© 2014 by john wiley & sons, inc. all rights reserved. chapter 8 alkenes and alkynes ii:...

© 2014 by John Wiley & Sons, Inc. All rights reserved.

Chapter 8

Alkenes and Alkynes II:

Addition Reactions


C C E Nu

E

C C

Nu

+

1. Addition Reactions of Alkenes

Classification of reagents and alkenes by symmetry with regard to addition

reaction

Carbocation

Hydrogen halides, for example, react with alkenes by donating a proton to the bond. The proton uses the two electrons of the bond to form a bond to one of the carbon atoms. This leaves a vacant p orbital and a +ve charge on the other carbon. The overall result is the formation of a carbocation and a halide ion from the alkene and HX:

Being highly reactive, the carbocation then combines with the halide ion by accepting one of its electron pairs:

Electrophiles Are Lewis Acids Electrophiles are molecules or ions that can accept an electron pair. Nucleophiles are molecules or ions that can furnish an electron pair (i.e. Lewis Bases).

ADDITION OF HYDROGEN HALIDES TO ALKENES: MARKOVNIKOV’S RULE

The addition of HX to an unsymmetrical alkene could conceivably occur in two ways. The addition of HBr to propene could lead to either 1-bromopropane or 2-bromopropane. The main product, however, is 2-bromopropane.

Alkyl groups stabilize carbocations by donating electron density from C--H and C--C sigma bonds that can line up with the empty p orbital on the

positively charged carbon atom

BASE GRAPH: Reaction energy diagram for

the addition of HBr to ethene

BALL-AND-STICK MODEL OF TRANSITION STATE 1: Reaction energy diagram for the addition of HBr to

ethene

BALL-AND-STICK MODEL OF TRANSITION STATE 2: Reaction energy diagram for the addition of HBr to ethene

BALL-AND-STICK MODEL OF CARBOCATION INTERMEDIATE: Reaction energy diagram for the addition of HBr to ethene

BALL-AND-STICK MODEL OF REACTANTS: Reaction energy diagram for the addition of HBr to ethene

BALL-AND-STICK MODEL OF PRODUCT: Reaction energy diagram for the addition of HBr to ethene

Reaction energy diagram for formation of the isopropyl and propyl cations from propene


1A.How To Understand Additions

to Alkenes This is an addition reaction: E–Nu added across the double bond

C C E Nu

E

C C

Nu

+

Bonds broken Bonds formed

p-bond s-bond 2 s-bonds


C C E Nu

E

C C

Nu

+

1. Addition Reactions of Alkenes


Since p bonds are formed from the overlapping of p orbitals, p electron clouds are above and below the plane of the double bond

C Cp electron clouds


In an electrophilic addition, the p electrons seek an electrophile, breaking the p bond, forming a s bond and leaving a positive charge on the vacant p orbital on the adjacent carbon. Addition of a nucleophile to form a s bond provides an addition product.


C CE Nu

C C

E

+ Nu

Nu

C C

E


C C

E

CC

E

CC

NuNuE Nu

2. Electrophilic Addition ofHydrogen Halides to Alkenes: Mechanism & Markovnikov’sRule

Mechanism


C C

E

Mechanism●Sometimes do not go through

a “free carbocation”, may go via


C C

H

H

H

HE Nu

E

CC H

H

H

H

E

C CH

H

H

H

E

C CH

H

H

H

Nu E

CC H

H

H

H

Nu

Nu

same as

same as

Markovnikov’s Rule ●For symmetrical substrates, no

problem for regiochemistry


E NuC C

H

H3C

H

H E

CC H

H

CH3

H

E

C CCH3

H

H

H

E

C CCH3

H

H

H

Nu E

CC H

H

CH3

H

Nu

Nu Nu

or

different from

Markovnikov’s Rule ●But for unsymmetrical

substrates, two regioisomers are possible


Markovnikov’s Rule ●In the electrophilic addition of

an electrophile across an unsymmetrical alkene, the more highly substituted and more stabilized carbocation is formed as the intermediate in preference to the less highly substituted and less stable one


E NuE Nu E

Markovnikov’s Rule ●Thus

E

NOT (1o cation; less stable)

Note: carbocation stability 3o > 2o > 1o


BrBr

fast

Addition of Hydrogen Halides●Addition of HCl, HBr and HI

across a C=C bond●H+ is the electrophile

slow

r.d.s

H Br+

Br

NO

x


2A.Theoretical Explanation ofMarkovnikov’s Rule

H XC C

H

H3C

H

H

H

CC H

H

CH3

H

H

C CCH3

H

H

H

or

2o carbocation

(more stable)

1o carbocation

(more stable)

step 1(slowr.d.s.)

One way to state Markovnikov’s rule is to say that in the addition of HX to an alkene, the hydrogen atom adds to the carbon atom of the double bond that already has the greater number of hydrogen atoms


H Br

H H

Br

Br

fast(1o cation)(minor)

slow(r.d.s.)

