Acetylene, also called Ethyne, is the simplest compound of the alkyne series. It is the best-known member of the alkyne series consisting of triple bonds. Acetylene Formula is \({{\rm{C}}_{\rm{2}}}{{\rm{H}}_{\rm{2}}}\). It is a colourless, flammable gas widely used as a fuel in oxy-acetylene welding and cutting of metals and as raw material in the synthesis of many organic chemicals and plastics.

Acetylene Formula Structure

Acetylene Formula Ethyne

Acetylene formula structure is only composed of carbon and hydrogen atoms, so it is classified as a hydrocarbon. It is a two-carbon containing unsaturated hydrocarbon consisting of a triple bond between carbon atoms and two sigma bonds with hydrogen atoms.

The parent name is Eth-  and for a triple bond, the suffix ‘-yne’ is added. Hence, the IUPAC name is Ethyne. Its chemical formula is \({{\rm{C}}_{\rm{2}}}{{\rm{H}}_{\rm{2}}}\).

Acetylene Molar Mass

The molar mass of Ethyne, \({{\rm{C}}_{\rm{2}}}{{\rm{H}}_{\rm{2}}}\, = \,2\, \times \,({\rm{Atomic}}\,{\rm{mass}}\,{\rm{of}}\,{\rm{Carbon}}) + 2 \times ({\rm{Atomic}}\,{\rm{mass}}\,{\rm{of}}\,{\rm{hydrozen}})\)

\( = \,2\,(12.01)\,\, + \,2\,(1.007) = 26.04\,{\rm{g/mol}}\)

Hence, one mole of Acetylene formula Ethyne weighs \(26.04\,{\rm{grams}}\)

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Acetylene Formula Hybridisation

In Ethyne, each carbon atom is sp hybridised. The ground state of Carbon has two electrons in its \(2{\rm{s}}\) orbital and \(1\) electron each in \(2{\rm{Px}}\) and \({\rm{2Py}}\) orbital. The \(2{\rm{Pz}}\) orbital is empty. However, in its excited state, one paired electron from the \(2{\rm{s}}\) orbital jumps to occupy the empty \(2{\rm{Pz}}\) orbital. Hence, there are four orbitals, \(2{\rm{s}},\,\,2{\rm{Px,}}\,2{\rm{Py}}\) and \(2{\rm{Pz}}\), that are singly paired and readily accept an electron from other atoms. However, there lies a difference in the manner they overlap with each other.

In sp acetylene formula hybridisation,  the single electron present in the \(2{\rm{Py}}\) and \({\rm{2Pz}}\) orbital do not undergo hybridisation. Instead, they laterally overlap with the unhybridised \(2{\rm{Pz}}\) and \({\rm{2Py}}\) atomic orbital of another carbon atom.

This results in the formation of a \({\rm{C – C}}\) triple bond. In \({\rm{C – C}}\) triple bond:-

1. a sigma \((\sigma )\) bond is formed due to the overlap of one sp orbital of the carbon atom,
2. a pi \((\pi )\) bond is formed due to the lateral overlapping of unhybridised \(2{\rm{Py}}\) orbitals, and
3. a pi \((\pi )\) bond is formed due to the lateral overlapping of unhybridised \(2{\rm{Pz}}\) orbitals.

The \(2\,{\rm{sp}}\) hybrid orbitals in Ethyne \(({{\rm{C}}_{\rm{2}}}{{\rm{H}}_{\rm{2}}})\) form a linear structure with a bond angle of \(180\) degrees.

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Molecular Geometry

All the atoms in Ethyne lie in the same plane, and there is no asymmetry in the molecule. Hence,\({{\rm{C}}_2}{{\rm{H}}_2}\) has linear molecular geometry. 

Bond Angle

Acetylene is an unsaturated hydrocarbon. This is because its two carbon atoms are bonded through a triple bond. The carbon-carbon triple bond places all four atoms in the same straight line. This results in Acetylene having bond angles of \({180^ \circ }\).

