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Summary of Alkene Reactions Mechanism Cheat Sheet, Cheat Sheet of Organic Chemistry

Chem 350 Organic Chemistry II, Jasperse Chapter 8 Handouts, Minnesota State University Moorhead

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Chem 350 Jasperse Ch. 8 Handouts
1
Summary of Alkene Reactions, Ch. 8.
Memorize Reaction, Orientation where Appropriate, Stereochemistry where
Appropriate, and Mechanism where Appropriate.
-all are drawn using 1-methylcyclohexene as a prototype alkene, because both orientation and
stereochemistry effects are readily apparent.
Orientation
Stereo
Mechanism
1
Br
HBr
(no peroxides)
Markovnikov
None
Be able to
draw
completely
2
Anti-Markovnikov
Nonselective.
Both cis
and trans
Be able to
draw
propagation
steps.
3
OH
CH3
H2O, H+
Markovnikov
None
Be able to
draw
completely
4
OH
CH3
1. Hg(OAc)2, H2O
2. NaBH4
Markovnikov
None
Not
responsible
5
H
CH3
OH
1. BH3•THF
2. H2O2, NaOH
Anti-Markovnikov
Cis
Not
responsible
6
OR
CH3
1. Hg(OAc)2, ROH
2. NaBH4
Markovnikov
None
Not
responsible
7
H
CH3
H
DD
H2, Pt
None
Cis
Not
responsible
pf3
pf4
pf5
pf8
pf9
pfa
pfd
pfe
pff
pf12
pf13
pf14
pf15
pf16
pf17
pf18
pf19
pf1a
pf1b
pf1c
pf1d
pf1e
pf1f
pf20
pf21

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Chem 350 Jasperse Ch. 8 Handouts

Summary of Alkene Reactions, Ch. 8.

Memorize Reaction, Orientation where Appropriate, Stereochemistry where

Appropriate, and Mechanism where Appropriate.

- all are drawn using 1-methylcyclohexene as a prototype alkene, because both orientation and

stereochemistry effects are readily apparent.

Orientation Stereo Mechanism

Br HBr

(no peroxides)

Markovnikov None Be able to

draw

completely

H

CH

3

Br

both cis and trans

HBr

peroxides

Anti-Markovnikov Nonselective.

Both cis

and trans

Be able to

draw

propagation

steps.

OH

CH

3

H

2

O, H

Markovnikov None Be able to

draw

completely

OH

CH

3

  1. Hg(OAc) 2

, H

2

O

  1. NaBH 4

Markovnikov None Not

responsible

H

CH

3

OH

1. BH

3

• THF

2. H

2

O

2

, NaOH

Anti-Markovnikov Cis Not

responsible

OR

CH

3

  1. Hg(OAc) 2

, ROH

  1. NaBH 4

Markovnikov None Not

responsible

H

CH

3

H

D

D

H

2

, Pt

None Cis Not

responsible

Chem 350 Jasperse Ch. 8 Handouts

Orientation Stereo Mechanism

Br

CH

3

H

Br

Br 2

(or Cl 2

None Trans Be able to

draw

completely

OH

CH

3

H

Br

Br 2

, H

2

O

(or Cl 2

Markovnikov Trans Be able to

draw

completely

O

CH

3

H

PhCO 3

H

None Cis Not

responsible

OH

CH

3

H

OH

CH

3

CO

3

H

H

2

O

None Trans

Be able to

draw

acid-

catalyzed

epoxide

hydrolysis

OH

CH

3

OH

H

OsO 4

, H

2

O

2

None Cis Not

responsible

O

H

H

O

1. O

3

  1. Me 2

S

Note: H-bearing alkene carbon

ends up as aldehyde.

