Alkynes: Preparation, Nomenclature and Properties

Alkynes: Hydrocarbons are the most basic of the organic molecules that are predominantly made up of Carbon and Hydrogen. Hydrocarbons are found in nature in the form of things we regularly use. It is for this reason that they are considered as the parent organic compounds.

All other compounds are created when one or more Hydrogen atoms are replaced with functional groups. Read this article to find out about Alkynes which are unsaturated hydrocarbons with at least a triple bond between carbon atoms.

What are Alkynes?

In organic chemistry, alkynes are the unsaturated hydrocarbons having at least a triple bond between carbon-carbon atoms \({\rm{( – C}} \equiv {\rm{C – )}}{\rm{.}}\) The general formula of this unsaturated hydrocarbon is \({{\rm{C}}_{\rm{n}}}{{\rm{H}}_{{\rm{2n – 2}}}},\) where \({\rm{n – 2,3,4,5}}…….\)

Alkynes are traditionally known as acetyls. Like other hydrocarbons, alkynes are also hydrophobic.

Structure of Alkynes

A carbon-carbon triple bond consists of one strong \({\rm{\sigma }}\) bond and two weak \({\rm{\pi }}\) bonds.

Nomenclature of Alkynes

The general formula of this unsaturated hydrocarbon is \({{\rm{C}}_{\rm{n}}}{{\rm{H}}_{{\rm{2n – 2}}}},\) where \({\rm{n – 2,3,4,5}}…….\) According to the IUPAC system, the names of alkenes have a suffix-yne, and the prefix depends on the number of carbon atoms. Some names of the alkenes are as follows:

The chain’s numbering is always done to give the lowest number to the carbon atom carrying the triple bond. A few examples are shown below:

Isomerism in Alkynes

Alkynes show the following structural isomerism:

1. Chain Isomerism

Alkynes having five or more carbon atoms show chain isomerism due to different carbon chain structures. For example,

2. Position Isomerism

In this, the isomers differ with respect to the position of the triple bond. For example, the two isomers of butene-

3. Functional Isomerism

Alkynes are functional isomers of dienes-the compounds containing two double bonds.

4. Ring Chain Isomerism

Alkynes show ring chain isomerism with cycloalkanes. For example,

Methods of Preparation of Alkynes

The important methods of preparation of alkynes are as follows:

1. By the Action of Water on Calcium Carbide

\({\rm{Ca}}{{\rm{C}}_2} + 2{{\rm{H}}_2}{\rm{O}} \to {\rm{HC}} \equiv {\rm{CH}} + {\rm{Ca}}{({\rm{OH}})_2}\)

Calcium carbide needed for this purpose is manufactured by heating limestone with coke in an electric furnace at \({\rm{2275}}\,{\rm{K}}{\rm{.}}\)

Procedure: Small lumps of calcium carbide are placed on a layer of sand in a conical flask which is provided with a dropping funnel and a delivery tube for the gases to escape. The air present in the glass flask is replaced by oil gas since acetylene forms an explosive mixture with air. Water is now dropped from the dropping funnel, and the acetylene gas thus formed is collected over water.

The obtained gas contains impurities of hydrogen sulfide and phosphine. Hydrogen sulfide is removed by bubbling the gas through an acidified copper sulfate, while phosphine is removed by passing the gas through a suspension of bleaching powder.

2. By Dehydrogenation of Dihaloalkanes

Alkynes can be prepared by dehydrohalogenation of vicinal-di haloalkanes by heating them in an alcoholic solution of potassium hydroxide. For example,

3. By Dehalogenation of Tetrahalides

Tetra haloalkanes, when heated with zinc dust in methanol, undergo dehalogenation to yield alkynes. For example,

4. By Dehalogenation of Haloforms

Chloroform and iodoform on heating with silver powder undergo dehalogenation to form ethyne.

5. By Kolbe’s Electrolysis Reaction

Acetylene can be prepared by the electrolysis of a concentrated solution of sodium or potassium maleic acid or fumaric acid.

6. Synthesis from Carbon and Hydrogen

Ethyne can also be prepared by passing steam of hydrogen gas through an electric arc struck between two carbon electrodes. This is called Berthelot synthesis.

Physical Properties of Alkynes

1. Physical State– The first three members of alkynes are colorless gases. The following eight members are liquids, while higher alkynes are solids.

2. Smell– All alkynes are odorless. However, acetylene has a garlic smell due to the presence of the impurity phosphine.

3. Melting and Boiling Points- The melting and boiling points of alkynes increase with an increase in the molecular mass of the alkynes. However, the melting and boiling points of alkynes are higher than the corresponding alkanes and alkenes. This is because alkynes are linear and their molecules are more closely packed in space.

4. Solubility- Alkynes are soluble In non-polar solvents and insoluble in polar solvents like water.

5. Density- The densities of alkynes increase with the increase in molecular weight. However, they are lighter than water as their density range between \((0.69 – 0.77)\,{\rm{g}}\,{\rm{c}}{{\rm{m}}^{ – 3}}\)

Chemical Properties of Alkynes

Alkynes shows acidic character, addition, and polymerization reactions.

1. Acidic Nature

Alkynes are slightly electronegative. This is because the triple bonded carbon atoms in alkynes are sp hybridized, making it easier for them to attract the shared electron pair.
So when a strong base like \(\left( {{\rm{NaN}}{{\rm{H}}_{\rm{2}}}} \right)\) reacts with ethyne, and sodium acetylide is obtained along with the liberation of hydrogen \(\left( {{{\rm{H}}_{\rm{2}}}} \right)\) gas. But such reactions do not happen in alkanes and alkenes. 

