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Alkynes

Last Updated on By Mrs Shilpi Nagpal

Alkynes or Acetylene

Acyclic unsaturated hydrocarbons containing a carbon-carbon triple bond are called alkynes or acetylenes. Their general formula is CnH2n-2 where n=2,3,4….etc. 

Structure of Triple bond 

Structure of triple bond

Ethyne is the first member of alkynes series.

Each carbon atom of ethyne is sp-hybridized and hence has two sp-hybridized orbitals. One sp-hybridized orbital of each carbon undergoes head on overlap with sp-hybridized orbital of another carbon to form a sp-sp, C-C, σ – bond. 

The second sp-hybridized orbital of each carbon overlaps along the internuclear axis with 1s-orbital of each of the two hydrogen atoms forming two sp-s, C-H, σ-bonds. Each carbon is now left with two unhybridized p-orbitals (2px and 2py) which are perpendicular to each other.

The two 2px-orbitals, one on each carbon, are parallel to each other and hence overlap sideways to form a π bond. Similar overlap between 2py orbitals, one on each carbon, results in the formation of a second π bond.

Since a p-orbital has two lobes, the electron cloud of a π-bond has two halves. If the two halves of one π bond are considered to lie in the plane of the paper, then one of the two halves of the second π-bond would lie above the plane of the paper and the other below the plane of the paper.

The four halves of the electron clouds of two π-bonds do not stay as such but merge together to form a single electron cloud which has cylindrical symmetry about the internuclear axis. It is because of this cylindrical symmetry of the electron cloud between two carbon atoms that ethyne is a linear molecule with H-C-C bond angle of 180°.

Carbon carbon triple bond consists of one strong σ – bond and two weak π-bonds. Due to smaller size of sp-orbitals and sideways overlap of p-orbitals, the carbon-carbon bond length in ethyne is shorter than those of C = C and C -C .

Alkynes are less reactive than alkenes towards addition reactions. Alkynes unlike alkenes do not exhibit geometrical isomerism due to their linear structure.

Isomerism in Alkynes

Position isomerism

The first two members i.e., ethyne and propyne exist in one form only. Butyne and higher alkynes exhibit position isomerism due to different positions of the triple bond on the carbon chain.

CH3-CH2-C≡CH  But-1-yne

CH3-C≡C-CH3    But-2-yne

Chain isomerism 

Alkynes having or five or more carbon atoms show isomerism due to different structures of the carbon chain.

CH3-CH2-CH2-C≡CH Pent-1-yne

3-Methylbut-1-yne

3-Methylbut-1-yne

Functional isomerism 

Alkynes are functional isomers of dienes i.e. compounds containing two double bonds.

CH3-CH2-C≡CH But-1-yne

CH2=CH-CH=CH2 But-1,3-dienes

CH2=C=CH-CH3 But-1,2-diene

Ring chain isomerism 

Alkynes show ring chain isomerism with cycloalkenes.

CH3-C≡CH Propyne

Cyclopropene

Classification of Alkynes

Alkynes are further classified as terminal or non-terminal alkynes according as the triple bond is present at the carbon chain or within the carbon chain.

Terminal alkynes

CH3C≡CH Propyne

CH2-CH2-C≡CH But-1-yne

Non-Terminal alkynes

CH3-C≡C-CH3 But-2-yne

CH3-C≡C-CH2-CH3 Pent-2-yne 

Methods of preparation of Alkynes 

1.By the action of water on calcium carbide 

Ethyne (acetylene) is prepared in the laboratory as well as on a commercial scale by the action of water on calcium carbide.

CaC2 + 2 H2O → HC≡CH + Ca(OH)2

Calcium carbide needed for the purpose is manufactured by heating limestone (calcium carbonate) with coke in an electric furnace at 2275 K.