Dipole Moment

\({{\rm{C}}_{\rm{2}}}{{\rm{H}}_{\rm{2}}}\) is made up of two types of atoms: Carbon and Hydrogen. The difference in electronegativity between Carbon and Hydrogen is almost \(0.35,\) which is less than \(0.4.\) Hence, \({\rm{C – H}}\) bonds are nonpolar. Moreover, the atoms in Ethyne are arranged in a linear geometry, which means the distribution is even on both sides. So, even if there is a dipole moment, it will get nullified due to the opposite directions. Hence, ethyne is a nonpolar molecule.

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Acetylene Lewis Structure

The carbon atom has four valence electrons in its outer shell. In ethyne, there are two carbon atoms. Hence there are eight valence electrons for Carbon atoms in \({{\rm{C}}_{\rm{2}}}{{\rm{H}}_{\rm{2}}}\)
A Hydrogen atom has one valence electron in its outer shell. Hence, there are two valence electrons for Hydrogen atoms. 
Valence electrons in \({{\rm{C}}_2}{{\rm{H}}_2} = 8 + 2 = 10\)

There are ten valence electrons in \({{\rm{C}}_2}{{\rm{H}}_2}\) molecule.
In \({{\rm{C}}_2}{{\rm{H}}_2}\) Acetylene Lewis structure, both the carbon atoms are placed at the centre as carbon atoms have higher valency than the hydrogen atoms. Hence, both the carbon atoms take the central position, and the hydrogen atoms are arranged around it.

Both the hydrogen atoms will share one valence electron of the carbon atom and form a bond.

The duplets of both the hydrogen atoms are now complete, but the octet of carbon atoms is not complete. To attain a stable structure, the Carbon atoms will share their remaining three valence electrons by forming a triple bond.

Out of \(10\) valence electrons, a total of six valence electrons is used to form a triple bond between both the carbon atoms and four electrons are used in single bond formation between carbon and hydrogen atoms.
In Lewis structure of \({{\rm{C}}_2}{{\rm{H}}_2}\) the octets of all the atoms are complete, and there are no lone pairs of electrons in the molecule.

Skeletal Structure

A chemical structure of a molecule includes the arrangement of atoms and the chemical bonds that hold the atoms together. The Ethyne molecule contains two types of bond \(({\rm{s}})\)- triple bond \((1)\) and sigma bonds \((2)\). The carbon atoms in the chemical structure of Ethyne are arranged in a straight line, and hydrogen atoms attached to carbon atoms are present at the terminals.

Preparation of Ethyne

In the laboratory, Ethyne is prepared from calcium carbide. Calcium carbide on reaction with water produces ethyne gas.

\({\rm{Ca}}{{\rm{C}}_{\rm{2}}}{\rm{(s) + 2}}{{\rm{H}}_{\rm{2}}}{\rm{O(l)}} \to {{\rm{C}}_{\rm{2}}}{{\rm{H}}_{\rm{2}}}{\rm{(g) + Ca(OH}}{{\rm{)}}_{\rm{2}}}{\rm{(aq)}}\)

Commercial Preparation

Ethyne is prepared commercially by thermal decomposition or pyrolysis of methane or natural gas at \(1500\,^\circ {\rm{C}}.\)

Physical Properties

The following are the physical properties of Acetylene.

1. Ethyne is a Colourless, poisonous gas.
2. It resembles the odour of ether, i.e., a faint garlic odour.
3. It is lighter than air.
4. It is slightly soluble in water and completely soluble in organic solvents
5. Ethyne has a Melting point around \(- 82\,^\circ {\rm{C}}\) and a boiling point around \(- 75\,^\circ {\rm{C}}\)
6. It has a sublimation temperature of  \(- 84\,^\circ {\rm{C}}\)

Chemical Properties

\( – {\rm{C}}\, \equiv \,{\rm{C – }}\) is the site of most chemical reactions in Ethyne.

