Chapter 6 Outline: Alkenes

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Ezra Gallagher
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1 Chapter 6 Outline: Alkenes 1. Structure of Carbocations 2. Addition Reactions to Alkenes A. Addition of HX (hydrohalogenation) B. Addition of H 2 O (hydration) C. Addition of X 2 (halogenation) D. Addition of X 2 /H 2 O (halohydrin) E. Addition of H 2 O without rearrangement (oxymercuration) F. Hydroboration-Oxidation 3. Oxidation/Reduction Reactions A. Reduction of Alkenes B. Oxidative Cleavage of Alkenes C. Forming Cis Diols There are only 5 primary types of reactions in organic chemistry 1. Acid-Base (chapter 4) involves identifying the acids & bases 2. Oxidation/reduction (ch. 6) change in oxidation number at C 3. Addition (ch. 6-7) involves identifying NuӨ, E+ 4. Substitution (ch. 9) involves identifying NuӨ, E+ & LG 5. Elimination (ch. 9) involves identifying the base & LG Your success in organic chemistry will be based on whether you can understand and can predict the reactions we do, NOT JUST MEMORIZE THEM! Although there may be some memorization, I will try and minimize this and teach you to understand why reactions occur as they do. It would be BEST if you did ALL the textbook problems, but at the very least do these: , 6.27, , 6.34a, 6.35, 6.36, 6.39, 6.41, 6.44, 6.47, 6.49 Page 1 of 29

2 Road Map to where we are going: Addition Reactions: X (+ E) X OH (OR) (+ E) X X Chapter 9 strong base X 2 CCl 4 X 2 H 2 OorROH H 3 PO 4, H 2 O OH heat Chapter 10 H 3 PO 4 H 2 O OH (+ E) OH POCl 3, pyr 1.BH 3 (THF) 2. NaOH, H 2 O 2 1.Hg(OAc) 2,H 2 O 2.NaBH 4 Oxidation/Reduction Reactions: H H H 2 Pd, high P (+ E) OH OH (+ E) 1. O 3 2. Me 2 S H O O KMnO 4, NaOH, H 2 O or 1. OsO 4 2. NaHSO 3,H 2 O OH +E OH Page 2 of 29

They are less stable than the reactants and products; hence, they

3 1. Structure of Carbocations Intermediates are the species formed on the way to products. They are less stable than the reactants and products; hence, they are short-lived and always proceed to products. What is the geometry/hybridization around a carbocation? Page 3 of 29

4 Resonance in the carbocation adds stability (I think you could have guessed this!) Allylic carbons are identified as carbocations that are adjacent to a double bond. For example: What kind of carbocations are these and which form is more stable? Page 4 of 29

5 Skeletal Rearrangements Intermediates can and will undergo hydride and methyl rearrangements during the reactions: Page 5 of 29

2. Addition Reactions - Electrophilic Addition Reactions The bulk of this chapter is dictated by Addition Reactions: SYN ANTI Two modes of addition are possible syn

6 2. Addition Reactions - Electrophilic Addition Reactions The bulk of this chapter is dictated by Addition Reactions: SYN ANTI Two modes of addition are possible syn or anti addition Alkenes are nucleophiles, so we will react them with electrophilic reagents. What is the driving force of addition reactions to alkenes? Page 6 of 29

7 A. Hydrohalogenation Electrophilic Addition of HX This is a Regioselective Reaction: Br H + HBr H Br Regioselective reaction = Markovnikov s Rule = Page 7 of 29

8 Mechanism? + HBr + HBr Page 8 of 29

9 Predict the products for the following reactions: Page 9 of 29

10 Provide the mechanism explaining the following reaction: Page 10 of 29

11 B. Hydration Electrophilic Addition of Water o In the presence of a non-halide containing acid, like H 2 SO 4, water adds to an alkene to give an alcohol. (This reaction occurs in equilibrium and is heat dependent.) o Mechanism occurs through a carbocation; hence, the most substituted alkene product is always favored (rearrangements can occur). General Reaction: Could you use these conditions to make primary alcohols? Practice drawing products before we try the mechanism: Page 11 of 29

12 Mechanism? Acid-catalyzed hydration H 2 SO 4 H 2 O OH A catalyzed reaction mechanism always regenerates the catalyst. Page 12 of 29

13 C. Halogenation Addition of X 2 o Usually Br 2 and Cl 2 is added to alkenes at room temperature by adding across the double bond. o The reaction can be run neat or in an inert solvent like CCl 4 or CH 2 Cl 2. General Reaction: o Halogens add to π-bonds because halogens are polarizable (can become electrophiles). o The electron rich double bond induces a dipole in an approaching halogen molecule, making one halogen atom electron deficient and the other electron rich o The electrophilic halogen atom is then attracted to the nucleophilic double bond, making addition possible. Page 13 of 29

14 The reaction is stereoselective yielding the halogens anti addition. Br CCl 4 + Br 2 Br + Br Br Mechanism? anti-addition What is the relationship between these two? Page 14 of 29

15 D. Halohydrin Reaction - Addition of X 2 /H 2 O General Reaction: What observation(s) do you notice about how the substituents added? Mechanism? Page 15 of 29