CaCO3 → CaO + CO2

Procedure

Lumps of calcium carbide are placed on a layer of sand in a conical flask fitted with a dropping funnel and a delivery tube. The air present in the flask is replaced by oil gas since acetylene forms an explosive mixture with air. Water is now dropped from dropping funnel and the acetylene gas thus formed is collected over water.
Laboratory preparation of acetylene
 
Purification

Acetylene gas contains impurities of hydrogen sulphide and phosphine due to the contaminations of calcium sulphide and calcium phosphide in calcium carbide. Hydrogen sulphide is removed by bubbling the gas through an acidified solution of copper sulphate while phosphine is removed by passing the gas through an acidified solution of copper sulphate while phosphine is removed by passing the gas through a suspension of bleaching powder. Pure acetylene is finally collected over water.
 

2. By dehydrohalogenation of dihaloalkanes 

Alkynes are prepared by  dehydrohalogenation of vicinal dihaloalkanes by heating them with an alcoholic solution of potassium hydroxide.

BrCH2-CH2Br +2 KOH(alc) → CH≡CH +2 KBr + 2 H2O

The reaction, occurs in two steps and each step involves the loss of a molecule HBr.

BrCH2-CH2Br +KOH(alc) → CH2=CHBr + KBr + H2O

CH2=CHBr + KOH (alc) → CH≡CH +KBr + H2O

In ethylene dibromide, Br is present on a saturated carbon atom. Therefore, like alkyl halides, it is a reactive molecule. On heating with alcoholic KOH, it readily eliminates a molecule of HBr to form vinyl bromide in good yield. Due to the presence of Br on a doubly bonded carbon atom, vinyl bromide is a highly unreactive molecule and hence on heating with alcoholic KOH, it does not easily loss a molecule of HBr to form acetylene.

With alcoholic KOH, the yield of acetylene is low. Therefore to obtain acetylene in fairly good yield from vinyl bromide, a much stronger base than alcoholic KOH such as NaNH2 in liquid NH3 is usually used. Dehydrohalogenation of ethylene dibromide to acetylene is preferably carried out in the following two stages.

The reaction is usually carried out in one step using NaNH2 in liquid NH3.

BrCH2-CH2Br + 2 NaNH2 → CH≡CH + 2NaBr +2 NH3

CH3– CHBr -CH2Br + 2 NaNH2 → CH3-C≡CH + 2NaBr +2 NH3
 
Alkynes can also be prepared from gem-dihalides by the action of alcoholic KOH followed by treatment with NaNH2 in liquid NH3 or preferably by the action of sodamide in liquid NH3.


CH3-CHBr2 → CH2=CHBr   in presence of KOH(alc)

CH2=CHBr → HC≡CH   in presence of NaNH2 / liq NH3

CH3-CHBr2 +2 NaNH2 → HC≡CH  + 2NaBr +2 NH3 in presence of liq NH3 at 196 K

3. By dehalogenation of terahalides

Tetrahaloalkanes when heated with zinc dust in methanol undergo dehalogenation to yield alkynes.
CHBr2 -CHBr2 +2 Zn → HC≡CH +2 ZnBr2

1,1,2,2-tetrabromoethane

4.  By dehalogenation of haloforms 

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

CHCl3 +6 Ag + Cl3CH → HC≡CH  + 6 AgCl
CHI3 +6 Ag + I3CH → HC≡CH  + 6 AgI

5. Kolbe’s electrolytic reaction

Acetylene can be prepared by electrolysis of a concentrated solution of sodium or potassium salt of maleic or fumaric acid.
Kolbe's Electrolytic reaction

6. Synthesis from carbon and hydrogen

Acetylene can be prepared by passing a stream of hydrogen through an electric are struck between carbon electrodes.
2C + H2 → HC≡CH

7. Synthesis of higher alkynes from acetylene 

Acetylene is first treated with sodium metal at 475 K or with sodamide in liquid ammonia at 196 K to form sodium acetylide. This upon treatment with alkyl halides gives higher alkynes.

