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Organic Chemistry I Reactions of Alkenes and Alkynes Unit 7 Dr. Ralph C. Gatrone Department of Chemistry and Physics Virginia State University Fall, 2009 1 Chapter Objectives • Present reactions of alkenes and alkynes • Reactions related to those found in biology • Must know reactions Fall, 2009 2 Preparation of Alkenes • Precursors • Alcohols (especially in biological chemistry) • Alkyl Halides (industrial chemistry) OH strong acid H dehydration X strong base H dehydrohalogenation Fall, 2009 3 Biological Dehydration • Rarely done on free alcohol • Generally done on molecules containing carbonyl and hydroxyl groups H2O - HO O2C Fall, 2009 CO2CO2- aconitase O2C CO2CO2- 4 Reaction with X2 • Halogenation • Reaction with Cl2 and Br2 Cl2 Cl Cl Br2 Br Br Fall, 2009 5 Stereochemistry • Reaction provides the trans product Fall, 2009 6 Explanation • Not a carbocation intermediate as shown • Bromonium ion intermediate forms H H Br + Br H H Br Br- Fall, 2009 7 Biological Halogenation • Marine organisms • Haloperoxidase • H2O2 oxidizes Cl- or Br- to X+ Cl Br Cl Fall, 2009 Br 8 Reaction with X2 in H2O • Cl2 in water yields HO-Cl (hypochlorous acid) • Br2 in water yields HO-Br (hypobromous acid) Br2/H2O OH Br Fall, 2009 9 Hydration of Alkenes • Alkenes react with water to give alcohols • Require high temperatures and pressures H2O CH3CH2OH • Does not work well in the laboratory Fall, 2009 10 Biological Hydration of Alkenes O O O- -O O fumarate fumarase OH -O O maleate • Relatively rare reaction • Cellular constraints are not present. Fall, 2009 11 Cellular Constraints • Solvent is water • Narrow pH range • Fixed temperature • Limited elemental choice Fall, 2009 12 Laboratory Hydration of Alkenes Oxymercuration Mercuric Acetate in THF Markovnikov Product Fall, 2009 13 Laboratory Hydration of Alkenes • Hydroboration • Non-Markovnikov Product Fall, 2009 14 Mechanism of Hydroboration • Borane is a Lewis acid • Alkene is Lewis base • Transition state involves anionic development on B • The components of BH3 add across C=C Fall, 2009 15 Reduction and Oxidation • Carbon always has 4 bonds – Oxidation changes are more difficult to see • Reduction: – Increase in H content – Decrease in O content • Oxidation: – Decrease in H content – Increase in O content Fall, 2009 16 Reduction of Alkenes: Hydrogenation • • • • Addition of H2 Requires Pt or Pd catalyst (or NR) Heterogeneous Reaction Process is not in solution Fall, 2009 17 Mechanism of Catalytic Hydrogenation • Heterogeneous – reaction between phases • Addition of H-H is syn Fall, 2009 18 Biological Reductions • Rare Reaction • Uses NADPH as reducing agent NH2 N O O N H HO H OH O P O- O O O P O N N OHO O N -2 OPO2 NH2 Nicotinamide Adenine Dinucleotide Phosphate Fall, 2009 19 Oxidation of Alkenes: Epoxides mcpba O CH2Cl2 H O OOH peroxide mcpba = Cl • Reaction with a peracid • Epoxide or oxirane • Cyclic ether Fall, 2009 20 Epoxide Preparation • From Halohydrin Br2/H2O OH base O Br bromohydrin Fall, 2009 21 Biological Epoxidation • Present in variety of processes • Does not involve peracids • Peroxides formed by reaction with O2 • Very selective reaction (see Figure 7.8) Fall, 2009 22 Hydroxylation of Alkenes • Diol formation H3O+ OH O OH • Laboratory and Biological Reaction • Biological process useful for detoxification Fall, 2009 23 Laboratory Hydroxylation • Reaction with osmium tetroxide • Stereochemistry of addition is syn (product is cis) • Product is a 1,2-dialcohol or diol (also called a glycol) Fall, 2009 24 Reaction with Carbenes • H2C: • The carbene functional group • Carbenes are electrically neutral with six electrons in the outer shell • They add symmetrically to double bonds giving cyclopropanes Fall, 2009 25 Formation of Dichlorocarbene • Base removes proton • • from chloroform Stabilized carbanion remains Unimolecular Elimination of Clgives electron deficient species, dichlorocarbene Fall, 2009 26 Reaction of Dichlorocarbene • Addition of dichlorocarbene is stereospecific cis Fall, 2009 27 Simmons-Smith Reaction • Equivalent of addition of CH2: • Reaction of diiodomethane with zinc-copper alloy • produces a carbenoid species Forms cyclopropanes by cycloaddition Fall, 2009 28 Radical Reactions • Mechanism of addition of HBr was hotly debated • Non-Markovnikov