Basic Organic ChemistryCourse code: CHEM 12162

(Pre-requisites : CHEM 11122)

Chapter – 08Chapter – 08

Aromatic Compounds

Dr. Dinesh R. Pandithavidana

Office: B1 222/3

Phone: (+94)777-745-720 (Mobile)

Email: dinesh@kln.ac.lk

Discovery of Benzene

• Isolated in 1825 by Michael Faraday who

determined C:H ratio to be 1:1.

• Synthesized in 1834 by Eilhard Mitscherlich

who determined molecular formula to be who determined molecular formula to be

C6H6.

• Other related compounds with low C:H ratios

had a pleasant smell, so they were classified

as aromatic.

Resonance Structure

C

C

CC

C

C

H

H

H

H

H

H Proposed in 1866 by Friedrich

Kekulé, shortly after multiple bonds

were suggested.

Each sp2 hybridized C in the ring has an unhybridized

p orbital perpendicular to the ring which overlaps

around the ring.

Unusual Reactions

• Alkene + KMnO4 → diol (addition)

Benzene + KMnO4 → no reaction.

• Alkene + Br2/CCl4 → dibromide (addition)

Benzene + Br /CCl → no reaction.Benzene + Br2/CCl4 → no reaction.

• With FeCl3 catalyst, Br2 reacts with benzene

to form bromobenzene + HBr (substitution!).

Double bonds remain.

Annulenes

• All cyclic conjugated hydrocarbons were proposed to be aromatic.

• However, cyclobutadiene is so reactive that it is so reactive that it dimerizes before it can be isolated.

• And cyclooctatetraene adds Br2 readily.

Hückel’s Rule

• If the compound has a continuous ring of

overlapping p orbitals and has 4N + 2 electrons, it

is aromatic (molecule should be planar).

• If the compound has a continuous ring of • If the compound has a continuous ring of

overlapping p orbitals and has 4N electrons, it is

antiaromatic.

• Nonaromatic compounds do not have a

continuous ring of overlapping p orbitals and may

be nonplanar

[N]Annulenes

• [4]Annulene is antiaromatic

• [6]Annulene is aromatic, it’s a

planar, molecule.

• [8]Annulene would be antiaromatic,

• [10]Annulene is aromatic except for

the isomers that are not planar.

Cyclopentadienyl Ions

• The cation has an empty p orbital, 4 electrons, so antiaromatic.

• The anion has a nonbonding pair of electrons in a p orbital, 6 electrons, aromatic.

Acidity of Cyclopentadiene

pKa of cyclopentadiene is 16, much more

acidic than other hydrocarbons.

pKa = 19pKa = 16

HOC(CH3)3+

H

OC(CH3)3_

+

H H

Tropylium Ion

• The cycloheptatrienyl cation has 6 p electrons

and an empty p orbital.

• Aromatic: more stable than open chain ion

H OH

H+ , H2O

H

+

Dianion of [8]Annulene

• Cyclooctatetraene easily forms a 2- ion.

• Ten electrons, continuous overlapping porbitals, so it is aromatic.

+ 2 K + 2 K+

Pyridine

• Heterocyclic aromatic compound.

• Nonbonding pair of electrons in sp2 orbital, so

weak base, pKb = 8.8.

Pyrrole

Also aromatic, but lone pair of electrons is

delocalized, so much weaker base.

Basic or Nonbasic-.???

NN

Pyrimidine has two basic

nitrogens.

N N HImidazole has one basic

nitrogen and one nonbasic.

N

N

N

N

H

PurineB?

Other Heterocyclic Compounds

Fused Ring Hydrocarbons

• Naphthalene

• Anthracene

• Phenanthrene

Common Names of Benzene

Derivatives

OH OCH3NH2CH3

phenol toluene aniline anisolephenol toluene aniline anisole

C

H

CH2 C

O

CH3

C

O

HC

O

OH

styrene acetophenone benzaldehyde benzoic acid

Disubstituted Benzenes

The prefixes ortho-, meta-, and para- are commonly

used for the 1,2-, 1,3-, and 1,4- positions,

respectively.

BrBr

Br

o-dibromobenzene or1,2-dibromobenzene

HO

NO2

p-nitrophenol or4-nitrophenol

3 or More Substituents

Use the smallest possible numbers, but the

carbon with a functional group is #1.