HC≡CH + NaNH2 → HC≡C¯Na+ + NH3

HC≡C¯Na+ +CH3Br → HC≡C-CH3 + NaBr

HC≡C¯Na+ + CH3CH2-I → HC≡C-CH2CH3 + NaI

CH≡CH → Na+¯C≡C¯Na+ → CH3-C≡C-CH3

Physical properties of Alkynes 

 (i) Physical state: The first three members of this family (ethyne, propyne and butyne) are colourless gases, the next eight are liquids while the higher ones are solids.

(ii) Smell: All the Alkynes are odourless. However, acetylene has garlic smell due to the presence of phosphine as impurity.

(iii) Melting and boiling points The melting points and boiling points of alkynes are slightly higher than those of the corresponding alkenes. This is probably due to the reason that because of the presence of a triple bond, alkynes have linear structures and hence their molecules can be more closely packed in the crystal lattice as compared to those of corresponding alkenes and alkanes.

(iv) Solubility Alkynes like alkanes being non – polar are insoluble in water but readily dissolve in organic solvents such as petroleum ether, benzene, carbon, tetrachloride, etc.

(v) Density Densities of alkynes like those of alkenes and alkanes increase as the molecular size increases. However, they are all lighter than water since their densities lie in the range 0.69  – 0.77  g/cm3. 

Reactivity of alkynes versus Alkenes 

Alkynes contain two π-bonds, yet they are less reactive than alkenes towards electrophilic addition reactions. Most of the addition reactions of alkynes are catalysed by heavy metal ions such Hg2+, Ba2+, etc while no such catalysts are needed in case of electrophilic
additions to alkenes. This lower reactivity of alkynes as compared to alkenes towards electrophilic addition reactions is due to the following two reasons :
 
(i) Due to greater electronegativity of sp-hybridized carbon atoms of a double bond, the π-electrons of alkynes are more tightly held by the carbon atoms than π- electrons of alkenes and hence are less easily available for reaction with electrophiles. As a result, alkynes are less reactive towards electrophilic addition reaction.
 
(ii) Due to cylindrical nature of the π- electron cloud of alkynes, the π-electrons of a triple bond are more deloacalized than π-electrons of a double bond. In other words, the π-electrons of a triple bond are less readily available for addition reactions than those of the double bond. Consequently alkynes are less reactive than alkenes. 

Chemical Reaction Of alkynes

Acidic character of Alkynes 

The hydrogen atoms attached to the the triple bond of alkynes are acidic in nature.

(1) Formation of alkali metal acetylides : Ethyne and other terminal alkynes or 1-alkynes react with strong bases such as sodium metal at 475 K or sodamide in liquid ammonia at 196 K to form sodium acetylides with evolution of H2 gas.

2HC≡CH +2 Na → 2CH≡C¯Na+ + H2

R-C≡CH + NaNH2 → R-C≡C¯Na+ + NH3

During these reactions, the acetylenic hydrogen is removed as a proton to form stable carbanions.

Sodium acetylide is decomposed by water regenerating acetylene. This shows that water is stronger acid than acetylene and thus displaces acetylene from sodium acetylene.

HC≡C¯Na+  + H2O → HC≡CH + NaOH


(2) Formation of heavy metal acetylene : Acetylenic hydrogens of alkynes can also be replaced by heavy metal ions such as Ag+ and Cu+ ions. When treated with ammoniacal silver nitrate solution (Tollen’s reagent), alkynes form white precipitate of silver acetylides.

CH≡CH + 2[Ag(NH3)2]+OH¯ → AgC≡CAg + 2 H2O + 4 NH3
R-C≡CH + 2[Ag(NH3)2]+OH¯ → R-C≡C-Ag + H2O + 2 NH3

With ammoniacal cuprous chloride solution, terminal alkynes form red ppt. of copper acetylides.

HC≡CH + 2[Cu(NH3)2]+OH¯ → CuC≡C-Cu + 2 H2O + 4 NH3R-C≡CH + [Cu(NH3)2]+OH¯ → R-C≡C-Cu +H2O + 2 NH3

Silver and copper acetylides are not decomposed by water. They can however, be decomposed with dilute mineral acids to regenerate the original alkynes.

