Chapter 09 Benzene and Its Derivatives -...

©2016 Gregory R Cook

cook.chem.ndsu.nodak.edu/chem240

Chapter 09Benzene and Its

Derivatives

CHEM 240: Fall 2016

Prof. Greg Cook


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




Benzene

• First isolated in 1825 from whale oil by Michael Faraday

• Unsaturated hydrocarbon but did not have the typical reactivity of alkenes or alkynes.

2

C6H6

Br2or

KMnO4no reaction

prismane Dewar benzene




Benzene

• In 1865 Kekulé proposed a structure having a six-membered ring with alternating double bonds.

3

1.34 Å1.54 Å

Kekulé structures

H

HH

H

H H

modern view of benzene's structurefully conjugated

RESONANCE - not equilibrium

bonds identical1.39 Å




Aromatic Reactivity

4

+ Br2

Br

BrH

H

NOT formed

+ Br2

FeBr3 catalyst Br+ HBr




Aromatic Stability

5




Aromatic Stability

• To have aromatic stabilization, a molecule must have three things:

• A planar ring

• A fully conjugated pi-system (a p orbital on every atom in the ring)

• Have 4n+2 number of electrons in the pi-system (Hückel Rule)

6




Aromatic Stability

7

AROMATIC2 pi electrons

4 pi electrons 4 pi electrons AROMATIC6 pi electrons



8 pi electrons




Aromatic Compounds

8

HO

H

OCH3

H H

estrone

N

S

H3C

O O

OCH2CH3

NH

NNCH3

O

sildenafil (viagra)

CHO CO2H CH3 CH3

CH3

OH NH2

benzaldehyde benzoic acid toluene xylene phenol aniline

naphthalene

polyaromatic hydrocarbons (PAH's)carcinogens formed from combustionThis PAH is the main carcinogen found in Tobacco smoke (so quit for goodness sake!)cinnamaldehyde

CHO

almond

cinnamon moth balls




Aromatic Heterocycles

9

N NH

O

N

NH

lone pairin resonancewith aromaticpi system



N H O N HN N

N

NN

N

NH2

HN

NN

N

O

H

H

NH2 N

N

NH2

OH

N

N

OH

O

H3C

HGuanine Thymine CytosineAdenine




Fused Aromatic Rings

10




Aromatic Carbon - Graphite

11




Aromatic Carbon - Graphite

12




Aromatic Carbon - other forms

13

carbon nanotubes

buckminsterfullerene




Naming Aromatic Compounds

14




Naming Substituted Benzenes

15




Naming Substituted Benzenes

16

Cl

orthodichlorobenzeneor

1,2-dichlorobenzene

Cl

CO2H

paranitrobenzoic acidO2N

2,4,6-trinitrotoluene

CH3

O2N NO2

NO2

Ph

4-phenyloctanebiphenylwe often abbreviate phenyl

with Ph when we draw structures

4-phenyloctane




Oxidation and Reduction

17

KMnO4CH3 CO

OH

KMnO4 CO

OH

KMnO4 CO

OH

O

KMnO4 No Reaction

H2

Pd/C

H2

Pt, ethanol130 atm

or H2CrO4 or H2CrO4

or H2CrO4or H2CrO4

benzylic carbon




Electrophilic Aromatic Substitution

18

+ Br2

Br

BrH

H

NOT formed

Br+ HBr

FeBr3




Electrophilic Aromatic Substitution

19

BrBr

HBr

HBr

H-HBr

Br

FeBr4

FeBr4

Br Br

BrH

HH

Br

NOT formed

reaction progress

energy

Rate Determing Step (RDS)




Chlorination and Iodination

20

+ Cl2Cl

+ HClFeCl3

+ Br2Br

+ HBrFeBr3

+ I2I

+ HICuI2




Nitration

21

+ HNO3

NO2H2SO4




Sulfonation

22

+ SO3

SO3HH2SO4




Friedel-Crafts Alkylation

23

+ RRAlCl3

Cl




Limitations of the Friedel-Crafts Alkylation

• Only alkyl halides work, not aryl or vinyl halides.

• Aromatic rings with electron withdrawing groups already on them will not work (-NO2, -CN, -SO3H, -COR, -CO2H).

• Aromatic rings with amine groups will not work (-NH2) because the amine will react with the alkyl halide.

24

+ RRAlCl3

Cl




Friedel-Crafts Acylation

25

+AlCl3

R Cl

O

O

R




Summary of EAS

26

Br2FeBr3

BrBr2 + FeBr3 Br + FeBr4

electrophile

Cl2FeCl3

Cl

I2CuI2

I

HNO3H2SO4

NO2

Cl2 + FeCl3 Cl + FeCl4

electrophile

I2 + CuI2 I + CuI3

electrophile

SO3H2SO4

SO3H

H2SO4NO

OHO + O N O

electrophile

H2SO4SO

O+O S

O

OHO

electrophileR-ClAlCl3

R

R Cl

O

AlCl3 R

O

AlCl3+

electrophile

+

electrophile

R Cl R + AlCl4

R Cl

OAlCl3 R Cl

O+ AlCl4




Substituent Effects

• How does Y affect the reactivity of the benzene ring?

• How does Y affect the ratio of products?

27

+

Y

E+

Y Y Y

E

E

E

+ +




Substituent Effect on Reaction Rate

28

Electron Donating Groups Electron Withdrawing Groups

OR NR2alkyl X SO3HNO2SR CN

Activating Deactivating

O

R

> > >

OH Cl NO2

relativereactivity 1000 1 0.033 6x10-8




Electron Density in the Ring

29




Substituent Effect on Regioselectivity

30




Electron Donating Groups

31

OHE+

ortho additionOH

E

H

OHE

H

OHE

H

OHE

H

especially stable

OHE+

para additionOH OH OH OH

especially stable

OHE+

meta additionOH OH OH

none especially stable

E H E H E H E H

E

H

E

H

E

H

OHE+

OH OH

+E

E

OH

+

E




Electron Withdrawing Groups

32

NO2

E+NO2 NO2

+E

E

OH

+

E

NE+

ortho addition

E

H

E

H

E

H

especially UNstable

E+

para addition

E+

meta addition

none especially UNstable - the lesser of the evils

E H E H E H

E

H

E

H

E

H

OON

OON

OON

OO

NOO

NOO

NOO

NOO

NOO

NOO

NOO

NOO

especially UNstable




Halogens - deactivating, but o,p-directors?

33

EH

ClEH

Cl halogens, while inductively electron withdrawing groups, they can stabilze plus charge through resonance. Thus, they are an exception and are ortho-para directors




Summary of Substituent Effects

34

Electron Donating GroupsElectron Withdrawing Groups

OR

NR2

alkyl

XSO3H

NO2

SR

CN

ActivatingDeactivating

O

R

ortho, para directorsstrongest (resonance)

weakest (inductive)

strongest (resonance)

meta directors

ortho, para directorsbut deactivating

weak




Organic Synthesis

• Create molecules by design

• Compare target and starting material

• Consider reactions that allow you to build molecules - may need to change functional groups along the way

• Retrosynthetic Analysis

• Work backwards from the product - immediate precursors

35




Organic Synthesis

• Starting from benzene, how would you synthesize p-bromobenzoic acid?

36

Br

CO2H




Organic Synthesis

• Starting from benzene, how would you synthesize p-bromobenzoic acid?

37

O2N

CH3

SO3H


Chapter 09 Benzene and Its Derivatives -...

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