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Contents
Chapter- 5 : HYDROCARBONS
Alkanes :
Preparation of alkanes by Wurtz Reaction :
When an alkyl halide( haloalkane) is heated with sodium metal in presence of dr ether, an alkane containing double number of carbon atoms than in haloalkane is formed. This reaction is called wurtz reaction.
Alkene :
Preparation of alkenes 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.
Laboratory preparation of ethene(ethylene) :
Ethene gas is prepared in laboratory by heating ethanol with excess amount of conc. sulphuric acid at about 1700C.
This reaction takes place in two steps as:
Procedure :
A mixture of 50 ml ethanol and 100 ml conc. sulphuric acid is taken in a round bottom flask and all the apparatus are fitted as shown in figure. Small amount of aluminium sulphate and sad is added to prevent frothing. The flask is then heated to about 1700C. The ethylene gas formrd is passed through NaOH solution which absorbs CO2 and SO2 present as impurities.
2NaOH + CO2 → Na2CO3 + H2O
2NaOH + SO2 → Na2SO3 + H2O
The pure ethylene gas is then collected in a gas jar by downward displacement of water.
Chemical properties of alkenes :
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.
Peroxide effect :
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. 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.
5. 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.
Alkyne :
Laboratory preparation of ethyne (acetylene) :
Ethyne is prepared in laboratory by the action of water on calcium carbide.
Procedure : Small pieces of calcium carbide are taken in a conical flask and all the apparatus are fitted as shown in figure. When water is dropped through dropping funnel, reaction of calcium carbide and water takes place to give acetylene gas. Acetylene gas thus obtained is passed through aqueous copper sulphate solution acidified with dilute HCl to remove impurities like H2S, PH3 and NH3.
The pure acetylene gas is then collected in the gas jar by downward displacement of water.
Chemical properties of alkynes :
1. Addition of hydrogen ( Catalytic hydrogenation) : 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, alkene is formed. Eg.
2. 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 aldehyde.
Propyne gives ketone.
3. Reaction with bromine solution :
Alkynes react with bromine in water or CCl4 to give tetrahalo compound. Here, red colour of bromine is discharged. This is test reaction of alkyne(unsaturated compound).
4. 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.
5. Reaction with sodium(Na) metal : Acetylene reacts with Na metal in Liq. NH3 to form sodium acetylide.
References :
- 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.
- 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