None None Not

responsible

O

H

OH

O

KMnO 4

H-bearing alkene carbon

ends as carboxylic acid

None None Not

responsible

Chem 350 Jasperse Ch. 8 Handouts

OH

CH

3

  1. Hg(OAc) 2

, H

2

O

  1. NaBH 4

O

H

H

H

Cation

Capture

HgOAc

HgOAc

H

H

- H

OH

H

HgOAc

Deprotonate

HgOAc

NaBH 4

OH

H

H

Hg(OAc) 2

  • OAc

OH

2

H

CH

3

OH

1. BH

3

• THF

2. H

2

O

2

, NaOH

H

CH 3

OH

H

BH 2

H

CH 3

BH 2

H 2

O 2

, NaOH

Notes

a. concerted addition of B-H across C=C

  • explains the cis stereochemistry

b. the B-H addition is Markovnikov; the

B is !+, the H is !-

c. The H 2

O 2

, NaOH process is complex,

but replaces the B with OH with complete

retention of stereochem

  • the explains why the cis stereochemistry

established in step one is preserved in step 2.

OR

CH

3

  1. Hg(OAc) 2

, ROH

  1. NaBH 4

O

H

H

H

Cation

Capture

HgOAc

HgOAc

H

CH

3

- H

OCH

3

H

HgOAc

Deprotonate

HgOAc

NaBH 4

OCH

3

H

H

Hg(OAc) 2

  • OAc

HOCH

3

Chem 350 Jasperse Ch. 8 Handouts

Br

CH

3

H

Br

Br 2

(or Cl 2

H

H

H

Cation

Capture

Br

Br

Br Br Br Br

3 Notes

  1. Cation intermediate is cyclic

bromonium (or chloronium) ion

  1. The nucleophile captures the

bromonium ion via backside attack

  • this leads to the trans stereochemistry
  1. The nucleophile attacks the bromonium

ion at the more substituted carbon

OH

CH

3

H

Br

Br 2

, H

2

O

(or Cl 2

H

H

H

Cation

Capture

Br

Br

Br Br O

OH

2

H

H

H

Br

OH

- H

4 Notes

1. Cation intermediate is cyclic bromonium (or chloronium) ion

2. The nucleophile captures the bromonium ion via backside attack (ala SN2)

  • this leads to the trans stereochemistry

3. The nucleophile attacks the bromonium ion at the more substituted carbon

  • this explains the orientation (Markovnikov)

a. There is more + charge at the more substituted carbon

b. The Br-C bond to the more substituted carbon is a lot weaker

H

CH

3

H

Br

Br

O

H

H

H

Br

OH

- H

More

Substituted

End

H

Br

O

H

H

H

Br

OH

- H

Less

Substituted

End

4. Alcohols can function in the same way that water does, resulting in an ether OR rather than

alcohol OH.

Chem 350 Jasperse Ch. 8 Handouts

Chapter 7 Reactions and Mechanisms, Review

E

On

R-X,

Normal

Base

CH

3

Br

OCH

3

H

H H OCH

3

Br

NaOCH 3

H OCH

3

Mech:

  • Br

(Normal

base)

Notes

1. Trans hydrogen required for E

2. Zaytsev elimination with normal bases

3. For 3º R-X, E2 only. But with 2º R-X, S

N

2 competes (and usually prevails)

4. Lots of “normal base” anions.

E2,

On

R-X, Bulky

Base

Br NEt 3

or

KOC(CH

3

3

(Bulky

bases)

H

2

C

Mech: Br

H

NEt 3

  • Et 3

NH Br

Notes:

1. Hoffman elimination with Bulky Bases

2. E2 dominates over S

N

2 for not only 3º R-X but also 2º R-X

3. Memorize NEt

3

and KOC(CH

3

3

as bulky bases.

Acid-

Catalyzed

E1-

Elimination

Of

Alcohols

OH

H

2

SO

4

+H OH

H

2

SO

4

+ HSO

4

+ OH

2

- H

2

O

HSO

4

+ H

2

SO

4

Protonation Elimination

Deprotonation

OH

OH

2

H

H

H

Mech

Notes:

1. Zaytsev elimination

2. Cationic intermediate means 3º > 2º > 1º

3. 3 - Step mechanism

Chem 350 Jasperse Ch. 8 Handouts

Ch. 8 Reactions of Alkenes

8 - 1,2 Introduction

CH

3

A B

CH

3

B

A

H

H

Addition Reaction

1. Thermodynamics: Usually exothermic

 1 π + 1 σ  2 σ bonds

2. Kinetics: π bond is exposed and accessible

Generic Electrophilic Addition Mechanism

CH

3

A B

CH

3

B

A

H

H

CH

3

A

H

+ B

CH

3

A

H

+ B

or

CH

3

B

A

H

vs

CH

3

A

B

H

Cation

Formation

Cation

Capture

CH

3

H

A

A

B

C

D

E

E

F

Doesn't Happen

Because

Inferior Cation

Product

Forms

2 Steps: Cation formation and cation capture

  • Cation formation is the slow step

o Cation stability will routinely determine the orientation in the first step

 Which is preferred, A  B or A  C?

