Preparation of Alkenes – Class 12 Chemistry Chapter Hydrocarbons Notes

Methods of Preparation of Alkenes are as follow:

By Dehydration of Alcohols:

The removal of water is called dehydration.

Alcohols when heated with dehydrating agents like H2SO4 or Al2O3 or P4O10 (also written as P2O5) they lose water so they are dehydrated to corresponding alkenes.

The ease of dehydration in alcohols is in the order of:

                      Tertiary alcohols > Secondary alcohols > Primary alcohols

Thus primary-alcohols on dehydration gives their corresponding 1-alkenes (terminal alkenes). Primary alcohols are less reactive so they need a high concentration of acid and also more heating.

Secondary-alcohols are comparatively more reactive so they need lower concentration of acid and also less heating.          

Tertiary-alcohols are most reactive among alcohols so they need still lower concentration of acid and also least heating.

Dehydration of alcohols having more than one un-symmetric β-carbons, follow Saytzeff’s rule.

Saytzeff’s rule:                                                                                                      

 The rule states, “If more than one un-symmetric β-carbons are their then hydrogen is eliminated from that β-carbon which has less hydrogens.”        

                           OR

When alternative un-symmetric β-carbons exist, hydrogen is preferentially eliminated from the β- carbon atom with a fewer number of hydrogen atoms.”

Dehydrohalogenation of Alkyl Halides:

The removal of hydrogen and halogen is called dehydrohalogenation.

When alkyl halides are heated with alcoholic solution of KOH, hydrogen and halogen atoms are removed from adjacent carbons and alkenes are produced.

Dehydrohalogenation of alkyl halides having more than one un-symmetric β-carbons, follow Saytzeff’s rule.

Dehalogenation of vicinal dihalides:

The removal of halogens is called dehalogenation. Vicinal means adjacent. When alkyl halides having two halogens on adjacent (vicinal) carbons (known as vicinal dihalides or vicinal dihaloakanes) are heated with zinc dust in anhydrous alcohol, dehalogenation occurs and alkenes are obtained.

By Hydrogenation of Alkynes:

Controlled addition of hydrogen to alkynes produces alkenes. When alkynes are heated with hydrogen in presence of palladium catalyst poisoned by heavy metal salts like BaSO4 , partial hydrogenation occurs and alkenes are produced.

Kolbe’s Electrolysis:

When electrolysis of aqueous solution of sodium or potassium salt of a dicarboxylic acid is carried out, alkenes are produced. This method is used to prepare gaseous alkenes with an even number of carbons. Electrolysis of disodium succinate produces ethene.

The mixture of ethene and carbon dioxide obtained at anode is made to pass through KOH(aq) solution, CO2 will be absorbed by the solution while ethene will pass on and will then be collected over surface of water.

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