AgC≡CAg +2 HNO3 → HC≡CH + 2 AgNO3

CuC≡CCu + 2 HCl → HC≡CH + 2 CuCl

(3) Formation of alkynyl Grignard reagent : Acetylene and other terminal alkynes react with Grignard reagents to form the corresponding alkynyl Grignard reagents.

HC≡CH + RMgX → HC≡CMgX + RH
R’-C≡CH + RMgX → R’-C≡CMgX + RH
 
Importance. The  formations of metal acetylides can be used :

(i) For separation and purification of terminal alkynes from non terminal alkynes, alkanes and alkenes.(ii) To distinguish terminal alkynes from non terminal alkynes or alkenes.

Electrophilic Addition Reactions

The electrophilic addition reactions of alkynes take place in two steps in two stages as shown below :

-C≡C- + X2 → -CX=CX-  + X2 → -X2C-CX2–
 
1. Addition of halogens : Chlorine and bromine readily add to alkynes first forming 1,2-dihaloalkenes and then 1,1,2,2-tetrahaloalkanes.

HC≡CH  + CCl2 →  ClHC≡CHCl + CCl2 → Cl2HC-CHCl2

CH3-C≡CH + Br2 → BrCH3C=CCH3Br + Br2 → Br2CH3C-CCH3Br2
 
During this reaction, the reddish brown colour of Br2 is decolourised and hence this reaction is used as a test for unsaturation i.e. for double and triple bonds.
However, if the reaction is carried out in ethanolic solution, the reaction stops after addition of one molecule of iodone.
HC≡CH + I2 → IHC=CHI in presence of C2H5OH
 

2. Addition of halogen halides or halogen acids : Halogen halides add to alkynes, their order of reactivity being HI > HBr  > HCI > HF . These additions occur in accordance with Markovnikov’s rule from first vinyl halides and then alkyliedene halides.

HC≡CH + HCl → CH2=CHCl + HCl → CH3CHCl2

When acetylene is passed through dil. HCl at 338  K in presence of Hg2+ ions as catalyst, only vinyl chloride is formed.

HC≡CH + HCl → CH2=CHCl

With  hydrogen bromide, first a vinyl bromide and then an alkylidene dibromide is formed. 


HC≡CH + HBr → CH2=CHBr + HBr → CH3CHBr2

CH3-C≡CH + HBr → BrCH3C=CH2 + HBr → (CH3)2CBr2

In presence of peroxides, anti- Markovnikov’s addition of HBr occurs.

 

CH3C≡CH → CH3CH=CHBr       in presence of HBr and RCOOR

CH3CH=CHBr →  CH3CH2CHBr2      in presence of HBr and RCOOR
Mechanism
mechanism of addition of halogen halides to alkynes

Since carbocation (I) is stablized by + R-effect of the X atom while carbocation (II) is destabilized by – I effect of X, therefore, reaction occurs through  the more stable carbocation (I) forming 1, 2-dihaloethane. 
3. Addition of the elements of hypohalous acids : Like alkenes, alkynes also add chlorine or bromine in presence of water. The overall reaction occurs in two stages, each stage involves the addition of the elements of the hypophalous acid to form dihalocarbonyl compound as the final product.

HC≡CH + HOCl → [Cl-CH=CH-OH] → [Cl2CH=CH(OH)2] → Cl2CH-CHO

CH3-C≡CH + HOCl → [CH3-COH=CHCl] → [CH3-C(OH)2-CCl2] → CH3-CO-CHCl2

4. Addition of H2O – Hydration of alkynes :Alkynes cannot be hydrated as easily as alkenes because of their lower reactivity towards electrophilic addition reactions. Further, dilute H2SO4 as catalyst, hydration occurs readily. Under these conditions, H2O adds to the triple bond to form an enol which readily tautomerises to the corresponding carbonyl compound.
Addition of water to alkynes

Unsymmetrical terminal alkynes, addition occurs in accordance with Markovnikov’s rule. If the unsymmetrical alkyne is non-terminal, a mixture of two isomeric ketones is obtained in which the methyl ketone predominates.