Br

fast

H

Br

H

(2o cation)

(major)

☓

Step 1 Step 2


x


Examples

+(1)H Cl

Cl

H

H

Cl

(95 : 5)

+(2)H Br

(98 : 2)

H

Br

Br

H


2B.General Statement of Markovnikov’s Rule

In the ionic addition of an unsymmetrical reagent to a double bond, the positive portion of the added reagent attaches itself to a carbon atom of the double bond so as to yield the more stable carbocation as an intermediate


Examples

(1)Cl OH

Cl

OH

Cl

OH

(major)more stable

3o cation

Cl Cl

OH

OH

(minor)less stable1o cation


Examples

(2)I Cl

Cl

(major)more stable

3o cation

I I

Cl

(minor)less stable1o cation

ClCl

II

ADDITION OF SULFURIC ACID TO ALKENES

The addition of sulfuric acid is regioselective, and it follows Markovnikov’s rule

Alcohols from Alkyl Hydrogen Sulfates

Alkyl hydrogen sulfates can be easily hydrolyzed to alcohols by heating with water.


2C. Regioselective Reactions When a reaction that can potentially

yield two or more constitutional isomers actually produces only one (or a predominance of one), the reaction is said to be regioselective

+H Cl

Cl

H

H

Cl

95 : 5

(major) (minor)

Regioselectivity:

regioisomers


2D. An Exception to Markovnikov’s Rule

Via a radical mechanism (see Chapter 10)

This anti-Markovnikov addition does not take place with HI, HCl, and HF, even when peroxides are present

H Br Br(anti-Markovnikovproduct)RO OR

heat H

With peroxides


4. Addition of Water to Alkenes:Acid-Catalyzed Hydration

Overall process●Addition of H–OH across a C=C

bond●H+ is the electrophile●Follows Markovnikov’s rule

OH HH2O

dilute H3O+

(e.g. dilute H2SO4, H3PO4)


H O

H

H

more stable

3o cation

slow

(step 1)

H

H2O

fast

(step 2)

H

OH H

H2Ofast(step 3)

H

OH

+H O

H

H

4A.Mechanism

ADDITION OF WATER TO ALKENES: ACID-CATALYZED HYDRATION

Example

The rate-determining step in the hydration mechanism is step 1; the formation of the carbocation.


5. Alcohols from Alkenes throughOxymercuration–Demercuration:Markovnikov Addition

Step 1: Oxymercuration

C C

HO HgOAc

C CHg(OAc)2

THF-H2O

Step 2: Demercuration

C C

HO HgOAc

NaBH4

HOC C

HO H


5A.Regioselectivity of Oxymercura-tion–Demercuration Oxymercuration–demercuration is

also highly regioselective and follows Markovnikov’s rule

HO

HgOAcHg(OAc)2

THF-H2O

HO

H

NaBH4

HO


HgOAc

+AcO

HO

HgOAc

O

HgOAc

HH

5C. Mechanism of Oxymercuration Does not undergo a “free carbocation”

H2O attacks the carbon of the bridged Hg ion that is better able to bear the partial +ve charge

OAc

Hg

OAc

H2O

H2O


hydroboration-oxidation-hydrolysis

OHH

1. BH3-THF

2. H2O23. NaOH, H2O

●Overall: anti-Markovnikov addition of H–OH across a C=C bond

●Opposite regioisomers as oxymercuration-demercuration


Example

H3C H

BH3-THFH

CH3

BH2

H

H2O2HO

H

CH3

OH

H

anti-Markovnikovsyn addition

This oxidation step occurs with retention of configuration


8A.Regiochemistry and Stereo-chemistry of Alkylborane Oxidation and Hydrolysis

Hydroboration–oxidation reactions are regioselective; the net result of hydro-boration–oxidation is anti-Markovnikov addition of water to an alkene

As a consequence, hydroboration–oxidation gives us a method for the preparation of alcohols that cannot normally be obtained through the acid-catalyzed hydration of alkenes or by oxymercuration–demercuration


H3O , H2O

H

OH

123

4

5

6

1

2

3

4

5

6

1-Hexene 2-Hexanol

OH

H

12

3

4

5

6

1

2

3

4

5

6

1-Hexene 1-Hexanol(90%)

1. BH3.THF

2. H2O2, HO

Markovnikov

Anti-Markovnikov


Examples

H

H2OH

OH

1. Hg(OAc)2, THF-H2O

2. NaBH4, HO

H

OH

1. BH3-THF

2. H2O2, HO

OH

H

with rearrangement

Markovnikov addition of H2O without rearrangement

anti-Markovni-kov, syn addition of H2O

H

Hvia1,2-hydride

shift


11. Electrophilic Addition of Bromine & Chlorine to Alkenes

Addition of X–X (X = Cl, Br) across a C=C bond

Br2C C

Br

C C

BrCCl4

(vicinaldibromide)


Examples

(anti addition of Br2)

Br

Br

Br

Br

+Br2

5oC

(racemate)

(1)

(anti addition of Cl2)

Cl2

10oC(2) Ph

PhPh

Ph

Cl

Cl

Cl

PhPh

Cl

same as

(rotation of C1-C2 bond)

