14.2 Unsaturated hydrocarbons- Alkenes & Alkynes

14.2 Unsaturated Hydrocarbons — Alkenes and Alkynes

🎯 Learning outcomes

By the end of this chapter, Students should be able to:

Define and describe unsaturated hydrocarbons (Alkenes & Alkynes).
Show preparation of alkenes by dehydration of alcohol, dehydrohalogenation, and catalytic hydrogenation of alkyne.
Explain the chemical properties of alkenes, i.e., addition reactions with HX (Markovnikov’s addition and peroxide effect), H₂O, O₃, and H₂SO₄ only.
Show preparation of alkynes from carbon and hydrogen, 1,2-dibromoethane, chloroform/iodoform only.
Describe chemical properties of alkynes, i.e., addition reaction with (H₂, HX, H₂O), acidic nature (action with Sodium, ammoniacal AgNO₃, and ammoniacal Cu₂Cl₂).
Test unsaturation of hydrocarbons (ethene & ethyne): bromine water test and Baeyer’s test.
Compare physical properties of alkane, alkene, and alkyne.
Describe Kolbe’s electrolysis methods for the preparation of alkanes, alkenes, and alkynes.

14.2 Alkenes & Alkynes — Unsaturated Hydrocarbons

1 14.2.1 General Methods of Preparation of Alkenes

1. From alkynes (Catalytic hydrogenation)

Alkynes can be hydrogenated to Alkenes with hydrogen gas in the presence of Pd/BaSO₄, which can be considered the catalyst.

e.g.,

Ethyne hydrogenated with Pd/BaSO4 to give Ethene

2. From Haloalkanes (dehydrohalogenation)

Haloalkanes undergo elimination with alcoholic KOH or alc. NaOH gives alkenes. e.g.,

2-bromopropane with alc. KOH gives propene

Saytzeff’s rule — when two different alkenes are possible, the one with the highest number of alkyl groups on the two sides of the double bond is the main product.

2-chlorobutane with alc. NaOH gives but-2-ene as major product and but-1-ene as minor product

3. From alcohols (dehydration)

When the calculated amount of ethyl alcohol is heated with an excess of conc. H₂SO₄ at about 170°C, intramolecular dehydration results in Ethene.

Ethanol dehydrated with conc. H2SO4 at 170C to give Ethene

2 Chemical Properties of Alkenes

1. Addition reactions

a) Addition of Hydrohalic acids (HCl, HBr, HI)

Alkenes undergo addition with hydrohalic acids (HCl, HBr and HI), resulting in haloalkanes. E.g.,

Markovnikov’s rule: When an unsymmetrical reagent is added to an unsymmetrical alkene, the positive part of the reagent is attached to the carbon atom with more hydrogen atoms; the negative part gets bonded with the carbon atom with a smaller number of hydrogen atoms. e.g.,

Propene with HCl gives 2-chloropropane as major product via Markovnikov's rule

Peroxide effect (Anti-Markovnikov’s rule)

When HBr is added to unsymmetrical Alkenes in the presence of organic peroxide, the positive part (H) gets bonded with the carbon having fewer hydrogens and the negative part gets bonded with the carbon having a higher number of hydrogens.

But-1-ene with HBr in presence of organic peroxide gives 1-bromobutane

b) Addition of Water

Alkenes react with water at high temperatures and pressures in the presence of phosphoric acid as a catalyst to form alcohols.

Propene with water and H3PO4 at 240C gives propan-2-ol

c) Addition of Ozone (Ozonolysis)

Alkenes, when treated with ozone, form unstable ozonides, which, upon hydrolysis with zinc and water, yield carbonyl compounds.

Alkenes of the type RCH=CH₂ or RCH=CHR give aldehydes.

Ozonolysis of 2-methylpropene giving propanone and methanal

Alkenes of the type R₂C=CH₂, R₂C=CHR or R₂C=CR₂ give at least one ketone.

Ozonolysis of 2,3-dimethylpent-2-ene giving propanone and butanone

d) Addition of Sulphuric acid

Concentrated sulphuric acid adds to alkene producing alkyl hydrogen sulphate.

Ethene with H2SO4 gives ethyl hydrogen sulphate

Alkyl hydrogen sulphate gives back alkene on heating and alcohol on boiling with water.

3 Preparation of Alkynes

1. From carbon and hydrogen (total synthesis)

Ethyne is prepared by passing an electric arc between two carbon electrodes in an atmosphere of hydrogen.

Carbon and hydrogen via electric arc give Ethyne

2. From haloforms (chloroform and iodoform)

When chloroform or iodoform is heated with silver powder Ethyne is produced.

Chloroform with silver gives Ethyne and silver chloride

2 CHCl₃ + 6 Ag → C₂H₂ + 6 AgCl

3. From 1,2-dibromoethane (By dehydrohalogenation)

Ethyne can be prepared by dehydrohalogenation (elimination) of 1,2-dibromoethane with alcoholic KOH or NaOH.

