3/2/18 CHEM 109A Organic Chemistry https://labs.chem.ucsb.edu/zakarian/armen/courses.html Chapter 6 1. The Reactions of Alkenes 2. The Stereochemistry of Addition Reactions Midterm 3, Wednesday March 7 Chapter 5: 5.6 – 5.12 inclusively (5.13, 5.14) Chapter 6: all sections Chapter 7: 7.1 – 7.6 (inclusively) 1 3/2/18 OH HO CO2H HO OH Practice Questions What is the major product obtained by addition of HBr to the following alkenes a. b. c. 2 3/2/18 summary of reactions electrophilic addition; H element carbocation? HBr Br addition of H ⎯ X Yes H2O, H2SO4 OH addition of H2O Yes CH3OH, H2SO4 OCH3 addition of ROH Yes 1. R B-H 2 2. H2O2, HO⎺ hydroboration/ BR2 OH oxidation No summary of reactions intermediate/ electrophilic addition; element element transition state Br Br2 Br halogen addition Br (I) common mechanism OH Br2, H2O Nu Br bromohydrin formation O R R O O OH H O (TS) peroxyacid O O epoxidation O O O O O3; CH3SCH3 O O O H O + ozonolysis ozonide H (I) (I) 3 3/2/18 summary of reactions Br H H + H+ + Br Br H Racemic Mixture is Formed from Simple Alkenes E HBr or + Br Br Z H + 50:50 racemic mixture carbocation intermediate If the reactant does not have an asymmetric center and the reaction forms a product with an asymmetric center, the product will be a racemic mixture. 4 3/2/18 Racemic Mixture is Formed from Simple Alkenes E HBr or + Br Br Z H + 50:50 racemic mixture carbocation intermediate If the reactant does not have an asymmetric center and the reaction forms a product with an asymmetric center, the product will be a racemic mixture. Alkene now has an asymmetric center HBr 5 3/2/18 Alkene now has an asymmetric center HBr Br + Br slightly preferred If the starting compound has an asymmetric center and the reaction forms a product with a new asymmetric center, the product will be a pair of diastereomers. Alkene now has an asymmetric center HBr Br + Br slightly preferred top + or bottom H If the starting compound has an asymmetric center and the reaction forms a product with a new asymmetric center, the product will be a pair of diastereomers. 6 3/2/18 How many new Asymmetric Centers are formed? HCl Cl H Syn and Anti Additions syn addition + Cl H Cl H + anti addition Cl H Cl H syn addition: new bonds on the same side of the double bond anti additions: new bonds on the opposite side of the double bond 7 3/2/18 Four Stereoisomers are Obtained if the Reaction Forms a Carbocation Intermediate step 1 syn addition + Cl H Cl H H Cl + anti addition + Cl H Cl H + + H H The stereoisomers obtained as products depend on the mechanism of the reaction. Four Stereoisomers are Obtained if the Reaction Forms a Carbocation Intermediate syn addition step 1 step 2 + + Cl H Cl H H Cl H Cl anti addition + + + Cl H Cl H + + H H Cl H Cl H same for the enantiomeric cation The stereoisomers obtained as products depend on the mechanism of the reaction. 8 3/2/18 syn and anti additions: same with acyclic substrates syn addition syn addition + + Cl H Cl H Cl H Cl H HCl HCl anti addition anti addition + + Cl H Cl H Cl H Cl H Same with acyclic substrates syn addition syn addition + + Cl H Cl H Cl H Cl H HCl HCl anti addition anti addition + + Cl H Cl H Cl H Cl H 9 3/2/18 Same with acyclic substrates syn addition syn addition + + Cl H Cl H Cl H Cl H HCl HCl anti addition anti addition + + Cl H Cl H Cl H Cl H syn or anti addition? MCPBA O + O 10 3/2/18 epoxidation: syn or anti addition? MCPBA O + O R O H O (TS) O epoxidation: syn or anti addition? MCPBA O + O R O H O (TS) O 11 3/2/18 epoxidation: syn or anti addition? MCPBA O + O MCPBA O + O only syn addition syn or anti addition? 1. BH3 (or 9-BBN) 2. H2O2, HO OH OH OH + H 12 3/2/18 hydroboration/oxidation: syn or anti addition? 1. BH3 (or 9-BBN) 2. H2O2, HO OH OH OH + H R H B R preferred more stable transition state hydroboration/oxidation: syn or anti addition? 1. BH3 (or 9-BBN) 2. H2O2, HO OH OH OH + H R H B R preferred more stable transition state only syn addition 13 3/2/18 The Stereochemistry of Hydrogen Addition only syn addition Syn H2 Addition Forms Only the Cis Stereoisomers.. H2, Pd/C + H H H H mirror The products can also be seen as mirror images. 14 3/2/18 If the Substituents are the Same, The Cis Stereoisomer gives a Meso compound H H2, Pd/C H H H2, Pd/C H syn or anti addition? ? syn Br Br Br Br Br2, CH2Cl2 (solvent) or same for Br2, H2O anti Br Br Br Br or Br2, CH3OH mechanism: 15 3/2/18 bromonium ion: anti addition ? syn Br Br Br Br Br2, CH2Cl2 (solvent) or same for Br2, H2O anti Br Br Br Br or Br2, CH3OH mechanism: Br + Br Br step 1 step 2 Br H H H H H H Br Br only anti addition bromonium ion: anti addition Br2, CH2Cl2 (solvent) + Br Br Br Br (Z)-2-pentene Br2, CH2Cl2 (solvent) Br Br + (E)-2-pentene Br Br 16 3/2/18 bromonium ion: anti addition Br2, CH2Cl2 (solvent) + Br Br Br Br (Z)-2-pentene same as Br Br + Br Br Br2, CH2Cl2 (solvent) Br Br + (E)-2-pentene Br Br Br2 addition is a stereospecific reaction If the Substituents are the Same, the Cis (or Z) alkene gives is a Meso Compound Br2, CH2Cl2 (solvent) + Br Br Br Br (Z)-2-butene enantiomers Br2, CH2Cl2 (solvent) Br (E)-2-butene Br meso (achiral) 17 3/2/18 Anti Addition to a Cis Isomer Forms Only the Trans Stereoisomers © 2017 Pearson Education, Inc. 18 3/2/18 Compounds That Can Be Formed From an Alkene 19 .