Hydrocarbons : Alkanes, Alkenes and Alkynes – Preparation and Properties

 Hydrocarbons : Alkanes, Alkenes and Alkynes – Preparation and Properties

Hydrocarbons are the organic compounds containing carbon and hydrogen only. Hydrocarbons are broadly classified as – aliphatic, alicyclic and aromatic hydrocarbons. In this note, we are going to study about preparation and properties of aliphalic hydrocarbons i.e open chain hydrocarbons. Aliphatic hydrocarbons are further classified as – saturated hydrocarbons (alkanes) and unsaturated hydrocarbons (alkenes and alkynes).

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Contents [hide]

Alkanes (saturated hydrocarbons)Preparation of alkanes
Chemical properties of alkanes
AlkenesPreparation of alkenes
Chemical properties of alkenes
AlkynesPreparation of alkynes
Chemical properties of alkynes
Test of unsaturation ( i.e Test of alkenes and alkynes)
Kolbe’s electrolysis method for the preparation of Alkanes, alkenes and Alkynes
References

Alkanes (saturated hydrocarbons)

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Preparation of alkanes

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1. From haloalkanes

a. By Wurtz reaction: When an alkyl halide (haloalkane) is heated with sodium metal in presence of dry 
ether, an alkane containing double number of carbon atoms than in haloalkane is formed. 
This reaction is called Wurtz reaction.

Q) Identify A.

b. By reduction: Alkyl halides when reduced with Zn/HCl , H2/Ni, etc. give alkanes. Eg.

2. By catalytic hydrogenation of alkenes and alkynes:

Hydrogenation of unsaturated hydrocarbons (i.e. alkenes and alkynes) in presence of nickel or platinum 

as catalyst results in the formation of alkanes. Eg.

3. By decarboxylation of (salt of) carboxylic acid:

When sodium salt of carboxylic acid is heated with soda-lime (NaOH+CaO), a molecule of carbondioxide 
is eliminated from the molecule to give alkane. This reaction is called decarboxylation reaction. Eg.

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Chemical properties of alkanes

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1. Halogenation: This reaction involves the substitution of hydrogen atoms of alkanes by halogen atoms. 
For example, chlorine reacts with methane in presence of sunlight or heat to form four different halogen
 derivatives.

2. Nitration: Alkane reacts with nitric acid at high temperature to form nitroalkane. Eg.

3. Sulphonation : When alkane is heated with fuming sulphuric acid, alkane sulphonic acid is formed. Eg.

4. Oxidation: If burnt in air (Oxygen), alkanes are completely oxidized to carbon dioxide and water with 
large amount of heat.

When burnt in insufficient supply of oxygen, alkane forms carbon monoxide and carbon (carbon black). 
Eg.

Lower alkanes when heated with limited supply of air at 350-5000C form aldehydes. Eg.

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Alkenes

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Preparation of alkenes

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1. By dehydrohalogenation of alkyl halide ( elimination reaction):

When alkyl halide is heated with alcoholic solution of sodium or potassium hydroxide, hydrogen and 
halogen atom is eliminated from adjacent carbon atoms to give alkene. Eg.

If there is possibility of the formation of two alkenes, major product is formed according to Saytzeff’s rule.
 This rule states that when there is a chance of formation of more than one alkene, then the more substituted 
alkene is formed as major product.
Eg. In the dehydrohalogenation of 2-bromobutane, but-2-ene is the major product over but-1-ene.

2. By dehydration of alcohols:

Removal of water molecule from a molecule is called dehydration. Alcohol undergoes dehydration to form 
alkene when it is heated with dehydrating agent like sulphuric acid(H2SO4), phosphoric acid(H3PO4), 
alumina(Al2O3) etc. eg.

If there is possibility of the formation of two alkenes, major product is formed according to Saytzeff’s rule. 
eg.

3. By controlled hydrogenation of alkynes:

When alkyne is treated with hydrogen in presence of catalyst Pd on BaSO4 poisoned by quinoline, alkene is
 formed. Eg.

