Chapter 6 Outline: Alkenes - Indiana University Bloomingtoncourses.chem.indiana.edu/c341/documents/C341S2010CH6Alkenes.pdf · C341/Spring 2010 Chapter 6 Page 1 of 29 Chapter 6 Outline:

C341/Spring 2010 Chapter 6

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Chapter 6 Outline: Alkenes

1. Structure of Carbocations

2. Addition Reactions to Alkenes

A. Addition of HX (hydrohalogenation)

B. Addition of H2O (hydration)

C. Addition of X2 (halogenation)

D. Addition of X2/H2O (halohydrin)

E. Addition of H2O 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: 6.14-6.20, 6.27, 6.29-6.33, 6.34a, 6.35, 6.36, 6.39, 6.41, 6.44, 6.47, 6.49


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Road Map to where we are going: Addition Reactions:

X

X

X2CCl4

X

OH

OH

strong base

H3PO4, H2O

heat

POCl3, pyr

(+ E)

OH

OH

OH (+ E)

(+ E)

H3PO4H2O

1.Hg(OAc)2, H2O2.NaBH4

1.BH3(THF)2. NaOH, H2O2

OH (OR)

X(+ E)

X2H2O or ROH

Chapter 9

(+ E)

Chapter 10

Oxidation/Reduction Reactions:

H2Pd, high PH

H

1. O32. Me2S

O

O

H

KMnO4, NaOH, H2Oor1. OsO42. NaHSO3, H2O

OH

OH+ E


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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?


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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?


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Skeletal Rearrangements Intermediates can and will undergo hydride and methyl rearrangements during the reactions:


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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?


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A. Hydrohalogenation – Electrophilic Addition of HX

This is a Regioselective Reaction:

HBr+

Br

H

H

Br

Regioselective reaction = Markovnikov’s Rule =


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Mechanism?

HBr+

HBr+


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Predict the products for the following reactions:


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Provide the mechanism explaining the following reaction:


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B. Hydration—Electrophilic Addition of Water o In the presence of a non-halide containing acid, like H2SO4, 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:


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Mechanism? Acid-catalyzed hydration

H2O

H2SO4

OH

A catalyzed reaction mechanism always regenerates the catalyst.


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C. Halogenation—Addition of X2

o Usually Br2 and Cl2 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 CCl4 or

CH2Cl2. 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.


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The reaction is stereoselective yielding the halogens anti addition.

+ Br2

Br

Br

+

Br

Br

CCl4

“anti-addition” What is the relationship between these two?

Mechanism?


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D. Halohydrin Reaction - Addition of X2/H2O General Reaction:

What observation(s) do you notice about how the substituents added?

Mechanism?


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E. Oxymercuration-Reduction (NOT IN THE TEXTBOOK) Reaction is:

Compare to:

Reagents are:

1. Hg(OAc)2 is: 2. NaBH4 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.


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Mehcanism?


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F. Hydroboration-Oxidation (last type of hydration of an alkene) Reaction:

1. BH3(THF)

2. NaOH, H2O2 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 BH3(THF)? BH3 does not exists as a single species, but it exists as a Lewis acid/Lewis base adduct:


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Mechanism?

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


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Compare all three hydration reaction conditions:


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3. Oxidation/Reduction Reactions How to Recognize Redox Reactions:

• Oxidation: loss of C—H, the gain of C—Z, or both

• Reduction: 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:


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Identify the following reactions as oxidation (O), reduction (R) or neither (N).

a.

b.

c.

d.

e.


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Recognizing Oxidizing & Reducing Reagents

Reducing agents contain hydrogen or hydride:

o H2 in the presence of a heavy metal, Pt, Pd Ni

Oxidizing agents contain O, an O—O, or an M—O bond:

o O2, O3 (ozone), H2O2 (hydrogen peroxide), (CH3)3COOH (tert-butyl hydroperoxide), and peroxyacids.

o The most common oxidizing agents with metal-oxygen bonds are H2Cr2O7,

Na2Cr2O7 and K2Cr2O7 and pyridinium chlorochromate (PCC)

o Also common reagents are KMnO4, OsO4 and Ag2O.


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A. Reduction of Alkenes

Most alkenes react with H2 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%


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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 (O3) is called ozonolysis.

Second step can be: o Reductive workup: Zn (in H2O) or dimethylsulfide (CH3SCH3) are

two common reagents used are used in the second step to afford the carbonyl compounds.

o Oxidative workup: H2O2 in ROH


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Ozonolysis – cleavage of a C=C bond to form carbonyls

1. O32. (CH3)2S


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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 KMnO4 or OsO4.


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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.

Br2, CCl4

SYN or ANTI ? SYN or ANTI ? SYN or ANTI ?

Br2, H2O

HBr 1. BH3(THF)2. NaOH, H2O2

SYN or ANTI ? SYN or ANTI ? SYN or ANTI ?

H2SO4, CH3OH

A B

HO OH

O O

H

CH3CO3H


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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.

Chapter 6 Outline: Alkenes - Indiana University Bloomingtoncourses.chem.indiana.edu/c341/documents/C341S2010CH6Alkenes.pdf · C341/Spring 2010 Chapter 6 Page 1 of 29 Chapter 6 Outline:

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