2/26/18. Practice Questions. Practice Questions B F. How many steps are there in this reaction?

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1 Practice Questions Practice Questions D B F C E A G How many steps are there in this reaction? 1

2 Practice Questions D B F C E A G What is the highest-energy transitions state? Practice Questions D B F C E A G What is the highest-energy intermediate? 2

3 Practice Questions D B F C E A G Is this reaction exergonic or endergonic? Practice Questions D B F C E A G Which steps determines the reaction s rate? 3

4 CHEM 109A Organic Chemistry Chapter 6 1. The Reactions of Alkenes 2. The Stereochemistry of Addition Reactions summary of reactions electrophilic addition; H element HBr Br addition of H X H 2 O, H 2 SO 4 OH addition of H 2 O CH 3 OH, H 2 SO 4 OCH 3 addition of ROH 1. R 2 B-H 2. H 2 O 2, HO BR 2 OH hydroboration/ oxidation 4

5 summary of reactions electrophilic addition; element element Br 2 Br Br halogen addition Br 2, H 2 O OH Br bromohydrin formation O R O OH peroxyacid O epoxidation O 3 ; CH 3 SCH 3 O + O H ozonolysis H summary of mechanisms E + E products E + or E δ+ E = electrophiles new bond many form reactive intermediates in the first step: carbocations now, are these nucleophiles or electrophiles? 5

6 summary of reaction selectivity 1. regioselectivity HBr Br + Br which is the major product? 2. stereoselectivity Br 2 Br Br + Br Br which is the major product? summary of reaction selectivity 3. stereospecificity Br 2 Br stereoisomers Z Br meso product Br 2 Br Br + Br Br E racemic mixture of enantiomers 6

7 The Mechanism of an Electrophilic Addition Reaction Addition of Hydrogen Halides 7

8 Which sp 2 Carbon Gets the H +? The Mechanism Formation of the carbocation is the rate-limiting step. 8

9 Relative Stabilities of Carbocations Alkyl groups decrease the concentration of positive charge on the carbon. Hyperconjugation 9

10 Why the Difference in Rate? The more stable carbocation is formed more rapidly. The Hammond Postulate The transition state resembles what it is closer to in energy. exergonic reaction: The transition state resembles the reactants. endergonic reaction: The transition state resembles the products. 10

11 Similar Energies = Similar Structures The Relative Rates of Carbocation Formation Determine the Relative Amounts of Products 11

12 What Product Is Formed? The Electrophile Adds to the sp 2 Carbon Bonded to the Most Hydrogens 12

13 Regioselective Reaction A regioselective reaction is a reaction that forms more of one constitutional isomer than of another. The reaction can be: moderately regioselective highly regioselective completely regioselective Not Regioselective 13

14 Acid-Catalyzed Addition of Water A reaction takes place if an acid is added. Mechanism for the Acid-Catalyzed Addition of Water 14

15 Acid-Catalyzed Addition of an Alcohol Mechanism for the Acid-Catalyzed Addition of an Alcohol 15

16 The Major Product is a Surprise The Major Product is a Surprise 16

17 A Carbocation Rearrangement (a 1,2-hydride shift) A Carbocation Rearrangement (a 1,2-methyl shift) 17

18 The Carbocation Does Not Rearrange (No Improvement in Carbocation Stability) BH 3 is an Electrophile 18

19 Hydroboration Oxidation The Electrophile Adds to the sp 2 Carbon Bonded to the Most Hydrogens 19

20 Mechanism for Hydroboration Addition of BH 3 and Addition of HBr Follow the Same Rule The electrophile adds to the sp 2 carbon bonded to the most hydrogens. 20

21 BH 3 Has Three Potential Hydride Ions, so a Dialkylborane and a Trialkyleborane Can Be Formed Only One Hydride Ion is Needed for Hydroboration Because of its bulky R groups, R 2 BH has a stronger preference for the less substituted sp 2 carbon. 21

22 The Mechanism is the Same Hydroboration is Followed by Oxidation In the oxidation reaction, OH replaces BR 2 It is an oxidation reaction because the number of C-O bonds increases. 22

23 Mechanism for the Oxidation Reaction No Carbocation Rearrangement A carbocation is not formed, so a carbocation rearrangement cannot occur. 23