  • Often the cation is a normal cation B. Sometimes 3-membered ring cations D will be involved.
  • In some cases, the cation will be captured by a neutral species (like water), in which case an

extra deprotonation step will be involved

4 Aspects to Watch For

1. Orientation

  • Matters only if both of two things are true:

a. The alkene is unsymmetrical, and

b. The electrophile is unsymmetrical

2. Relative Stereochemistry

o Matters only if both the first and the second alkene carbons are transformed into chiral

centers

3. Mechanism

4. Relative Reactivity of Different Alkenes

o Stability of cation formed is key

Chem 350 Jasperse Ch. 8 Handouts

Mechanism

Br

H

H

H Br

H

H

  • Br

Br

Protonate Cation

Capture

H

o Protonate first

o Capture cation second

o Cation formaton (step 1) is the slow step

Rank the Reactivity of the following toward HBr addition.

3 (2º) 2 (3º) 1 (3º allylic)

Issue: Cation stability

Why Does Markovnikov’s Rule Apply? Product/Stability Reactivity Rule.

o Formation of the most stable carbocation results in Markovnikov orientation

H Br

For unsymmetrical alkenes,

protonation occurs at the

less substituted alkene carbon

so that the more stable cation forms

( 3 º > 2 º > 1 º), in keeping with the

product stability-reactivity principle

or

H

H

Br

Br

H

Br

H

Br

Markovnikov Product

anti-Markovnikov Product

Slow Step

o This same logic applies anytime something adds to an alkene.

o You want to make the best possible intermediate in the rate-determining step.

Draw the mechanis for the following reaction:

HBr

Br

H

2

C

H Br

H

3

C

  • Br

H

3

C

Br

Chem 350 Jasperse Ch. 8 Handouts

8.3B Free Radical Addition of HBr with Peroxide Initiator: Anti-Markovnikov Addition (Rxn 2)

H

CH

3

Br

both cis and trans

HBr

peroxides

Anti-Markovnikov Nonselective.

Both cis

and trans

Be able to

draw

propagation

steps.

  • Peroxides are radical initiators, and cause the mechanism to shift to a radical mechanism
  • With peroxides, the orientation is reversed to anti-Markovnikov: now the Br adds to the less

substituted end and the H adds to the more substituted end of an unsymmetrical alkene

o No peroxides: Br goes to more substituted end

o With peroxides: Br goes to less substituted end

  • The anti-Markovnikov radical process works only with HBr, not HCl or HI
  • The radical process is faster, and wins when peroxides make it possible. In the absence of

peroxides, the slower cationic process happens.

Mechanism, and Reason for AntiMarkovnikov Orientation

Br

For unsymmetrical alkenes,

bromination occurs at the

less substituted alkene carbon

so that the more stable radical forms

( 3 º > 2 º > 1 º), in keeping with the

product stability-reactivity principle

or

Br

Br

2 º radical

Br

H

Br

H

1 º radical

Markovnikov Product

anti-Markovnikov Product

Slow Step

H Br

H Br

Examples, Predict the Products.

Does Markovnikov’s

Rule matter?

HBr, peroxides

HBr, no peroxides

Br

Br

Yes

2 HBr, peroxides

HBr, no peroxides

Br

Br

Yes

HBr, peroxides

HBr, no peroxides

Br

Br

No

Chem 350 Jasperse Ch. 8 Handouts

Examples, Predict the Products.

Does Markovnikov’s

Rule matter?