OH

NO2

NO2

O2N

1,3,5-trinitrobenzene

NO2

NO2

O2N

OH

2,4,6-trinitrophenol

Common Names for

Disubstituted Benzenes

CH3 CH3

CH

CO OH

OH

CH3 CH3H3C

CH3OH

H3Cm-xylene mesitylene o-toluic acid p-cresol

Phenyl and Benzyl

Phenyl indicates the benzene ring attachment.

The benzyl group has an additional carbon.

Br

phenyl bromide

CH2Br

benzyl bromide

=>

Physical Properties

• Melting points: More symmetrical than

corresponding alkane, pack better into

crystals, so higher melting points.

• Density: More dense than nonaromatics, less

dense than water.

• Solubility: Generally insoluble in water.

Electrophilic Aromatic Substitution

Electrophile substitutes for a hydrogen on

the benzene ring.

Mechanism

Bromination of Benzene

• Requires a stronger electrophile than Br2.

• Use a strong Lewis acid catalyst, FeBr3.

Br Br FeBr3 Br Br FeBr3

Br Br FeBr3

H

H

H

H

H

H

H

H

H

H

HH

Br+ + FeBr4

_

Br

HBr+

Chlorination and Iodination

• Chlorination is similar to bromination. Use

AlCl3 as the Lewis acid catalyst.

• Iodination requires an acidic oxidizing agent,

like nitric acid, which oxidizes the iodine to an like nitric acid, which oxidizes the iodine to an

iodonium ion.

H+

HNO3 I21/2 I+

NO2 H2O+ ++ +

Nitration of Benzene

Use sulfuric acid with nitric acid to form the

nitronium ion electrophile.

H O N

O

O

H O S O H

O

O

+ HSO4

_H O N

OH

O+

H O N

OH

O+

H2O + N

O

O

+

Sulfonation

Sulfur trioxide, SO3, in fuming sulfuric acid is

the electrophile.

S

O

S

O

S

O

S

O

+ + +

_

_ _

2 H2SO4 SO3 + H3O+ + HSO4

-

O OS

O OS

O OS

O O

+ +_ _

S

O

OO

H

S

O

O

OH

+

_

S

HOO

O

benzenesulfonic acid

Nitration of Toluene

• Toluene reacts 25 times faster than benzene. The

methyl group is an activator.

• The product mixture contains mostly ortho and para

substituted molecules.

Sigma Complex

Intermediate

is more

stable if

nitration nitration

occurs at the

ortho or para

position.

Energy Diagram

Activating, Ortho-, Para- Directing

Substituents

• Alkyl groups stabilize the sigma complex by

induction, donating electron density through the

sigma bond.

• Substituents with a lone pair of electrons stabilize

the sigma complex by resonance.

OCH3

H

NO2

+

OCH3

H

NO2

+

The Amino Group

Aniline reacts with bromine water (without a

catalyst) to yield the tribromide.

Sodium bicarbonate is added to neutralize the

HBr that’s also formed.

NH2

Br23

H2O, NaHCO3

NH2

Br

Br

Br

Summary of Activators

Deactivating Meta-Directing

Substituents

• Electrophilic substitution reactions for nitrobenzene are 100,000 times slower than for benzene.

• The product mix contains mostly the meta isomer, • The product mix contains mostly the meta isomer, only small amounts of the ortho and para isomers.

• Meta-directors deactivate all positions on the ring, but the meta position is less deactivated.

Ortho Substitution on Nitrobenzene

Para Substitution on Nitrobenzene

Meta Substitution on Nitrobenzene

Meta-Deactivators

The atom attached to the aromatic ring will have a partial

positive charge.

Electron density is withdrawn inductively along the sigma

bond, so the ring is less electron-rich than benzene.

More Deactivators

Halobenzenes

• Halogens are deactivating toward electrophilic

substitution, but are ortho, para-directing!

• Since halogens are very electronegative, they

withdraw electron density from the ring inductively

along the sigma bond.

• But halogens have lone pairs of electrons that can

stabilize the sigma complex by resonance.

Summary of Directing Effects

Multiple Substituents

The most strongly activating substituent will

determine the position of the next substitution.