24 Addition of Br 2 or Cl 2 The Mechanism for the Addition of a Halogen 24

25 A Cyclic Bromonium Ion Intermediate The carbons atoms connected to the oxygen atom are the most electrophilic atoms. No Carbocation Rearrangement A carbocation is not formed, so a carbocation rearrangement cannot occur. 25

26 Alkenes Do Not Add I 2 Formation of Halohydrins 26

27 Mechanism for Halohydrin Formation Why Does It Follow the Same Rule? The electrophile adds to the sp 2 carbon bonded to the most hydrogens. 27

28 More Reactions Addition of a Peroxyacid 28

29 An Oxygen Oxygen Single Bond is a Weak Bond MCPBA is a Commonly Used Peroxyacid 29

30 Mechanism for Epoxidation The mechanism is similar to that for the addition of Br 2. Nomenclature of Epoxides 30

31 Ozonolysis Mechanism for Ozonide Formation 31

32 The Ozonide is Converted to Ketones and/or Aldehydes Ketones and Aldehydes 32

33 What Alkene Formed the Ozonolysis Products? Ozonolysis 33

34 A Regioselective Reaction A Stereoselective Reaction 34

35 A Stereospecific Reaction The Product Does Not Have Stereoisomers 35

36 The Product is a Racemic Mixture Why a Racemic Mixture is Formed 36

37 Forming a Racemic Mixture 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. Forming a Second Asymmetric Center If the reactant has an asymmetric center and the reaction forms a product with a new asymmetric center, the product will be a pair of diastereomers. 37

38 Reactions That Form Two Asymmetric Centers The stereoisomers obtained as products depend on the mechanism of the reaction. Four Stereoisomers are Obtained if the Reaction Forms a Carbocation Intermediate syn and anti additions 38

39 The Stereochemistry of Hydrogen Addition only syn addition Syn Addition to a Cis Isomer Forms Only the Erythro Stereoisomers 39

40 If the Substituents are the Same, the Erythro Stereoisomer is a Meso Compound Syn Addition to a Trans Isomer Forms Only the Threo Stereoisomers 40

41 Syn Addition to a Cis Isomer Forms Only the Cis Stereoisomers The products can also be written as mirror images. If the Substituents are the Same, The Cis Stereoisomer is a Meso Compound 41

42 Cyclic Alkenes are Cis, Unless They Have at Least Eight Ring Atoms The Stereochemistry of Peroxyacid Addition only syn addition 42

43 Syn Addition to a Cis Isomer Forms Only the Cis Enantiomers The products can also be written as mirror images. Syn Addition to a Trans Isomer Forms Only the Trans Enantiomers The products can also be written as mirror images. 43

44 If the Substituents are the Same, the Cis Stereoisomer is a Meso Compound The Stereochemistry of Hydroboration Oxidation only syn addition 44

45 The H and OH Add to the Same Side of the Ring The products can also be written as mirror images. Stereochemistry of Reactions That Form a Cyclic Bromonium (or Chloronium) Ion Intermediate only anti addition 45

46 Anti Addition to a Cis Isomer Forms Only the Threo Stereoisomers Anti Addition to a Trans Isomer Forms Only the Erythro Stereoisomers 46

47 If the Substituents are the Same, the Erythro Stereoisomer is a Meso Compound Anti Addition to a Cis Isomer Forms Only the Trans Stereoisomers 47

48 CIS-SYN-(ERYTHRO or CIS) You can change two terms but not one. CIS-ANTI-(THREO or TRANS) OK TRANS-ANTI-(ERYTHRO or CIS) OK CIS-ANTI-(ERYTHRO or CIS) not OK 2017 Pearson Education, Inc. 48

49 An Enzyme Forms Only One Stereoisomer Enzyme-catalyzed reactions are completely stereoselective. Only the S stereoisomer is formed. An Enzyme Can Block One Side of the Reactant 49

50 An Enzyme Catalyzes the Reaction of Only One Stereoisomer Enzyme-catalyzed reactions are stereospecific. The Stereospecificity of an Enzyme-Catalyzed Reaction Allows Enantiomers to be Separated 50

51 When A Reacts, B is Synthesized Compounds That Can Be Formed From an Alkene 51