1,2-dihaloethane with alc. KOH gives Ethyne

4 Chemical Properties of Alkynes

1. Addition reactions

a) Addition of Hydrogen (catalytic hydrogenation)

Alkynes undergo addition with hydrogen in the presence of finely divided Nickel (Ni), Platinum (Pt) or Palladium (Pd) as a catalyst at a temperature of about 200–300°C producing alkenes and finally alkanes.

Ethyne with hydrogen and Ni gives Ethene

Ethene with hydrogen and Ni gives Ethane

b) Addition of Hydrohalic acid (HX)

Alkynes undergo an addition reaction with hydrohalic acids in two steps giving dihaloalkanes. The addition occurs according to Markovnikov’s rule.

Propyne with HBr in two steps gives 2,2-dibromopropane

c) Addition of Water

Water can be added to alkynes in the presence of 1% HgSO₄ and 42% H₂SO₄ to give vinyl alcohols, which tautomerize to form carbonyl compounds (aldehydes and ketones). This addition also occurs according to Markovnikov’s rule. Ethyne gives Ethanal; other alkynes give ketones.

Propyne with water, HgSO4 and H2SO4 gives propen-2-ol which tautomerizes to propanone

2. Acidic nature

a) Reaction with Sodium

When Ethyne (Acetylene) gas is passed over molten sodium, monosodium acetylide and disodium acetylide are produced.

Ethyne with sodium gives monosodium acetylide then disodium acetylide

Or, the reaction may be written in a single step as;

Ethyne with 2 sodium gives disodium acetylide in a single step

b) Action with ammonical AgNO₃

Ammonical silver nitrate solution is commonly called Tollen’s reagent. When acetylene is passed through Tollen’s reagent, a white precipitate of silver acetylide is obtained. Hence this reaction is also used as a test to detect Ethyne.

Ethyne with AgNO3 and NH4OH gives silver acetylide white precipitate

Alkynes, having no acidic hydrogen, do not react in such a way.

c) Action with ammonical Cu₂Cl₂ (CuCl)

When acetylene is passed through an ammonical solution of cuprous chloride, a red precipitate of copper acetylide is obtained.

Ethyne with Cu2Cl2 and NH4OH gives copper acetylide red precipitate

3. Test of unsaturation for (Ethene and Ethyne)

a) Bromine water test

The presence of unsaturation (double or triple bond) can be tested by using bromine water.

Ethyne with reddish brown bromine gives 1,2-dibromoethene

b) Oxidation with alkaline KMnO₄ (Bayer’s test) — hydroxylation

When Ethene is treated with an alkaline solution of dilute KMnO₄, the pink colour of KMnO₄ is discharged and Ethylene glycol is obtained. Such a reaction is also shown by other alkenes and alkynes. A dilute alkaline solution of KMnO₄ is called Bayer’s reagent and this reaction is used as a test for the detection of unsaturation in organic compounds in the name of Bayer’s test.

Ethene with KMnO4 and KOH gives ethane-1,2-diol

Ethyne with KMnO4/KOH Bayer's reagent gives oxalic acid

5 Comparative Physical Properties of Alkanes, Alkenes and Alkynes

Alkanes, Alkenes and Alkynes are hydrocarbons; basically, their physical properties are similar. These can be studied comparatively as:

Physical properties	Alkanes	Alkenes	Alkynes
State	C₁–C₄ – colourless gases C₅–C₁₅ – liquids Higher members – waxy solids	C₂–C₄ – colourless gases C₅–C₁₂ – liquids Higher members – waxy solids	C₂–C₄ – colourless gases C₅–C₁₅ – liquids Higher members – waxy solids
Solubility	Insoluble in water (polar solvent); soluble in organic solvents (non-polar)	Insoluble in water (polar solvent); soluble in organic solvents (non-polar)	Insoluble in water (polar solvent); soluble in organic solvents (non-polar)
Density	Lighter than water; increases with molecular weight	Lighter than water; increases with molecular weight	Lighter than water; increases with molecular weight
Melting and boiling points	Increases with increasing molecular mass	Higher compared to the corresponding alkanes.	Higher compared to the corresponding alkanes and alkenes.
Polarity	Non-Polar	Non-Polar	Non-Polar

6 14.6 Kolbe’s Electrolysis Method for Preparation of Alkane, Alkene and Alkyne

Alkanes

Electrolysis of a concentrated aqueous sodium or potassium salt of carboxylic acid (fatty acid) yields an alkane.

Potassium alkanoate electrolysis at anode gives alkane

At cathode potassium and water give potassium hydroxide and hydrogen

Alkene

When an aqueous solution of sodium or potassium succinate is electrolyzed, Ethene liberates at the anode.

Potassium succinate electrolyzed to succinate ion

At anode:

Succinate ion loses electrons to form unstable intermediate giving ethene and CO2

At cathode:

Alkyne

When an aqueous solution of sodium or potassium salt of Maleic acid (or Fumaric acid) is electrolyzed, Ethyne liberates at the anode.

Maleic acid and fumaric acid cis trans geometric isomer structures

Potassium maleate electrolyzed to maleate ion

At anode:

Maleate ion loses electrons to give ethyne and CO2

At cathode:

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