16 E. Oxymercuration-Reduction (NOT IN THE TEXTBOOK) Reaction is: Compare to: Reagents are: 1. Hg(OAc) 2 is: 2. NaBH 4 is: The reaction brings about a hydration of an alkene, with the OH moiety on the most substituted carbon. This reaction is regiospecific: o The product is a result of Markovnikov addition o These reactions occur WITHOUT rearrangement. Page 16 of 29

17 Mehcanism? Page 17 of 29

18 F. Hydroboration-Oxidation (last type of hydration of an alkene) Reaction: 1. BH 3 (THF) 2. NaOH, H 2 O 2 OH o These reactions occur with syn addition and WITHOUT rearrangement due to a concerted mechanism that does not go through a carbocation. What is BH 3 (THF)? BH 3 does not exists as a single species, but it exists as a Lewis acid/lewis base adduct: Page 18 of 29

19 Mechanism? First step is syn addition; the second step (oxidation) shows retention of configuration (we will not discuss the mechanism): Page 19 of 29

20 Compare all three hydration reaction conditions: Page 20 of 29

C H, the loss of C Z, or both What is Z?

21 3. Oxidation/Reduction Reactions How to Recognize Redox Reactions: Oxidation: Reduction: loss of C H, the gain of C Z, or both the gain of C H, the loss of C Z, or both What is Z? One direction of a reaction will be oxidation and the opposite direction will be reduction: Page 21 of 29

22 Identify the following reactions as oxidation (O), reduction (R) or neither (N). a. b. c. d. e. Page 22 of 29

Recognizing Oxidizing & Reducing Reagents Reducing agents contain hydrogen or hydride: o H 2 in the presence of a heavy metal, Pt, Pd Ni Oxidizing agents contain O, an O O, or an M O bond: o O 2, O 3

23 Recognizing Oxidizing & Reducing Reagents Reducing agents contain hydrogen or hydride: o H 2 in the presence of a heavy metal, Pt, Pd Ni Oxidizing agents contain O, an O O, or an M O bond: o O 2, O 3 (ozone), H 2 O 2 (hydrogen peroxide), (CH 3 ) 3 COOH (tert-butyl hydroperoxide), and peroxyacids. o The most common oxidizing agents with metal-oxygen bonds are H 2 Cr 2 O 7, Na 2 Cr 2 O 7 and K 2 Cr 2 O 7 and pyridinium chlorochromate (PCC) o Also common reagents are KMnO 4, OsO 4 and Ag 2 O. Page 23 of 29

24 A. Reduction of Alkenes Most alkenes react with H 2 in the presence of a late transition metal catalyst like Pd, Pt, Ru or Ni. o Yields are nearly quantitative (i.e. ca. 99%). o Process is catalytic, needing only small amounts of metal What kind of addition do you observe below? ca. 85% 15% Page 24 of 29

B. Oxidative Cleavage of Alkenes o Oxidative cleavage of an alkene breaks both the σ and π bonds of the double bond to form two carbonyl compounds. o Cleavage with ozone (O 3 ) is called ozonolysis.

25 B. Oxidative Cleavage of Alkenes o Oxidative cleavage of an alkene breaks both the σ and π bonds of the double bond to form two carbonyl compounds. o Cleavage with ozone (O 3 ) is called ozonolysis. Second step can be: o Reductive workup: Zn (in H 2 O) or dimethylsulfide (CH 3 SCH 3 ) are two common reagents used are used in the second step to afford the carbonyl compounds. o Oxidative workup: H 2 O 2 in ROH Page 25 of 29

26 Ozonolysis cleavage of a C=C bond to form carbonyls 1. O 3 2. (CH 3 ) 2 S Page 26 of 29

27 C. Dihydroxylation of alkenes o Depending on the reagent, the two new OH groups can be added to the opposite sides (anti addition) or the same side (syn addition) of the double bond. o Only syn hydroxylation is learned now and anti addition will be addressed in chapter 9. Syn hydroxylation results when an alkene is treated with either KMnO 4 or OsO 4. Page 27 of 29

28 Alkene Reactions. Provide correct product(s) for the following reactions demonstrating correct stereochemistry in your products. If the product has an enantiomer or diastereomer, then just write +E or +D as necessary. Finally, circle if the reaction undergoes ANTI or SYN addition (if a mechanism undergoes both then circle both words). Provide reagents for reactions A and B. SYN or ANTI? SYN or ANTI? SYN or ANTI? HO OH HBr A Br 2, CCl 4 B 1. BH 3 (THF) 2. NaOH, H 2 O 2 H O O CH 3 CO 3 H Br 2,H 2 O H 2 SO 4,CH 3 OH SYN or ANTI? SYN or ANTI? SYN or ANTI? Page 28 of 29

29 Mechanisms. Using mechanistic arrows provide step-by-step mechanisms for BOTH of the following reaction to prepare the major product. As always, correct placement of arrows and formal charges are necessary for full credit. Page 29 of 29

Electrophilic Addition Reactions

Electrophilic Addition Reactions Electrophilic addition reactions are an important class of reactions that allow the interconversion of C=C and C C into a range of important functional groups. Conceptually,