Reactions of Alkenes and Alkynes - NDSUcook.chem.ndsu.nodak.edu/.../uploads/2014/08/240-Chapter04.pdf · Chapter 04 Reactions of Alkenes and Alkynes CHEM 240: Spring 2014 ... 10 •

©2014 Gregory R Cook

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Chapter 04Reactions of Alkenes and

Alkynes

CHEM 240: Spring 2014

Prof. Greg Cook

cook.chem.ndsu.nodak.edu/chem240

Thursday, September 18, 14

http://cook.chem.ndsu.nodak.edu/chem341





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0Electrophilic Addition of HX

2

C CH

H

H

H+ HBr C C

H

BrHH

H

H





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0Electrophilic Addition of HX

3

C CH

H

H

H+ HBr C C

HHH

H

H

E

reaction progress

transition state ‡

ΔG°

transition state ‡

C CH

H

H

H+ HBr

C C

H

Br

HH

H

H

C CH

HH

H

H

activationenergyΔG‡

C CH

BrHH

H

HBrRDS(slow step)





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0Regioselectivity

4

HBr

Br

HH

Br+

80% yield not formed

HBr HBr

90% yield

HBr

100% yield

Br

H





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0Markovnikov’s Rule

5

• “When an unsymmetrically substituted alkene reacts with HX, the hydrogen adds to the carbon that has the greater number of hydrogens, and the halogen adds to the carbon having fewer hydrogens.”





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0Carbocation Structure

6

• Carbocations are sp2 hybridized with a trigonal planar geometry (empty p orbital)

C

CC

CCH

HH

H

HH

HHH

C CCHH

H

H

H

H

H HH

tert-butyl





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0Carbocation Stability

7

• More substituted carbocations are more stable (lower in energy)

C

C

H

HHC

H

HCH

HH

C

H

CCH

HH

H

H

HC

C

CCH

HH

H

H

H

HHH

CHHH C

HHC

HH

HCH

CCHH

H

H

H

HC CC

HH

H

H

H

H

H HH

methyl ethyl isopropyl tert-butyl

stability (lower energy)





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8

• Spread out the positive charge (more stable)

• inductive effects and hypercongujugation





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9







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10







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0Mechanistic basis for Regioselectivity

11





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0Markovnikov’s Rule - Restated

12

• When an unsymmetrically substituted alkene reacts with HX, protonation occurs to form the more stable carbocation.





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0Addition of H2O - Hydration of Alkenes

13

H3PO4 cat.OH

250 °C+ H2O





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0Halogenation of Alkenes

14

+C C C CX

XX X

+ Br BrBr

BrCHCl3

anti addition





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0Mechanism of Electrophilic Halogenation

15

Br Br

Br

Br

Br

Br

A bromonium ion

H

OH

O

Br

H

HOH

Br

-H+

a halohydrin





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0Generalities of Electrophilic Addition

16

A+ B-

A

orA

A

B

A+ B-overall equivalent

reagents notes

HX H+ X- MarkovnikovX2 X+ X- bridged, anti addnX2/H2O X+ HO- bridged, anti addn, Markovnikov

H2O (H+ cat) H+ HO- MarkovnikovBH3 OH+ H- syn addn, anti-Markovnikovthen H2O2/OH-





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0Addition of H2 - Hydrogenation

17

C C

H

H

H

H

H

HC C

H

H H

H

+ H Hcatalyst

ΔH° = -136 kJ/mol (-32.6 kcal/mol)

carbon

Pd H H

carbon

Pd

H H

Hydrogen is activated

C C

H

H

H

H

C C

H

H

H

H

H

H

The catalyst activates hydrogen gas by splitting the H-H bond. It then transfers the H's to the alkene and this regenerates the metal catalyst to activate more hydrogen





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0Heat of Hydrogenation

18





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0Hydrogenation Stereochemistry

19

CO2CH3

CO2CH3

H2Pt

CO2CH3

CO2CH3

H

H

syn addition of H2

CO2CH3

H

H

CO2CH3

only product

H

H

H

H

H

H

H

C C

H

H

H

H

H

H

H

C C H

H

H

H

H

H

CCH

CCH

H





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0Hydrogenation Stereochemistry

20

H

HH3C

H

H3C CH3H

HH

CH3

H3C CH3

H

H3C

H3C CH3

α-pinene cis-pinene trans-pinene

H2Ni





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0Oxidation-Reduction

21

• Oxidation of carbon - increase the number of bonds to oxygen (more electronegative atom)

• Reduction of carbon - decrease the number of bonds to oxygen (more electronegative atom)

C

H

H H

H

C OOoxidation

C-4 C+4





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0Oxidation-Reduction

22

CH

HH

H CH

HH

OH

CH

HH

Cl

CHH

O

CH

HCl

Cl

CHOH

O

CH

ClCl

Cl

CO

CCl

ClCl

Cl

O

Lower Oxidation State

Higher Oxidation State

CO

HO OH

CH

HH

Cl + 2 Li CH

HH

Li + LiCl





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0Epoxides

23

water

HO

H RO

H RO

Ralcohol ether

C C

O





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0Expoxidation of Alkenes

24

peroxy acid

C C

O

+C C R O

OO

H

epoxide





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0Formation of 1,2-diols

25

mCPBAO

H3O+ OH

OH

OH

OH





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0Oxidation of Alkenes

26

OH

OH

+ KMnO4

H2ONaOH

O

O

+ KMnO4

H3O+





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0Reactions of Alkenes with Alkenes - Polymers