5. Addition of caboxylic acids : When acetylene is passed into warm acetic acid in presence of mercury salts, first vinyl acetate and then ethylidene diacetate is formed.

HC≡CH + CH3COOH → H2C=CH-OCOCH3 → CH3-CH(OCOCH3)2

Vinyl acetate is used for manufacture of vinyl resin. Ethylidene diacetate, when heated rapidly to 573 – 673 K, gives  acetic anhydride and acetaldehyde.

CH3-C-(OCOCH3)2 → (CH3COO)2O

6. Addition of hydrogen cyanide : Acetylene adds on hydrogen cyanide in presence of Ba(CN)2 or CuCl in HCl as catalyst or acrylonitrile.

HC≡CH + HCN → CH2=CH-CN in presence of Ba(CN)2 or CuCl/HCl

Nucleophilic Addition Reaction

 

Because of the greater electronegativity of the sp-hybridized carbons as compared to sp2-hybridized carbons, alkynes are more susceptible to nucleophilic addition reactions than alkenes. When acetylene is passed into methanol at 433-473 K in presence of a small amount (1-2%) of potassium methoxide under pressure, methyl vinyl ether is formed.

HC≡CH + CH3OH → CH2=CH-OCH3

Methyl vinyl ether is used for making polyvinyl ether plastics.

Reduction of Alkynes

Addition of dihydrogen to alkynes in presence of nickel at 523 – 573 gives alkanes.

HC≡CH + H2 → CH2=CH2 + H2 → CH3-CH3 in presence of Ni catalyst.

It is possible to stop the reduction at the alkene stage by using specific catalysts such as Lindlar’s catalyst i.e. Pd supported over CaCO3 or BaSO4 and partially poisoned by addition of sulphur or quinoline.

Catalytic reduction of alkynes with dihydrogen in presence of Lindlar’s catalyst always gives cis- alkenes.

Since during hydrogenation, the addition of dihydrogen occurs on the same face of the alkyne molecule.
Catalytic reduction of alkynes with dihydrogen in presence of Lindlar's catalyst

Chemical reduction i.e. Birch reduction of non-terminal alkynes with Na or Li in Liquid NH3 at 196-200 K gives trans-alkenes.
Birch reduction
Mechanism
Mechanism of Birch reduction

Oxidation Reaction of Alkynes

1. Oxidation with air or oxygen, Combustion

When heated in air, alkynes undergo combustion to form CO2 and H2O accompanied by release of large amount of heat and light energy.
2CH≡CH + 5 O2 → 4 CO2 + 2 H2O       ΔH= -1300 KJ/mol

Acetylene burns with a luminous yellow sooty flame due to the presence of higher carbon content than hydrogen. However, with air or oxygen under high pressure, acetylene burns with a blue flame (oxyacetylene flame) producing high temperature of the order of 3000 K which is used for cutting and welding of metals.

2. Oxidation with cold dilute potassium permanganate

Alkynes are readily oxidised by cold dilute alkaline KMnO4 solution to give α-dicarbonyl compounds i.e. 1,2-dialdehydes , 1,2-diketones, 2-oxoacids, or 1,2-dioic acids depending upon the nature of alkyne.

In case of terminal alkynes, ≡CH part is oxidised to -COOH group while in case of non terminal alkynes, ≡CR part is oxidised to R-C=O group.

CH3-C≡CH + 3[O] → CH3COOH + CO2   in presence of KMnO4 , H2O at 288-303K

CH3-C≡C-CH3 + 2 [O] → CH3COCOCH3  in presence of KMnO4 , H2O at 288-303K

HC≡CH + 2[O] → OHC-CHO +[O] → HOOC- COOH

The pink colour of the KMnO4 solution is discharged and a brown precipitation of manganese dioxide is obtained. This reaction is therefore used as test for unsaturation under the name Baeyer’s test.

3. Oxidation with hot KMnO4 solution

Terminal alkynes when treated with KMnO4 solution, undergo cleavage of the C≡C bond leading to the formation of carboxylic acids and carbon dioxide depending upon the nature of the alkyne. ≡CH is oxidised to CO2 and H2O and ≡CR is oxidised to RCOOH.