H

2

O, H

OH

Yes

H

2

O, H

HO

Yes

H

2

O, H

OH

No

H

2

O, H

OH

No

H

2

O, H

OH

Yes

Problems with Acid-Catalyzed Addition of Water to Alkenes

1. Alkenes with poor water solubility often don’t add very well.

  • Can’t drive the equilibrium strongly to the alcohol side in that case
  • Solvent mixtures can often help, but not always good enough

2. Alcohol/Alkene equilibrium sometimes poor

3. Carbocation rearrangements can be a problem

4. The degree of Markovnikov selectivity isn’t always satisfactory

  • 99:1 isomer selectivity is a lot nicer than 90:10…

o Especially if you have to purify!

5. Obviously you can’t get the reverse, anti-Markovnikov alcohol products.

Each of these limitations, when they are a problem, can be solved by alternative recipes that

indirectly add H-OH.

Draw the mechanism for the following reaction:

H

2

O, H

HO

H

2

C

O

H

H

3

C

OH

2

H

H

OH

Chem 350 Jasperse Ch. 8 Handouts

8.5 Indirect Markovnikov Addition of H-OH via Oxymercuration/Demercuration. Reaction 4.

General: C C C C

H OH

  1. Hg(OAc) 2

, H

2

O

  1. NaBH 4

OH

CH

3

  1. Hg(OAc) 2

, H

2

O

  1. NaBH 4

Markovnikov

Stereo:

None

Mech:

Not

responsible

Notes:

1. Often higher yields, cleaner, faster, and easier

2. No restrictions

3. No cation rearrangements

4. Very strong , often superior Markovnikov selectivity

o OH adds to the more substituted end, H to the less substituted end

Does Markovnikov’s

Rule matter?

  1. Hg(OAc) 2

, H

2

O

  1. NaBH 4

H

2

O, H

+ OH

OH

Yes

  1. Hg(OAc) 2

, H

2

O

  1. NaBH 4

H

2

O, H

HO

HO

Yes

H

2

O/H

vs Oxymercuration/Demercuration: Which should I use?

  • Both normally give same product
  • For predict-the-product problems, be able to handle either recipe
  • For provide-the-right-recipe problems, I will accept either answer.

o H

2

O/H

is easier to write!

  • In the real lab, the choice is decided on a case-by-case basis.

o Default to H

2

O/H

o Go to oxymercuration/demercuration when direct acid-catalyzed hydration doesn’t

work as well as you’d like

Chem 350 Jasperse Ch. 8 Handouts

8.7 Indirect anti-Markovnikov Addition of H-OH via Hydroboration/Oxidation. Reaction 5.

C C

C C

H BH

2

Overall pathway:

1. BH

3

• THF 2. H

2

O

2

, NaOH

C C

H OH

"Hydroboration" "Oxidation"

H

CH

3

OH

1. BH

3

• THF

2. H

2

O

2

, NaOH

plus enantiomer

Anti-Markovnikov Cis Not

responsible

Notes:

1. Anti-Markovnikov orientation : the OH ends up on the less substituted end of an

unsymmetrical alkene; the H adds to the more substituted end

2. Cis addition. Both the H and the OH add from the same side.

3. When does cis/trans addition stereochemistry matter?

o Only when both alkene carbons turn into chiral centers in the product.

o If one does but not both, then the relative stereochemistry doesn’t matter

o For Markovnikov additions involving H-Br or H-OH, the H usually adds to a carbon that

already has an H, so that in the product it is not a stereocenter.

o In anti-Markovnikov additions, much more common for both carbons to become chiral

carbons

4. Chiral products are Racemic (two enantiomers form) but not optically active

o When only one chiral center forms (often in the Markovnikov additions), any chiral

product will always be racemic

o When two chiral centers form, as in the example above, of the four possible

stereoisomers, you get only two of them, in racemic mixture.

H

CH

3

OH

1. BH

3

• THF

2. H

2

O

2

, NaOH

H

CH

3

OH

H

CH

3

OH

H

CH

3

OH

A

B

C

D

Cis Addition Enantiomers

Do Form

Trans Addition Enantiomers

Do NOT Form

Examples, Predict the Products.

Does

Markov.

Matter?

Does

Stereo

Matter?