Ethyne readily undergoes electrophilic addition reactions due to a high degree of unsaturation.

A molecule of Ethyne is completely saturated by accepting a maximum of four atoms of hydrogen or its equivalent.

\({\rm{HC}}\, \equiv \,{\rm{CH + AB}}\, \to \,{\rm{AHC}}\,{\rm{ = }}\,{\rm{CHB}}\,{\rm{ + AB}}\, \to {{\rm{A}}_2}{\rm{HC – CH}}{{\rm{B}}_2}\)

Note:

The carbon atoms in Ethyne are similar. Hence \({\rm{AB}}\) can add up to any of the carbon atoms.

The reaction occurs in two stages – stage \(1\) is the formation of the corresponding alkene derivatives, while the second stage is the formation of the completely saturated alkane derivatives

(a) With hydrogen – Ethyne reacts with excess hydrogen in the presence of nickel catalyst at about \(200\,^\circ {\rm{C}}\) to form a completely saturated hydrocarbon, ethane.

\({{\rm{C}}_{\rm{2}}}{{\rm{H}}_{\rm{2}}} + {{\rm{H}}_2} \to {{\rm{H}}_2}{\rm{C}} = {\rm{C}}{{\rm{H}}_2} \to {{\rm{C}}_2}{{\rm{H}}_6}\)

In a limited supply of hydrogen, the reaction proceeds till the production of ethene (i.e., stage1).

(b). With the halogens – In the presence of a catalyst (metallic halide- \({\rm{FeC}}{{\rm{l}}_{\rm{3}}}\) ethyne combines rapidly with chlorine and bromine to produce halogenated compounds.The catalyst is used to polarise the halogen molecule and enhance its electrophilic character.

(c). With halogen acids or hydrogen halides: Ethyne reacts very readily with excess hydrogen iodide at room temperature to form \(1,\,1 – \) diiodoethane.

\({{\rm{C}}_2}{{\rm{H}}_2}{\rm{ + HI}} \to {\rm{IHC}}{\mkern 1mu} {\rm{ = }}{\mkern 1mu} {\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{ + HI}} \to {{\rm{I}}_{\rm{2}}}{\rm{HC – C}}{{\rm{H}}_{\rm{3}}}\)

Note:

For the second stage of reaction, the electronegative element of HI is added to the Carbon with a lower number of hydrogen. The order of reactivity of hydrogen halide with alkyne is \({\rm{HI}}\, > \,{\rm{HBr}}\, > {\rm{HCI}}\)

This is because the \({\rm{H – I}}\) bond is the easiest to break than \({\rm{HBr}}\) and \({\rm{H – CI}}\) Hence \({\rm{HBr}}\) reacts faster than \({\rm{HCI}}\).

(d). With water

Ethyne reacts with water in the presence of dilute \({{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4}\) and mercury \(({\rm{II}})\) tetraoxosulphate \(({\rm{VI}})\) as a catalyst at about \(60\,^\circ {\rm{C}}\) to produce Ethanal (acetaldehyde). This reaction gives the industrial preparation of Ethanal, which can then proceed to produce ethanoic acid.

\({{\rm{C}}_2}{{\rm{H}}_2}\, + \,{{\rm{H}}_2}{\rm{O }} \to \,{\rm{C}}{{\rm{H}}_3}{\rm{CHO}}\)

(e). With potassium tetraoxomanganate \(({\rm{VII}})\)

Ethyne, being unsaturated like the alkenes, decolourises acidified solution of \({\rm{KMn}}{{\rm{O}}_4}\) , while the alkaline solution of \({\rm{KMn}}{{\rm{O}}_4}\) is changed to green at room temperature. This reaction distinguishes Ethyne (i.e., alkynes) from all gaseous alkanes, e.g., methane and ethane, which do not react with \({\rm{KMn}}{{\rm{O}}_4}\) solution.