1

2


Br

+ Br

C C

Br

Br

11A. Mechanism of Halogen Addition

C C + Br Br

Br

Br

Br–Br bond becomes polarized when close to alkene

(vicinaldibromid

e)(bromoniu

m)


Br

Br

H H CCl4

Br Br

Stereochemistry●Anti addition

H

Br

Br

H

SN2 reaction

(anti)

enantiomer +

ADDITION OF BROMINE AND CHLORINE TO ALKENES


13. Halohydrin Formation

Addition of –OH and –X (X = Cl, Br) across a C=C bond

X+ is the electrophile Follows Markovnikov’s rule

X2C C

OH

C C

XH2O


H3C HH2O

Br Br

BrH3C H

H3CBr

H

H2O

OH

CH3

H

Br

Mechanism


OMe

Br

Br2

MeOH

e.g.

Other variation●If H2O is replaced by ROH, ●ROH will be the nucleophile


15. Oxidation of Alkenes:Syn 1,2-Dihydroxylation

Overall: addition of 2 OH groups across a C=C bond

Reagents: dilute KMnO4 / HO⊖ / H2O / cold or OsO4, pyridine then NaHSO3, H2O

C C

OH OH


15A. Mechanism for Syn Dihydroxylation of Alkenes

C C

dil. KMnO4HO , H2O

cold

C C

O O

MnO

HO

H2OC C

OH OH

+ MnO2

C C

O O

OsO O

NaHSO3

H2OC C

OH OH

+ Os

OsO4pyridine

O


Both reagents give syn dihydroxylation

H H

H H

OH OH

or OsO4, pyridinethen NaHSO3

(cis-diol)

dil. KMnO4

HO, H2O, cold

OXIDATIONS OF ALKENES: SYN HYDROXYLATION

Osmium tetroxide addition to cyclopentene


Comparison of the two reagents●KMnO4: usually lower yield and

possibly side products due to over-oxidation

O

OH

OH

O+1. KMnO4,

2. H+

●OsO4: usually much higher yield but OsO4 is extremely toxic

(oxidative cleavage of C=C)


16. Oxidative Cleavage of Alkenes

16A. Cleavage with Hot Basic Potassium Permanganate

KMnO4, HO, H2O

O

O

a2

or

O

OH2

H3O

b

a

b

a

b

ab a

b

ab


Other examples

1. KMnO4, HO, H2O, heat

2. H3OO

O C O

+

(1)

O

O

OH

1. KMnO4, HO , H2O, heat

2. H3O(2)


16B. Cleavage with Ozone

R'

R

H

R"

O

R'

R

O

H

R"1. O3+

2. Zn, AcOH or Me2S


Examples

(1)1. O3

2. Zn, AcOH

O

O

+

(2)O

H

O

1. O3

2. Me2S


Mechanism

C C

OO

O

CO

C

OO

C C

OO

O

initial ozonide

C

O

C

OO

+

O O

CO

C

ozonide+C O CO

+Zn(OAc)2Zn

AcOH


17. Electrophilic Addition of Bromine & Chlorine to Alkynes

C CR H

X

C C

X

R

X

H

XCH2Cl2

(X = Cl, Br, I)

X2 (excess)

C CR H

X

C C

X

R

X

H

X

C C

H

X

X

H X2X2

(anti-addition)


18. Addition of Hydrogen Halidesto Alkynes

C CR H

X

C C

H

R

X

H

H(X = Cl, Br, I)

X (excess)H

Regioselectivity●Follows Markovnikov’s rule

C C

Br

CH3

Br

H

H

Hgem-dibromide

C CH3C HHBr

C C

CH3

Br

H

H HBr


Mechanism

C CCH3 HH Br

C C

H

H

CH3Br

C C

CH3

Br

H

H

C C

H

H

HCH3

BrBrC

H

H

H

C

Br

CH3

Br

H Br


Anti-Markovnikov addition of hydrogen bromide to alkynes occurs when peroxides are present in the reaction mixture

H Br

peroxidesBr

H

(E) and (Z)

(74%)


19. Oxidative Cleavage of Alkynes

C CR R' + R'CO2H1. O3

2. HOAcRCO2H

C CR R' + R'CO2H1. KMnO4, HO

2. H3ORCO2H

Example1. O3

2. AcOHC CPh CH3 PhCO2H + CH3CO2H

OR


How to synthesize ?

OH

(target molecule) (precursor)

OH

●Retrosynthetic analysis

20A. Retrosynthetic Analysis


●Synthesis

OH

H

H2O

1. Hg(OAc)2, THF-H2O

2. NaBH4, HO

or

Markovnikov additionof H2O


How to synthesize ?

(target molecule) (precursor)

OH

OH

●Retrosynthetic analysis

●Synthesis

OH

1. BH3-THF

2. H2O2, HO

anti-Markovnikov addition of H2O

Summary of Addition Reactions of Alkenes

Summary of Addition Reactions of Alkynes


END OF CHAPTER 8

© 2014 by john wiley & sons, inc. all rights reserved. chapter 8 alkenes and alkynes ii:...

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