product was observed • Peroxides form readily in starting material HBr Br HBr Br On occasion Fall, 2009 + Br 29 Radical Reactions - HBr • If reaction is done with HBr/peroxides • Get the non-Markovnikov product HBr/peroxides Br Fall, 2009 30 Radical Reactions: Polymer Formation • Polymer – a very large molecule made of repeating units of smaller molecules (monomers) • Biological Polymers • Starch • Cellulose • Protein • Nucleic Acid Fall, 2009 31 Polymers • Alkene polymerization • Initiator used generally is a radical n repeating unit Fall, 2009 32 Mechanism • Initiation • Propagation • Termination • See page 241 in text for details • High reactivity of radicals limits usefulness • Not true in biological chemistry Fall, 2009 33 Biological Radical Reactions • Enzyme permits a single substrate at a time at the active site • Greater control over reactivity • Radical reactions are common • Example given on page 244 for biosynthesis of the PGAs Fall, 2009 34 Dienes • Contain two double bonds • Non-conjugated • Conjugated Fall, 2009 35 Common Feature in Nature Fall, 2009 36 Conjugation • Absorption of visible light produces color • Conjugated hydrocarbon with many double bonds are polyenes • Lycopene - red color in tomatoes • Carrotene – orange color • Extended conjugation in ketones (enones) found in hormones such as progesterone Fall, 2009 37 Conjugated Dienes • Chemistry is slightly different • More stable than non-conjugated dienes • Heat of hydrogenation Fall, 2009 38 Greater Stability • Why? • Orbital Picture of alkene bonding Fall, 2009 39 Fall, 2009 40 Conjugated Diene • Orbital picture of conjugated diene • Electrons are delocalized (spread-out) over the entire pi framework • Impact upon the chemistry Fall, 2009 41 Fall, 2009 42 Reactions • With HBr Br (71%) HBr H Br (29%) H • Why? Fall, 2009 43 Mechanism Fall, 2009 44 Allylic Cation Fall, 2009 45 Some Data X 1,2 product Nucleophile Bromide Chloride H H HX 1,2 Product 71% 30% X 1,4 product 1,4 Product 29% 70% If HBr is added at 0 oC we see the above data. If the reaction is done at 40 oC, we see 30% of the 1,2 product and 70% of the 1,4 product. How do we explain these results? Fall, 2009 46 A • • • • B+C B forms faster than C Energy of activation is lower for B than C C is more stable than B Constructing reaction energy diagram energy B A C reaction progress Fall, 2009 47 Thermodynamic Control • Transition state leading to more stable species is higher in energy, therefore, it is easier to get to the less stable product • Reaction is reversable • At high temperatures, sufficient E for both reactions to occur •A B (fast) and A C (slower) • or B A C • We see more stable product dominate. Fall, 2009 48 Kinetic Control • At low temperatures – Reaction is not reversable – Equilibrium is not reached – Insufficient energy for A to C – Sufficient energy for A to B – Less stable product dominates. Fall, 2009 49 Reactions of Alkynes • Alkynes are rare in biological chemistry • Chemistry is similar to alkenes • Generally less reactive than alkenes • Reactions can be stopped at alkene stage using one equivalent of the reagent Fall, 2009 50 Reactions with HX • Regiochemistry is Markovnikov Fall, 2009 51 Reactions with X2 • Initial addition gives trans intermediate • Product with excess reagent is tetra-halide Fall, 2009 52 Reactions with H2 • Reduction using Pd or Pt does not stop • Alkene is more reactive than alkyne Fall, 2009 53 Reactions with H2 • Lindler’s catalyst is poisoned • Not as reactive • Stops at cis-alkene Fall, 2009 54 Reduction using dissolving metals • • • • Anhydrous ammonia (NH3) is a liquid below -33 ºC Alkali metals dissolve in liquid ammonia Provide a solution of e- in NH3 Alkynes are reduced to trans alkenes with sodium or lithium in liquid ammonia Fall, 2009 55 Hydration of Alkynes • Hydration (Hg+2) of terminal alkynes provides methyl ketones • Hydration (BH3) of terminal alkynes provides aldehydes Fall, 2009 56 Alkyne Acidity: Acetylide Anion • • • • Terminal alkynes are weak Brønsted acids pKa is approximately 25 alkenes and alkanes are much less acidic Reaction of strong anhydrous bases with a terminal acetylene produces an acetylide ion Fall, 2009 57 Alkylation of Acetylide Anions • Acetylide ions are nucleophiles • Acetylide ions are bases • React with a primary alkyl halides Fall, 2009 58