3/2/18

1

https://labs.chem.ucsb.edu/zakarian/armen/courses.html

CHEM 109A Organic Chemistry

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)

3/2/18

2

OH

CO2H

HO

HO

OH

Practice Questions

What is the major product obtained by addition of HBr to the following alkenes

a.

b.

c.

3/2/18

3

summary of reactions

HBr Br

H2O, H2SO4 OH

CH3OH, H2SO4 OCH3

addition of H ⎯ X

addition of H2O

addition of ROH

1. R2B-H 2. H2O2, HO⎺BR2 OH

hydroboration/oxidation

electrophilic addition; H element carbocation?

Yes

Yes

Yes

No

summary of reactions

electrophilic addition; element element

Br2Br

Br

Br2, H2O OHBr

peroxyacid

RO

O OHO

O3; CH3SCH3

O O

H

H+

halogen addition

bromohydrin formation

epoxidation

ozonolysis

intermediate/ transition state

common mechanism

Br

Nu

(I)

O(TS)H O

OR

OO

O

O

O O

ozonide

(I) (I)

3/2/18

4

summary of reactions

H+

HBr

H +

HBr

+ Br

Racemic Mixture is Formed from Simple Alkenes

HBr+

Br Br

50:50

or

racemic mixture

E

Z H +

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.

3/2/18

5

Racemic Mixture is Formed from Simple Alkenes

HBr+

Br Br

50:50

or

racemic mixture

E

Z H +

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

3/2/18

6

Alkene now has an asymmetric center

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.

HBrBr Br+

slightly preferred

Alkene now has an asymmetric center

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.

HBrBr Br+

slightly preferred

H

+ topor

bottom

3/2/18

7

How many new Asymmetric Centers are formed?

Cl H

HCl

SynandAntiAdditions

syn addition: new bonds on the same side of the double bond anti additions: new bonds on the opposite side of the double bond

Cl H Cl H

syn addition

Cl H Cl H

+

+anti addition

3/2/18

8

FourStereoisomersareObtainediftheReactionFormsaCarbocationIntermediate

Cl H Cl H

syn addition

Cl H Cl H

+

+anti addition

The stereoisomers obtained as products depend on the mechanism of the reaction.

H Cl

H H

++ +

step 1

FourStereoisomersareObtainediftheReactionFormsaCarbocationIntermediate

The stereoisomers obtained as products depend on the mechanism of the reaction.

H Cl

H H

++ +

step 1

Cl H Cl H

syn addition

Cl H Cl H

+

+

anti addition

H

H

+

+

step 2

Cl

ClHCl

same for the enantiomeric cation

3/2/18

9

synandantiadditions:samewithacyclicsubstrates

Cl H Cl H

syn addition

Cl H Cl H

+

+

anti addition

HCl

Cl H Cl H

syn addition

Cl H Cl H

+

+

anti addition

HCl

Samewithacyclicsubstrates

Cl H Cl H

syn addition

Cl H Cl H

+

+

anti addition

HCl

Cl H Cl H

syn addition

Cl H Cl H

+

+

anti addition

HCl

3/2/18

10

Samewithacyclicsubstrates

Cl H Cl H

syn addition

Cl H Cl H

+

+

anti addition

HCl

Cl H Cl H

syn addition

Cl H Cl H

+

+

anti addition

HCl

syn or anti addition?

MCPBAO + O

3/2/18

11

epoxidation: syn or anti addition?

MCPBAO + O

O(TS)H O

OR

epoxidation: syn or anti addition?

MCPBAO + O

O(TS)H O

OR

3/2/18

12

epoxidation: syn or anti addition?

only syn addition

MCPBAO + O

MCPBAO + O

syn or anti addition?

1. BH3 (or 9-BBN)2. H2O2, HO OH

+OH OH

H

3/2/18

13

hydroboration/oxidation: syn or anti addition?

1. BH3 (or 9-BBN)2. H2O2, HO OH

+OH OH

H

H B

more stable transition state

preferred

RR

only syn addition

1. BH3 (or 9-BBN)2. H2O2, HO OH

+OH OH

H

hydroboration/oxidation: syn or anti addition?

H B

more stable transition state

preferred

RR

3/2/18

14

The Stereochemistry of Hydrogen Addition

only syn addition

SynH2AdditionFormsOnlytheCisStereoisomers..

The products can also be seen as mirror images.

mirror

H2, Pd/C

HH H H+

3/2/18

15

IftheSubstituentsaretheSame,TheCisStereoisomergivesaMesocompound

H2, Pd/C H

H

H2, Pd/C H

H

syn or anti addition?

Br2, CH2Cl2 (solvent)Br Br

?Br Br

syn

Br Br Br Branti

orsame for Br2, H2Oor Br2, CH3OH

mechanism:

3/2/18

16

bromonium ion: anti addition

Br2, CH2Cl2 (solvent)Br Br

?Br Br

syn

Br Br Br Branti

orsame for Br2, H2Oor Br2, CH3OH

H H

Br

Br

H H

Br+

Br

HH

Br

Br

step 1 step 2

only anti addition

mechanism:

bromonium ion: anti addition

Br2, CH2Cl2 (solvent)

(E)-2-pentene

Br

Br

Br

Br

+

Br2, CH2Cl2 (solvent)

Br Br Br Br+

(Z)-2-pentene

3/2/18

17

bromonium ion: anti addition

Br2, CH2Cl2 (solvent)

(E)-2-pentene

Br

Br

Br

Br

+

Br2, CH2Cl2 (solvent)

Br Br Br Br+

(Z)-2-pentene

Br

Br

Br

Br

+

same as

Br2 addition is a stereospecific reaction

IftheSubstituentsaretheSame,theCis(orZ)alkenegivesisaMesoCompound

Br2, CH2Cl2 (solvent)

Br Br Br Br+

(Z)-2-buteneenantiomers

Br2, CH2Cl2 (solvent)

(E)-2-butene

Br

Br

meso (achiral)

3/2/18

18

AntiAdditiontoaCisIsomerFormsOnlytheTransStereoisomers

© 2017 Pearson Education, Inc.

3/2/18

19

CompoundsThatCanBeFormedFromanAlkene