Note: Lindlar’s catalyst = Pd+BaSO4+quinoline

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Chemical properties of alkenes

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1. Addition of hydrogen (Catalytic hydrogenation):

When alkenes are heated with hydrogen gas in presence of metal catalyst like Ni, Pt or Pd, alkanes are 
formed. This reaction is called catalytic hydrogenation.

2. Addition of halogens:

Halogens react with alkene in presence of inert solvent like carbon tetrachloride to give dihaloalkane.
Eg. ethene reacts with Br2 in presence of CCl4 to give 1,2-dibromoethane. In this reaction reddish brown 
colour of bromine is discharged. Hence this is a test reaction of ethene (alkene).

3. Addition of hydrogen halides ( halogen acids)(HCl, HBr, HI):

Alkene reacts with halogen acids to give alkyl halide (haloalkane). Eg.

When alkene is unsymmetrical then the addition takes place according to Markovnikov’s rule.

Markovnikov’s rule:

This rule states that when an unsymmetrical reagent is added to an unsymmetrical alkene, the negative part 
of the reagent goes to that double bonded carbon which has lesser number of hydrogen atoms.
For example: The addition of HBr to propene gives 2- bromopropane instead of 1- bromopropane.

Other example:

Peroxide effect or anti-Markovnikov’s rule:

When HBr is added to an unsymmetrical alkene in presence of organic peroxide, bromine goes to the double
 bonded carbon atom having more number of hydrogen. This phenomenon of anti- Markovnikov’s addition 
of HBr caused by the presence of peroxide is known as peroxide effect or anti- Markovnikov’s rule.

4. Addition of water [Catalytic hydration]:

Alkenes react with water in presence of dilute mineral acid as catalyst to form alcohol. Eg.

5. Addition of sulphuric acid:

Alkenes react with concentrated sulphuric acid to give alkyl hydrogen sulphate. Eg.

6. Ozonolysis:

Alkene reacts with ozone to give ozonide. On warming ozonide with Zn in water, it breaks down to give two
 molecules of carbonyl compounds (aldehyde or ketone). This process of formation of ozonide and it’s 
decomposition to give carbonyl compounds is called ozonolysis.

7. Polymerization:

The process of making polymers from monomers is known as polymerization. Smaller molecules 
undergoing polymerization are called monomers. The polymers are high molecular weight large molecules
 made by the polymerization of monomers.

Ethene polymerizes to form polyethene.

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Alkynes

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Preparation of alkynes

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1. By direct combination of elements (i.e. carbon and hydrogen):

Ethyne (acetylene) gas is formed when an electric spark is struck between two carbon electrodes in an 
atmosphere of hydrogen gas.

2. By dehydrohalogenation of vicinal dihalides:

When vicinal dihalides are treated with alcoholic KOH, alkynes are formed by dehydrohalogenation. Eg.

Note: Vicinal dihalide– Compounds that contain two hydrogen atoms on adjacent carbon atoms.

3. By heating trihalogen derivatives with silver powder:

Trihaloalkanes like chloroform and iodoform when heated with silver powder form alkynes. Eg.

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Chemical properties of alkynes

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1. Addition of hydrogen ( Reduction):

When alkyne is heated with hydrogen in presence of Ni, Pt or Pd catalyst, alkane is formed. Eg.

However, alkyne reacts with hydrogen in presence of palladium catalyst deposited over barium sulphate 
poisoned by sulphur to give alkene. Eg.

2. Addition of halogen acids(HX):

Alkynes react with two molecules of halogen acids according to Markovnikov’s rule to give dihaloalkane. Eg.

3. Addition of water : Catalytic hydration:

Alkynes react with water in presence of mercuric sulphate and sulphuric acid to give vinyl alcohol which 
rearranges to give aldehyde or ketone.

For example, ethyne gives ethanal (i.e. aldehyde).

Propyne gives propanone (i.e. ketone).

4. Reaction with bromine solution:

Alkynes react with bromine in water or CCl4 to give tetrabromo compound. Here, red colour of bromine is
 discharged. This is test reaction of alkyne (unsaturated compound).

