Alkynes | Chemistry, Class 11, Hydrocarbons

Alkynes or Acetylene

Acyclic unsaturated hydrocarbons containing a carbon-carbon triple bond are called alkynes or acetylenes. Their general formula is C_nH_2n-2 where n=2,3,4….etc.

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 (2p_x and 2p_y) which are perpendicular to each other.

The two 2p_x-orbitals, one on each carbon, are parallel to each other and hence overlap sideways to form a π bond.Similar overlap between 2p_y 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

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-CH₃Pent-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.

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 NH₃or 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 NH₃in 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.

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 NH₃

R-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 —–> CH₃CH₂CHBr₂in presence of HBr and RCOOR

Mechanism

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.

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.

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

Mechanism

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 + CO₂in presence of KMnO₄ , H₂O at 288-303K

CH₃-C≡C-CH₃ + 2 [O] ——-> CH₃COCOCH₃in 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 + CO₂in 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.

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.

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.

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.