1. BH

3

• THF

2. H

2

O

2

, NaOH

OH

Yes No

Chem 350 Jasperse Ch. 8 Handouts

Does

Markov.

Matter?

Does

Stereo

Matter?

1. BH

3

• THF

2. H

2

O

2

, NaOH

HO

Yes No

1. BH

3

• THF

2. H

2

O

2

, NaOH OH

No No

1. BH

3

• THF

2. H

2

O

2

, NaOH

OH

No No

1. BH

3

- THF

  1. NaOH, H 2

O

2

  1. Hg(OAc) 2

, H

2

O

  1. NaBH 4

H

2

O, H

OH

H

H

3

C

OH

OH

H

Yes Yes

No

No

1. Which starting alkenes would produce the following products following hydroboration-

oxidation? Factor in the stereochemistry of the products in considering what starting materials

would work.

1. BH

3

- THF

  1. NaOH, H 2

O

2

1. BH

3

- THF

  1. NaOH, H 2

O

2

Ph

H

H

HO

H

3

C

Ph

OH

H

H

H

3

C

Ph

H

CH

3

H

Ph

CH

3

CH

3

Ph

CH

3

H

HO

H

H

3

C

2. Fill in recipes for converting 1-butene into the three derivatives shown.

1. BH

3

- THF

  1. NaOH, H 2

O

2

OH

  1. Hg(OAc) OH 2

, H

2

O

  1. NaBH 4

H

2

O, H

or

Chem 350 Jasperse Ch. 8 Handouts

8.6 Alkoxymercuration-Demercuration: Markovnikov Addition of H-OR (Reaction 6)

General: C C C C

H OR

  1. Hg(OAc) 2

, ROH

  1. NaBH 4

OR

CH

3

  1. Hg(OAc) 2

, ROH

  1. NaBH 4

Markovnikov Stereo:

None

Mech:

Not

responsible

Mechanism

OR

CH

3

  1. Hg(OAc) 2

, ROH

  1. NaBH 4

O

H

H

H

Cation

Capture

HgOAc

HgOAc

H

CH

3

- H

OCH

3

H

HgOAc

Deprotonate

HgOAc

NaBH 4

OCH

3

H

H

Hg(OAc) 2

  • OAc

HOCH

3

Notes:

1. Everything is the same as with oxymercuration-demercuration to form an alcohol, except

you use an alcohol instead of water

2. This results in an oxygen with it’s spectator carbon chain adding rather than an OH

3. Strong Markovnikov orientation

o The OR adds to the more substituted end of the alkene

o The Hydrogen ends up on the less substituted end of the alkene

4. The mechanisms are analogous.

Examples, Predict the Products.

Does Mark’s

Rule matter?

Does

Stereo?

  1. Hg(OAc) 2

, CH

3

CH

2

OH

  1. NaBH 4

O

Yes No

  1. Hg(OAc) 2
  1. NaBH 4

OH

O

Yes No

Chem 350 Jasperse Ch. 8 Handouts

Ether Synthesis: Two Routes

1. From Alkene and Alcohol: By Oxymercuration/Demercuration

2. From R-Br and Alkoxide Anion: By S

N

3. Multistep Syntheses : Design Reactants for the Following Conversions

  • Note: It is often most strategic to think backward from product to precursor.
  • Then think back how you could access the precursor from the starting material.
  • There may sometimes be more than one suitable route.

a.

OCH

3

  1. HBr, peroxides

  2. NaOCH 3

b.

  1. Hg(OAc) 2

, CH

3

OH

  1. NaBH 4

OCH

3

c.

OH

CH

3

OH

CH

3

1. H

2

SO

4

, heat

2. BH

3

- THF

  1. NaOH, H 2

O

2

d.

  1. Br 2

, hv

  1. NaOCH 3

(or other small, normal base)

e.

  1. Br 2

, hv

  1. NEt 3

(or KOCMe 3

f.

OH

CH

3

OH

CH

3

Br

CH

3

  1. H 2

SO

4

, heat

  1. HBr

g.

1. H

2

SO

4

, heat

  1. HBr, peroxides

OH

CH

3

CH

3

Br