Chapter 3 ALKENES Dr. Yasser Mostafa Abdallah For Dental Students Chapter 3 ALKENES Dr. Yasser Mostafa Abdallah

Learning outcomes: Students should be able to: Describe a homologous series and its general characteristics; Describe the alkanes as a homologous series of saturated hydrocarbons with the general formula CnH2n; Draw the structures of branched and unbranched alkenes, C1 to C6 and name the unbranched alkenes C1 to C10; Define different methods to prepare alkenes; Describe the properties of alkenes;

General methods for the Preparation of Alkenes 1-By cracking: 2- By dehydration of alcohols : Removal of water (H2O) from a substrate molecule by a suitable dehydrating agent e.g. conc. H2SO4, AI2O3, H3PO4, P2O5.

Examples:

3-By dehydrohalogenation of mono alkyl halides: Removal of HX from a substrate by alcoholic KOH or Zn-dust. Example:

4- By dehalogenations of dihalides: Removal of X2 from a substrate Zn dust/CH3OH or Zn—Cu couple in alcoholic solution. Example:

5- By Kolbe’s electrolysis : Electrolysis of aqueous solutions of sodium or potassium salts of saturated dicarboxylic acids gives alkene.

6- Reduction of alkynes: Hydrogenation of alkynes in presence of Lindlar catalyst (Lindlar catalyst is Pd on CaCO3 deactivated by lead acetate which prevents further hydrogenation) gives alkenes.

Reactions of alkenes 1- Addition of hydrogen (catalytic Hydrogenation): Hydrogenation of alkenes produces the corresponding alkanes. The reaction is carried out under pressure at a temperature of 200 °C in the presence of a metallic catalyst. Common industrial catalysts are based on platinum, nickel or palladium. Example:

2- Addition of Halogen: a- Addition of X2: In electrophilic halogenation the addition of elemental bromine or chlorine to alkenes yields vicinal dibromo- and dichloroalkanes (1,2-dihalides or ethylene dihalides), respectively. CH2=CH2 + Br2 → BrCH2-CH2Br 16

CH3CH=CH2 + HBr → CH3-CHBr-CH2-H b- Halohydrin formation: Alkenes react with water and halogens to form halohydrins by an addition reaction. Addition of Cl2 or Br2 in the presence of water yields compounds containing halogen and hydroxyl groups on adjacent carbon atoms. There are known as halohydrins. CH2=CH2 + X2 + H2O → XCH2-CH2OH 3- Addition of Hydrogen Halides: Hydrohalogenation is the addition of hydrohalic acids such as HCl or HBr or HI to alkenes to yield the corresponding haloalkanes. CH3CH=CH2 + HBr → CH3-CHBr-CH2-H

If the two carbon atoms at the double bond are linked to a different number of hydrogen atoms, the halogen is found preferentially at the carbon with fewer hydrogen substituents (Markovnikov's rule). But terminal olefin products don't yield by this method.  Mechanism: Example:

Markovnikov ’s Rule: Addition on carbon-carbon double bond of an alkene, the hydrogen attaches to the carbon atom that already holds the greater number of hydrogens.

Anti Markovnikov : In addition of HBr only the presence of peroxide follow anti Markovnilov rule so the hydrogen will attached to the carbon atom which contain lower number of hydrogen atoms

4- Addition of diluted H2SO4: This addition reaction follows markovnikov’s rule and convert alkenes into alcohols.

5- Oxidation of alkenes: a-Hydroxylation (Glycol formation): Alkenes are oxidized with a large number of oxidizing agents. In the presence of oxygen, alkenes burn with a bright flame to produce carbon dioxide and water. KMnO4 converts alkenes into glycerol. b-Oxidation with Oxygen: Oxidation using oxygen only by Ag2O or peracids (RCOOOH) leads to the formation of alkylene oxides (epoxides). e. g.

C-Cleavage of the carbon-carbon double bonds -Ozonolysis (O3) The double bond is cleaved by ozone and after hydrolysis the products are aldehyde or ketones.