2. Addition Reaction

Under suitable conditions, alkynes undergo hydration reactions. Alkynes react with halogens, hydrogen, and other such elements to give a saturated compound as a product. Since they have a triple bond, two hydrogen atoms or halides, or halogens can be added to their structure.

(i) Addition of Dihydrogen: The reaction occurs in the presence of a catalyst such as Nickel or Platinum, or Palladium. The addition of hydrogen to the alkyne results in the formation of an alkene.
\({{\rm{C}}_3}{{\rm{H}}_4} + 2{{\rm{H}}_2} \to {{\rm{C}}_3}{{\rm{H}}_8}\)

(ii) Addition of Halogens: The addition of halogens chlorine and bromine is carried in the presence of \({\rm{CC}}{{\rm{l}}_{\rm{4}}}\) solvents to form haloalkanes. The addition is completed in two steps and takes place according to Markownikov’s rule.

(iii) Addition of Water: In the presence of dilute \({{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4}(40\% )\) and very dilute \({\rm{HgS}}{{\rm{O}}_4}(1\% )\) solution, alkynes get hydrated at about \(330\,{\rm{K}}\) to form carbonyl compounds (aldehydes and ketones) as a product.

3. Oxidation Reaction

When heated in air, alkynes undergo combustion to form carbon dioxide and water vapors, accompanied by the release of a large amount of energy.

\({\rm{2HC}} \equiv {\rm{CH + 5}}{{\rm{O}}_{\rm{2}}} \to {\rm{4C}}{{\rm{O}}_{\rm{2}}}{\rm{ + 2}}{{\rm{H}}_{\rm{2}}}{\rm{O;\Delta H = – 1300}}\,{\rm{KJ}}\,{\rm{mo}}{{\rm{l}}^{{\rm{ – 1}}}}\)

4. Polymerization

Alkynes can undergo linear and cyclic polymerization under suitable conditions. They polymerize to give compounds that have a higher molecular weight than the original alkyne.

(i) Linear Polymerization: Ethyne polymerizes to give polymer polyacetylene \({{\rm{(CH = CH – CH = CH)}}_{\rm{n}}}{\rm{.}}\) It is of higher molecular weight polythene.

(ii) Cyclic Polymerization: It occurs at high temperatures and in the presence of a catalyst. For example, passing ethyne through a red-hot iron tube at a minimum of \(877\,{\rm{K}}\) gives benzene.

Uses of Alkynes

Let’s look at some of the uses of Alkynes:

1. Oxyacetylene flame is used in gas welding.
2. Acetylene is used as an illuminant in Hawker’s lamp and lighthouses.
3. Acetylene is used in the artificial ripening of fruits.
4. Acetylene is used to prepare several useful compounds like acetaldehydes, acetic acid, ethyl alcohol, synthetic rubber, etc.
5. Acetylene and its derivatives are widely used in organic chemistry for the synthesis of cis and trans alkenes, methyl ketones, etc.

Points to Remember

(i) Alkynes start from ethyne as the structure methyne is not possible.
(ii) Alkynes show structural isomerism.

Summary

Through this article, we studied that Alkynes are unsaturated hydrocarbons with at least one triple bond between carbon atoms. Also, we learned about the structure of alkynes- how sigma and pi-bonds are placed. We now know some important preparation methods like Kolbe’s-electrolysis method, preparation from calcium carbide, etc., and physical and chemical properties of alkynes.

Frequently Asked Questions (FAQ) about Alkynes

Let’s look at some of the commonly asked questions about Alkynes:

Q. What is an alkyne in chemistry?
Ans: In organic chemistry, alkynes are the unsaturated hydrocarbons having at least a triple bond between carbon-carbon atoms (–C≡C–).(–C≡C–).The general formula of this unsaturated hydrocarbon is
CnH2n–2 where n = 2,3,4,5…. Alkynes are traditionally known as acetyls. Like other hydrocarbons, alkynes are also hydrophobic.

Q. What is the main use of alkynes?
Ans: The main compound of alkynes is Acetylene or Ethyne. Some of the uses are:
Oxyacetylene flame is used in gas welding.
Acetylene is used as an illuminant in Hawker’s lamp and in lighthouses.
Acetylene is used in the artificial ripening of fruits.
Acetylene is used to prepare many useful compounds like acetaldehydes, acetic acid, ethyl alcohol, synthetic rubber, etc.
Acetylene and its derivatives are widely used in organic chemistry to synthesize cis and trans alkenes, methyl ketones, etc.

Q. In the ozonolysis reaction, acetylene forms?
Ans: Glyoxal, Formic Acid.

Q. What are the first ten alkynes?
Ans: The first ten alkynes are as follows:

Q. What are the properties of alkynes?
Ans: The first three members of alkynes are colourless gases, the following eight members are liquids, while higher alkynes are solids, they are odourless, they are insoluble in water but soluble in organic solvents as they are non-polar molecules, they have high melting and boiling points, and they are lighter than water.

Q. KMnO₄ oxidizes Acetylene to form?
Ans: Oxalic Acid.

Q. What is formed when Ethyne passes through a red hot tube?
Ans: When Ethyne passes through a red hot tube, Benzene is formed.