May have mixtures.

OCH3

O2N

SO3

H2SO4

OCH3

O2N

SO3H

OCH3

O2N

SO3H

+

Friedel-Crafts Alkylation

• Synthesis of alkyl benzenes from alkyl halides and

a Lewis acid, usually AlCl3.

• Reactions of alkyl halide with Lewis acid produces

a carbocation which is the electrophile.a carbocation which is the electrophile.

• Other sources of carbocations:

alkenes + HF or alcohols + BF3.

Examples of Carbocation Formation

CH3 CH CH3

Cl

+ AlCl3

CH3

C

H3C H

Cl AlCl3+ _

F_

H2C CH CH3

HFH3C CH CH3

F+

H3C CH CH3

OHBF3

H3C CH CH3

OH BF3+

H3C CH CH3

++ HOBF3

_

Formation of Alkyl Benzene

C

CH3

CH3

H+

H

H

CH(CH3)2+

H

H

CH(CH3)2

B

F

F

F

OH

CH

CH3

CH3

+

HF

BF

OHF+

-

Limitations of Friedel-Crafts Reactions

• Reaction fails if benzene has a substituent that is more

deactivating than halogen.

• Carbocations rearrange. Reaction of benzene with n-

propyl chloride and AlCl3 produces isopropylbenzene.

• The alkylbenzene product is more reactive than benzene,

so polyalkylation occurs.

(CH3)2CHCl

AlCl3

CH(CH3)2 CH(CH3)2

CH(CH3)2

+

Friedel-Crafts Acylation

• Acyl chloride is used in place of alkyl

chloride.

• The acylium ion intermediate is resonance

stabilized and does not rearrange like a stabilized and does not rearrange like a

carbocation.

• The product is a phenyl ketone that is less

reactive than benzene.

Mechanism of Acylation

R C

O

Cl AlCl3 R C

O

AlCl3Cl+ _

R C

O

AlCl3Cl+ _

AlCl4 +

_ +R C O R C O

+

C

O

R

+

H

C

H

O

R

+

Cl AlCl3

_C

O

R +

HCl

AlCl3

Clemmensen Reduction

Acylbenzenes can be converted to

alkylbenzenes by treatment with aqueous HCl

and amalgamated zinc.

+ CH3CH2C

O

Cl1) AlCl3

2) H2O

C

O

CH2CH3Zn(Hg)

aq. HCl

CH2CH2CH3

Nucleophilic Aromatic

Substitution

• A nucleophile replaces a leaving group on

the aromatic ring.

• Electron-withdrawing substituents activate

the ring for nucleophilic substitution.

Examples of Nucleophilic Substitution

Addition-Elimination Mechanism

Benzyne Mechanism

• Reactant is halobenzene with no electron-withdrawing groups on the ring.

• Use a very strong base like NaNH2.

Benzyne Intermediate

CH3

HH

H NH2

CH3

HH

H NH2

_

or

CH3

HH

H

NH2

CH3

HH

H

NH2_

meta-toluidine para-toluidine

Catalytic Hydrogenation

• Elevated heat and pressure is required.

• Possible catalysts: Pt, Pd, Ni, Ru, Rh.

• Reduction cannot be stopped at an

intermediate stage.

CH3

CH3

Ru, 100°C

1000 psi3H2,

CH3

CH3

Birch Reduction

Mechanism of Birch Reduction

Mechanism of Birch Reduction

Side-Chain Oxidation

Alkylbenzenes are oxidized to benzoic acid by

hot KMnO4 or Na2Cr2O7/H2SO4.

CH(CH3)2

CH CH2

KMnO4, OH-

H2O, heat

COO

COO

_

_

Side-Chain Halogenation

• Benzylic position is the most reactive.

• Chlorination is not as selective as bromination,

results in mixtures.

• Br2 reacts only at the benzylic position.

CHCH2CH3

Br

hνBr2,

CH2CH2CH3

SN1 Reactions

• Benzylic carbocations are resonance-

stabilized, easily formed.

• Benzyl halides undergo SN1 reactions.

CH2BrCH3CH2OH, heat

CH2OCH2CH3

SN2 Reactions

• Benzylic halides are 100 times more

reactive than primary halides via SN2.

• Transition state is stabilized by ring.