27

polymerization

polyethylene

polymerization

polystyrene

Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph

Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph

Ph =

Cl Cl Cl Cl Cl Cl Cl Cl

nPVC (polyvinylchloride)





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0Free Radical Polymerization Mechanism

28

• Initiation - A stable molecule is activated to generate free radical species

• Step 1 - Benzoyl peroxide breaks apart to form two radicals

OO

O

OO

O

benzoyl peroxide BzO•





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• Chain Propogation - a stable molecule (alkene) reacts with a radical to produce a new stable molecule and new radical

• Step 2 - Addition of BzO• to alkene

O

O

BzO•

+ BzO





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• Chain Propogation

• Step 3 - Alkyl radical reacts with another alkene

+ BzOBzO





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• Termination - two radicals combine to form a stable molecule. This stops chain propogation

+BzOR BzO

R





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32

+ BzOBzO

O

O

BzO•

+ BzO

OO

O

OO

O

benzoyl peroxide BzO•

+ BzOBzO R R

Initiation

Propagation

Termination





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0Conjugation

• Conjugation - conjugare (Latin) to link together - when pi bonds are extended beyond 2 carbons

33

conjugated not conjugated

CH

H

β-Carotene - orange colored constituent of carrots





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0Conjugation and Color

34

lycopene (red color of tomatos)





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0Conjugation and Color

35

O

OHR

R

OROH

HO NaOH O

OR

R

OROH

O





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0Electrophilic Addition to Dienes

36

HBr

Br

Br+H H

71% 29%

Br2

Br

Br+Br Br

55% 45%





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0Allylic Conjugation

37

allylic carbocation allylic free radical allylic carbanion





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0Allylic and Vinylic Positions

38

allylic alcohol

OH

allylic bromide

Br

CC

C

H

H

H

H

HH

allylic hydrogens

vinylic hydrogens





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0 ResonanceSection 4.9-10





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0Allylic Carbocations

40





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0Allylic Carbocations

41





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0Resonance

• Some molecules may have more than one correct Lewis structure

• These are NOT isomers. They are resonance forms.

42

OO

O OO

O





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0General Rules for Resonance Structures

• Individual resonance forms DO NOT EXIST - they represent the extremes. The actual chemical species is a hybrid of all the resonance forms.

• Resonance forms differ only in the distribution of pi or non-bonding electrons. Atoms do not move.

• Individual resonance forms must obey normal valency rules. You cannot break the octet rule.

• The actual hybrid is more stable than the individual resonance forms and the individual structures do not have to be equal in energy.

43





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• How many resonance forms are there?

Example HNO3

44

H3C C

O

O

O N

O

O





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0Example Benzene

45





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0Not all Resonance Forms are Equal

• Formamide

46

HC

NH

H

O





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0 Reactions of AlkynesSection 4.11





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0Hydrocarbons

• Alkanes: C-C single bondsAlkenes: C-C double bondsAlkynes: C-C triple bonds

• Aromatic hydrocarbons

48





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0Naming Alkynes

• Alkynes are named similar to alkenes

• They have equal priority

49

oct-1-ene-6-yne

propyne 1-butyne 2-butyne





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0Structure of Alkynes

50

C CH H





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0Hydrogenation of Alkynes

• Similar to alkenes, alkynes can be reduced by catalytic hydrogenation

51

H2Pd/C

H H H2Pd/C

H H

H H

H HH2

Lindlar's Catalyststops at alkene

Lindlar's CatalystPd / Pb(OAc)4

quinolineon CaCO3





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0Addition of HX to Alkynes

52

HBr1 equiv

H

Br

HBr1 equiv

H H

Br Br

rate = k [alkyne] [HBr]2





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0Halogenation of Alkynes

53

Br21 equiv

Br

Br

Br21 equiv

Br Br

Br Br





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0Hydration of Alkynes

54

an enol

OHH+, H2O O

a ketone

cat. HgSO4H2O

OH

Hg+

H2SO4OH

Henol





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0Acidity of Terminal Alkynes

55

C CH

HH

H

HH C C

H

H

H

HC CH H

pKa 62 45 26

base base base

C CH

HH

HH C C

H

H HC CH

carbanion carbanion carbanion





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0Deprotonation of Alkynes

56

C CH HpKa 26

C CH+ H O H + H O H

H

pKa -1.7

C CH HpKa 26

C CH+ O H + H O HpKa 15.7

C CH HpKa 26

C CH+ N H +pKa 36

H H N H

H





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0Alkylation of Alkynes

• Terminal alkynes can be alkylated to make new carbon-carbon bonds. Reaction proceeds via a SN2 substitution.

57

NaNH2H NaH3C Br

δ+ δ-

CH3




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1


Organic Synthesis

58

NaNH2H

Br

LiNH3

H HNaNH2

BrH

NaNH2

Br

1)

2)

2) 2)

1) 1)


Reactions of Alkenes and Alkynes - NDSUcook.chem.ndsu.nodak.edu/.../uploads/2014/08/240-Chapter04.pdf · Chapter 04 Reactions of Alkenes and Alkynes CHEM 240: Spring 2014 ... 10 •

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