2-bromobutane, as seen in the diagram below.

Addition of HX to Terminal Alkyne

This image depicts the addition of HBr to an asymmetric molecule. What happens if you have too much HBr? According to Markovnivov's rule, adding too much HX results in a geminal dihaloalkane.

Reaction Addition of Halogens (Cl₂, Br₂, I₂)

It involves the addition of halogen to an alkyne, which is similar to adding halogen to an alkene. First, the trans-alkene is obtained. A tetrahalogened alkane is formed when halogen is used in excess.

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Alkynes Homologous Series

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A homologous series is a type of series that involves the collection of carbon atoms in which the hydrogen atom replaces the same type of functional group. 

• As the same type of functional groups are added, compounds will have similar chemical properties.
• It exhibits the gradation of physical properties.

The table below explain different types of alkynes:

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Tests for the Presence of a Triple Bond

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Tests for presence of a triple bond in alkynes are represented by following test which are as follows:

With Ammoniacal Silver Nitrate

In this test, alkynes will form a white precipitate of silver acetylide in reaction with ammoniacal silver nitrate.

With Ammnoniacal Cuprous Chloride

In this test, alkynes will form a red precipitate of cuprous acetylide in reaction with ammoniacal cuprous chloride.

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Uses of Alkynes

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The uses of alkynes are as follows:

Fuel for Rockets

The dominant alkyne in acetylene is used as a fuel, with millions of kilos produced annually through fractional oxidation of natural gases.

Synthesis of Organic Substances

Organic chemicals such as ethanoic acid, acrylic acid, and ethanol are made from some of these alkynes.

Solvents made of organic materials

Ethyne, for example, is an alkyne that is used to make organic solvents.

Polymers

A polymer is a macromolecule made up of numerous subunits that are repeated. Vinyl chloride, for example, is used as a starting ingredient for PVC.

Fruits are artificially ripened

Alkynes are used to make toxic mustard gas by artificially ripening fruits and as a general anaesthetic.

Lighting that is transportable

Acetylene was first used to power portable lamps known as acetylene gas lamps, which were found in homes, bicycles, and automobiles.

Cutting and welding

Acetylene is used to weld or cut materials that demand high temperatures of 3500 degrees Celsius, and it is the gas that can produce the brightest flame of all the gases.

Plastics made of polyethylene

Ethylene is a derivative of acetylene that is used to make polyethylene polymers. Production of chemicals. Acetylene is also used to make a variety of inorganic chemicals, including vitamins A and E.

Manufacturing Products are made in industries.

Because of its capacity to undergo various chemical reactions, acetylene, also known as ethyne, is useful in the creation of products in industries.

Anti-tumor Agents

When used in pharmaceuticals, many alkynes are hazardous to humans. Specific alkynes known as ene-diynes, on the other hand, have a potent and aggressive anti-tumor chemical. An example of an anti-tumor acting agent is calicheamicin.

Production of a hot flame

Oxy-acetylene, also known as oxyethylene, is a gas made by combining ethyne with oxygen to produce high flames for cutting and welding.

Lamps provide illumination

Individual lamps, such as the hunter's and miner's lamp, are lit by ethyne.

Polypropene Nitrile production

Propenonitrile polymerizes in the presence of a peroxide initiator, yielding a substantial synthetic fibre known as polypropene nitrile.

Neoprene manufacturing

Neoprene is a synthetic rubber that will not burn or absorb oils.

Production of pharmaceuticals

Many medications on the market, such as the antiviral efavirenz and the antifungal terbinafine, include alkynes.

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Things to Remember

• Alkynes are valuable reagents in the synthesis of polymers, which is a major industrial use.
• It is an unsaturated hydrocarbon with at least one carbon—carbon triple bond between two carbon atoms.
• The chain is numbered to keep the number of double or triple bonds to a minimum.
• The structure of these compounds contains one in number σ bond and two feeble π bonds.
• Functional, ring chain or positional isomerism are types of isomerism of alkynes.