HC≡CH + 4 [O] → HOOC-COOH +[O] → 2CO2 + H2O in presence of KMnO4,KOH, 373-383 K

CH3-C≡CH +4[O] → CH3COOH + CO2 in presence of KMnO4,KOH, 373-383 K

Non-terminal alkynes on oxidation with hot KMnO4 solution give only carboxylic acids.

CH3-C≡CH + 4[O] → 2CH3COOH

CH3-CH2-C≡CH3 + 4[O] → CH3CH2COOH + HOOCCH3


Thus, by identifying the products formed during alkaline KMnO
oxidation, it is possible to determine the position of the triple bond in alkyne molecule.


4. Oxidation with ozone

Alkynes react with ozone in presence of some inert solvents such as CH2Cl2, CHCl3 or CCl4 at low temperature (196- 200 K) to form ozonides. These ozonides on decomposition with Zn dust and water or H2/Pd (reductive cleavage) give 1, 2-dicarbonyl compounds.

Oxidation with ozone
Reaction of But-2-yne with ozone

5. Polymerization Reactions of Alkynes

Alkynes also undergo polymerization reactions. But alkynes undergo two types of polymerization reactions.

(a) Linear Polymerization In this type of polymerization, two or more molecules of ethyne combine to form products which have linear structures.

(i) In presence of CuCl/NH4Cl, acetylene first gives vinylacetylene and then divinylacetylene.

2HC≡CH → HC≡CH=CH2 + HC≡CH → H2C=CH-C≡C-CH=CH2

Vinylacetylene is widely used in the manufacture of chloroprene which is the starting material for the synthetic rubber neoprene.

CH2=CH-C≡CH + HCl → CH2=CH-CCl =CH2
(ii) Polymerization of acetylene produces the linear polymer polyacetylene. It is a high molecular weight conjugated polyene containing the repeating unit. under proper conditions, this material conducts electricity. Thin films of polyacetylene can be used as electrodes in batteries. Further, since polyacetylenes have much higher conductance than metal conductors, these can be used to prepare lighter and cheaper batteries.

(b) Cyclic polymerization : In this type of polymerization, three or more molecules of an alkyne combine together to form products which have ring structures.

(i) When ethyne is passed through red hot iron tube at 873 K, it trimerises to give benzene.
Cyclic polymerization in alkynes
Cyclic polymerization of propyne
In presence of nickel cyanide as catalyst and under high pressure, four molecules of ethyne combine to form a tetramer called cycloocta-1,3,5,7-tetraene.
Cyclic polymerization of Ethyne

6. Isomerism of alkynes

When alkynes are heated with NaNH2 in an insert solvent such as kerosene oil or paraffin oil, they undergo isomerism i.e. 2-alkynes isomerize to 1-alkynes and vice versa.

CH3C≡CCH3 → CH3CH2C≡CH   NaNH2 in inert solvent, heat

CH3CH2C≡CH → CH3C≡CCH3    NaNH2 in inert solvent, heat

Uses of Alkynes 

(i) Acetylene and its derivatives are widely used in synthetic organic chemistry for synthesis of cis and trans-alkenes, methyl ketones etc.

(ii) Oxyacetylene flame is used for cutting and welding of metals.

(iii) Acetylene is used as illuminant in hawker’s lamp and in light houses.

(iv) Acetylene is used for ripening of fruits and vegetables.

(vi) Acetylene is used for manufacturing of ethyl alcohol, acetaldehyde, acetic acid, vinyl plastics, synthetic rubbers such as Buna-N and synthetic fibres such as Orlon.

About Mrs Shilpi Nagpal

Author of this website, Mrs. Shilpi Nagpal is MSc (Hons, Chemistry) and BSc (Hons, Chemistry) from Delhi University, B.Ed. (I. P. University) and has many years of experience in teaching. She has started this educational website with the mindset of spreading free education to everyone.

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