Chapter 4_Reactions of Alkenes Alkynes_063fd3b665744c7080c61ca28544eee8.pptx

Reactions of Alkenes and Alkynes CHAPTER 4 1 In This Chapter We will study about – •Hydrohalogenation •Markovnikov’s Rule •Carbocation •Hydration •Halogenation •Hydrogenation (Reduction) •Oxidation •Alkynes and their reactions 2 4.1. Addition of HX to Alkenes: Hydrohalogenation H Alkenes under...

Reactions of Alkenes and Alkynes CHAPTER 4 1 In This Chapter We will study about – •Hydrohalogenation •Markovnikov’s Rule •Carbocation •Hydration •Halogenation •Hydrogenation (Reduction) •Oxidation •Alkynes and their reactions 2 4.1. Addition of HX to Alkenes: Hydrohalogenation H Alkenes undergo addition reaction with halogen acids (H-Cl, H-Br, H-I) to produce Halosubstituted alkanes. C 2. Then attack of X- on the cation. + C H H H H This is a two step reaction. 1. First step is the formation of carbocation – intermediate step H C H Cl H Cl C C H H H Chloroethane H + C H H H H H C C H H H Carbocation Intermediate H H C C H H H + Cl H H Cl C C H H 3 H 4.1. Addition of HX to Alkenes: Hydrohalogenation H3C H C H3C H + C Cl Ether H H3C Cl H C C H CH3 H 2-Chloro-2-methylpropane CH3 + H Br Ether 4 4.2. Orientation of Alkene Addition Reactions: Markovnikov’s Rule Markovnikov’s Rule: In the addition of HX to an unsymmetrical alkene, H of H-X will attach to that carbon of alkene, which has higher number of hydrogens. Addition of HX to alkene is regiospecific. 5 4.2. Orientation of Alkene Addition Reactions: Markovnikov’s Rule Markovnikov’s Rule: If both, double bond, carbon atoms have same number of hydrogen atoms then a mixture of products obtained. 6 4.2. Orientation of Alkene Addition Reactions: Markovnikov’s Rule During intermediate step, Highly substituted carbocations are more favorable rather then less substituted cations. 7 4.2. Orientation of Alkene Addition Reactions: Markovnikov’s Rule E.g.: Predict the product OR reactants of following reactions? + H Br C2H5 H 2C C H C(CH3)2 + H I Br 8 4.3. Carbocation structure and stability Why Highly substituted cations are more favorable? Carbocations are planar and the tribonded carbon is surrounded by only 6 electrons in sp2 orbitals. All of these six electrons are used in bonding. The fourth orbital on carbon is a vacant p-orbital. 9 4.3. Carbocation structure and stability Why Highly substituted cations are more favorable? Stability of these cations are increased with the increase of alkyl substitutions. More substituted – More stable. This is due to the electron donating effect of alkyl substituents on the positively charged carbon atom. 10 4.3. Carbocation structure and stability Which carbocations will be formed for the following alkenes? 11 4.4. Addition of H2O to Alkenes: Hydration Addition of water molecule to the alkene to produce alcohol is called as hydration of alkenes. Acid catalysis and high temperature is required for this reaction to take place. This reaction also follow Markovnikov rule for an unsymmetrical alkene. Mechanism Water will act as nucleophile 12 4.5. Addition of X2 to Alkenes: Halogenation Halogens (Cl2, Br2, I2) can easily be added to alkenes to yield 1,2dihaloalkanes. H H C + Cl C H Ethylene H Cl H Cl H C C H H Cl 1,2-Dichloroethane 13 4.5. Addition of X2 to Alkenes: Halogenation Halogens (Cl2, Br2, I2) can easily be added to alkenes to yield 1,2Br dihaloalkanes. + Br Br Cyclopentene Br 1,2-Dibromocyclopentane Addition is exclusively trans. 14 4.5. Addition of X2 to Alkenes: Halogenation Br+ adds to an alkene producing a cyclic ion Bromonium ion, bromine shares charge with carbon ◦Gives trans addition 15 4.5. Addition of X2 to Alkenes: Halogenation Electrophilic addition of bromine to give a cation is followed by cyclization to give a bromonium ion This bromoniun ion is a reactive electrophile and bromide ion is a good nucleophile 16 4.6. Addition of H2 to Alkenes: Hydrogenation - Reduction Addition of H-H across C=C Addition of H2 in general is also called as Reduction Requires Pt or Pd as powders on carbon and H2 Hydrogen is first adsorbed on catalyst Reaction is heterogeneous (process is not in solution) 17 4.7. Oxidation of Alkenes: Hydroxylation and Cleavage •Hydroxylation is the addition of OH group to each of the alkene carbons. • Hydroxylation is called as oxidation and in basic conditions, product is 1,2-diol. (Glycol) •KMnO4 can be used as Oxidizing agent. H OH H2O + KMnO4 NaOH OH H Cyclohexene cis-1,2-Cyclohexanediol 18 4.7. Oxidation of Alkenes: Hydroxylation and Cleavage • Cleavage of double bond occurs if oxidation is carried out with KMnO4 in acidic conditions. •Products of cleavage are carbonyl compounds - If double bond is tetrasubstituted, two ketones will be obtained - If one hydrogen is present on the alkene carbon, one product will be carboxylic acid. - If two hydrogen is present on the same alkene carbon, CO2 will be formed. 19 4.7. Oxidation of Alkenes: Hydroxylation and Cleavage CH3 C + CH3 H3O O + KMnO4 CH3 O C CH3 Cuclohexanone 20