\({\rm{KMn}}{{\rm{O}}_4}\) oxidises Ethyne to ethanedioic acid (oxalic acid).

(f) With bromine water: Ethyne decolourises the reddish-brown colour of bromine vapour. Alkanes do not undergo any such change. Hence this is a more reliable test for an unsaturated compound than the use of \({\rm{KMn}}{{\rm{O}}_4}\) solution (because any reducing agent which might not be unsaturated can also decolourise \({\rm{KMn}}{{\rm{O}}_4}\)).

(g) Oxidation of Ethyne
Ethyne is oxidised to form Oxalic acid in the presence of dilute. \({\rm{KMn}}{{\rm{O}}_4}.\)

(h) Ozonolysis
Ethyne reacts with ozone to produce acetylene ozonide, which is further hydrolysed in the presence of Zinc to form Glyoxal.

Reactions of Acetylene

Let us go through some reaction properties of Acetylene:

Substitution Reaction in Acetylene

Acetylene can also undergo substitution reactions. This type of reaction occur with sodium in liquid ammonia and with certain salts of heavy metals to form metallic derivatives:

1. Ethyne reacts with sodium in liquid ammonia to displace hydrogen. This reaction accounts for the acidic behaviour of the terminal alkynes.

\(2{\rm{H}} – {\rm{C}} \equiv {\rm{C}} – {\rm{H}} + ({\rm{liq}}.{\rm{N}}{{\rm{H}}_3}){\rm{Na}} \to 2{\rm{HC}} \equiv {{\rm{C}}^ – }{\rm{N}}{{\rm{a}}^ + } + {{\rm{H}}_2}\)

2. Ethyne reacts with an ammoniacal solution of copper \(({\rm{I}})\) chloride at room temperature to form a reddish-brown precipitate of copper \(({\rm{I}})\) dicarbide.

\({{\rm{C}}_2}{{\rm{H}}_2} + 2{\rm{CuCl}} \to {\rm{C}}{{\rm{u}}_2}{{\rm{C}}_2} + 2{\rm{HCl}}\)

3. Ethyne reacts with an ammonical solution of \({\rm{AgN}}{{\rm{O}}_3}\) at room temperature to form a whitish-yellow precipitate of silver dicarbide.

\({{\rm{C}}_2}{{\rm{H}}_2} + 2{\rm{AgN}}{{\rm{O}}_3} \to {\rm{A}}{{\rm{g}}_2}{{\rm{C}}_2} + 2{\rm{HN}}{{\rm{O}}_3}\)

Both copper \(({\rm{I}})\) ) dicarbide and silver dicarbide give off Ethyne when warmed with dilute acid.

Ethene does not form any metallic derivative. Hence, this reaction is used to distinguish ethyne from ethene.

All alkynes with a triple bond at the end of their carbon chain (i.e. terminal alkynes) would undergo similar substitution reactions.

Combustion of Ethyne

Ethyne burns readily in oxygen, releasing enormous heat, which can be used in cutting and welding metals. This flame is called oxy-acetylene flame, used by welders.

The products of the reaction are carbon dioxide and steam.

\({\rm{2}}{{\rm{C}}_2}{{\rm{H}}_2} + 5{{\rm{O}}_2} \to 4{\rm{C}}{{\rm{O}}_2} + 2{{\rm{H}}_2}{\rm{O}}\)

Polymerisation of Ethyne

When ethyne gas is passed through the red hot iron tube at \({400^ \circ }{\rm{C}}\) it leads to the formation of a cyclic aromatic compound.

Three molecules of ethyne gas undergo cyclic polymerisation to produce an aromatic compound known as benzene.

Uses of Acetylene

The following are the uses of Acetylene:

1. Ethyne is used in artificial ripening and preservation of fruits.
2. Ethyne reacts with hydrogen chloride to form chloroethene, a Vinyl chloride \(({{\rm{H}}_2}{\rm{C = CHCl)}}\) The vinyl chloride is polymerised to give a plastic product that is used in electrical insulation, water-proofing and in the manufacture of pipes.