5. Polymerization reaction:

When alkynes are passed through a red hot iron or copper tube, they polymerize to form aromatic 
compounds.
Eg. Three molecules of ethyne (acetylene) polymerize to give benzene.

6. Formation of acetylides (Acidic nature of acetylene):

Acetylene is acidic in nature because it releases H+ easily.

a. Action with sodium metal:

Acetylene reacts with molten Na metal to form sodium acetylide.

b. Action with ammonical silver nitrate solution (Tollen’s reagent):

Acetylene reacts with ammonical silver nitrate solution ( i.e. Tollen’s reagent) to give silver acetylide 
which have white ppt.

c. Action with ammonical cuprous chloride solution:

Acetylene reacts with ammonical cuprous chloride solution to form copper acetylide which have red ppt.

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Test of unsaturation ( i.e Test of alkenes and alkynes)

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1. Bromine decolorization test:

Red colour of bromine is discharged when Br2 solution in water or carbon tetrachloride is added to 
unsaturated compounds (alkenes or alkynes). Therefore, this reaction is used to detect the presence of 
multiple bond in a molecule. Eg.

2. Baeyer’s test ( Oxidation with alkaline solution of KMnO4):

Alkaline solution of potassium permanganate is known as Baeyer’s reagent.
Alkene reacts with Baeyer’s reagent to form glycol, where pink colour of the potassium permanganate is
 discharged. Therefore this reaction is used as test reaction of alkenes.

Similarly, alkynes also discharge the pink colour of Baeyer’s reagent.

* Baeyer’s reagent oxidizes ethyne to oxalic acid and the pink colour of KMnO4 is discharged.

* Other alkynes react with Baeyer’s reagent to give two molecules of carboxylic acids. Eg.

Q) Give a suitable test to distinguish following pairs.

a. ethyne and ethane

b. ethene and ethyne.

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Kolbe’s electrolysis method for the preparation of Alkanes, alkenes and Alkynes

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Alkanes, alkenes and alkynes are prepared by electrolysis of salt of monocarboxylic acid, dicarboxylic acid 

and unsaturated dicarboxylic acid respectively.

1. Preparation of alkanes:

An alkane is obtained by the electrolysis of sodium or potassium salt of a carboxylic acid in aqueous 
solution. Eg.
Ethane is produced at anode during the electrolysis of an aqueous solution of sodium or potassium acetate
 as follows:

2. Preparation of alkenes:

An alkene is obtained by the electrolysis of sodium or potassium salt of a dicarboxylic acid in aqueous 
solution. Eg.
Ethene is produced at anode during the electrolysis of an aqueous solution of sodium or potassium 
succinate as follows:

3. Preparation of alkynes:

An alkyne is obtained by the electrolysis of sodium or potassium salt of an unsaturated dicarboxylic acid in aqueous 
solution. Eg.
Ethyne is produced at anode during the electrolysis of an aqueous solution of sodium or potassium maleate (i.e. salt 
of maleic acid) as follows:

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References

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• Finar, I. L., Organic Chemistry, Vol. I and Vol. II, Prentice Hall, London, 1995.
• Ghosh, S.K., Advanced General Organic Chemistry, Second Edition, New Central Book Agency Pvt. Ltd., Kolkatta, 2007.
• Sharma, M.L., Chaudary, P.N., A Text Book of B.Sc. Chemistry, Second edition (Volume one and two), Ekta Books, Kathmandu, 2004.
• Sthapit, M.K., Pradhananga, R.R., Foundations of Chemistry, Vol 1 and 2, Fourth edition, Taleju Prakashan, 2005.
• https://en.wikipedia.org/wiki/Acetylene
• https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Map%3A_Organic_Chemistry_(Smith)/Chapter_01%3A_Structure_and_Bonding/1.9%3A_Ethane%2C_Ethylene%2C_and_Acetylene
• https://www.britannica.com/science/hydrocarbon/Nomenclature-of-alkenes-and-alkynes.

•  Kolkatta, 2007.
• https://socratic.org/organic-chemistry-1/ways-to-draw-and-represent-molecules/condensed-structure
• https://www.britannica.com/science/homologous-series