3. For the production of hot flame:
Ethyne is widely used in producing oxy-acetylene flame used for cutting and welding metals. Ethyne reacts with oxygen to produce carbon dioxide and steam. Combustion of Ethyne is a highly exothermic reaction that releases an enormous amount of heat. This heat is used for cutting and welding metals.

4. As a source of important organic compounds:
Ethyne reacts with chlorine to form \(1,1,2,2 – \) tetrachloroethane used as a solvent for grease, fats, and oil.

5. Certain lamps, e.g. hunter’s and miner’s lamps, are fuelled by Ethyne.

6. In the production of polypropenonitrile:
Propenonitrile \(({\rm{C}}{{\rm{H}}_2} = {\rm{CHCN}})\) polymerises in the presence of a peroxide initiator to produce important synthetic fibre known as polypropenonitrile.
\(… – {\rm{C}}{{\rm{H}}_2} – {\rm{CHCN – C}}{{\rm{H}}_2} – {\rm{CHCN – }}…\)

7. Ethyne is used in the production of neoprene (polymer of \(2 – \)-chlorobuta- \(1,3\)-diene). Neoprene is an artificial rubber that does not burn or is attacked by oils.

Chemical Tests to Distinguish Between Alkanes, Alkenes and Alkynes

Listed below are the Chemical Tests to Distinguish Between Alkanes, Alkenes and Alkynes:

Test	Alkane	Alkene	Alkyne
Few drops of bromine solution are added to the hydrocarbon	No change observed	Decolourisation of bromine solution	Decolourisation of bromine solution
Add a few drops of alkaline. \({\rm{KMn}}{{\rm{O}}_4}\)	No change observed	Fade in the purple colour observed.	Fade in the purple colour observed.
Adding a few drops of ammonia silver nitrate	No change	No change	A white precipitate of silver acetylide is formed.
Adding a few drops of ammoniacal cuprous chloride	No change observed	No chaange is observed	A red precipitate of copper acetylide is formed.

Summary

Acetylene is the simplest alkyne, an important organic compound used extensively in welding and cutting metals. Hence, it is essential to learn about its structure. This article discussed concepts on Acetylene, starting from its formula to its structure and moving through its polarity towards its physical and chemical properties. We also learnt its uses and basic difference with its corresponding alkane and alkene.

PRACTICE QUESTIONS RELATED TO ACTYLENE

FAQs

Here are some FAQs on Acetylene formula:

Q.1. Why is Ethyne called Acetylene?
A.1. The name was invented by French chemist Marcelin-Pierre-Eugène Berthelot \(1823 – 1907\) in \(1864\) from the French word acétylène. It was derived from the chemical ending acetyl + ene, coined by German chemist Justus von Liebig in \(1839.\)

Q.2. Are Acetylene and ethylene the same?
A.2.The key difference between Acetylene and ethylene is that Acetylene has a triple bond between two carbon atoms, whereas ethylene has a double bond between two carbon atoms. The IUPAC name of Acetylene is Ethyne, whereas, for ethylene, it is ethene. The names acetylene and ethylene sound similar, but they have different homologous series.

Q.3. Why is Acetylene so dangerous?
A.3. Acetylene is an extremely flammable gas and can form an explosive atmosphere in the presence of air or oxygen. High pressure or temperatures can result in decomposition that can result in fire or explosion.

Q.4. Does Acetylene burn without oxygen?
A.4. On decomposition, Acetylene breaks down into its constituent elements, Carbon and hydrogen. This reaction gives out a lot of heat, which can cause the gas to ignite without the presence of air or oxygen effectively.

Q.5. What pressure should oxygen and Acetylene be set at for cutting?
A.5. The gas pressures should be set at \(3\) to \(5\) psi for Acetylene and \(20\) to \(30\) psi for the oxygen.

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