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Aromatic Compounds. Lecture Outline Aromatic vs aliphatic compounds Aromatic vs aliphatic compounds...

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Aromatic Compounds
Transcript

Aromatic Compoun

ds

Lecture OutlineLecture Outline

Aromatic vs aliphatic compoundsAromatic vs aliphatic compounds BenzeneBenzene

Structure/ stabilityStructure/ stability Electronic configurationElectronic configuration

The concept of aromaticityThe concept of aromaticity Huckel’s ruleHuckel’s rule Aromatic compounds other than benzeneAromatic compounds other than benzene

Nomenclature of benzene derivativesNomenclature of benzene derivatives

Aromaticity

hydrocarbons

aliphatic aromatic

alkanes alkenes alkynes

unsaturated cyclic hydrocarbons organic compounds had an aroma

or odourcontain single rings or groups of ringssimplest aromatic compound is benzenearomatic compounds include benzene or

are benzene based and have benzene-like structures and properties.

Aromatic Compounds

NOTE:

H2C

H2CCH2

CH2

CH2

H2C

C6H12

cyclohexane

HC

HCCH

CH

CH

HC

C6H6

benzene

♪♫Sweet Dreams are Made of Benzene♪♫

Kekulé The idea of the

benzene structure came to Kekulé in a dream

Benzene, C6H6

6 carbon ring with a hydrogen bonded to each carbon

one electron from each carbon is free to participate in a double bond

Resonance Theory

1.Resonance forms are imaginary benzene has a single hybrid structure which combines the

characteristics of both resonance forms

Resonance forms

Hybrid structure

C

CC

C

CCH

H

H

H

H

H

C

CC

C

CC

H

H

H

H

H

H

C

CC

C

CC

H

H

H

H

H

H

Structure of BenzeneStructure of Benzene Benzene - Kekulé The first structure for benzene was

proposed by August Kekulé in 1872

this structure, however, did not account for the unusual chemical reactivity of benzene

CH

CH

CH

CHC

H

C

H

CC

CC

C

C

H

H

HH

HH

Molecular Orbital

* electron cloud delocalized all over the ring

* the resonance picture this helps to explain lack of reactivity of benzene

* great stability (substitution not addition )

Benzene - Resonance Energy

Resonance energy:Resonance energy: the difference in energy between a resonance hybrid and its most stable hypothetical contributing structures in which electrons are localized on particular atoms and in particular bonds one way to estimate the resonance energy

of benzene is to compare the heats of hydrogenation of benzene and cyclohexene

Benzene - Resonance Energy

heats of hydrogenation for both cyclohexene and benzene are negative (heat is liberated)

these results are shown graphically on the next slide

+ H2Ni

1-2 atmH° = -28.6 kcal/mol (-120 kJ/mol)

+3H2Ni

200-300 atmH° = -49.8 kcal/mol (-208 kJ/mol)

The Concept of AromaticityThe Concept of AromaticityRequirements for aromaticityRequirements for aromaticity

Molecule must be cyclicMolecule must be cyclic Molecule must be planarMolecule must be planar Each atom of the ring (s) must have a p-orbital Each atom of the ring (s) must have a p-orbital

perpendicular to the plane of the ringperpendicular to the plane of the ring Huckel’s rule must be obeyedHuckel’s rule must be obeyed

Planar compounds must have (4n + 2) pi (Planar compounds must have (4n + 2) pi ( electrons, where n is an integerelectrons, where n is an integer

n= integer (1,2,3,…) Ring with 2, 6, 10 or 14 pi electrons Ring with 2, 6, 10 or 14 pi electrons maymay be be

aromaticaromatic Ring with 8, 12 or 16 pi electrons Ring with 8, 12 or 16 pi electrons will notwill not be be

aromaticaromatic

For aromaticity, all pi (For aromaticity, all pi ( electrons must electrons must be paired and all bonding orbitals filledbe paired and all bonding orbitals filled Maximum and complete overlap is required Maximum and complete overlap is required

for stabilizationfor stabilization With unpaired pi (With unpaired pi ( electrons, overlap is electrons, overlap is

not maximizednot maximized

The pi (The pi ( electrons in an aromatic electrons in an aromatic compound are delocalized over the entire compound are delocalized over the entire ring leading to stabilizationring leading to stabilization

02/19/1302/19/13 Farshid ZandFarshid Zand 1515

So what makes a molecule So what makes a molecule aromaticaromatic??

It must be cyclicIt must be cyclic It must be conjugatedIt must be conjugated It must be flat so that the It must be flat so that the pp orbital orbital

overlap can occuroverlap can occur It must also have It must also have

4n + 2 pi electrons…4n + 2 pi electrons…

Six electrons present

n = 1, Aromatic Eight electrons present

Not aromatic The Cyclopentadienyl anionThe Cyclopentadienyl anion

H H

- H+

H Two electronspresent in orbital for anion

H H

- H-

HNo electronspresent in orbital for cation

The Cyclopentadienyl cationThe Cyclopentadienyl cation

Structure

Resonance theory of benzeneResonance theory of benzene All bonds are equivalent! electrons are delocalised around the

ring

AromaticityExample 1: BenzeneExample 1: Benzene

cyclic

planar

conjugated

6 electrons

Aromaticity

Other Examples?

cyclic

planar conjugated 6 electrons XX

cyclic planar

conjugated 6 electrons XX

© Prentice Hall 2001© Prentice Hall 2001 Chapter 14Chapter 14 2020

AromaticityAromaticity

cyclooctatetraene cyclooctatetraene is is nonnonaromaticaromatic

It is It is notnot planar planar

Physical Properties of Physical Properties of BenzeneBenzene

MPMPbenzenebenzene = 5.5°C, BP = 5.5°C, BPbenzenebenzene=80.1°C=80.1°C

non-polar Moleculenon-polar Molecule

insoluble in waterinsoluble in water

less dense than waterless dense than water

Nomenclature of aromatic compounds

A] Mono substituted benzene:

B] Disubstituted benzene.

*because all C equal no numbers

*Substituent and then benzene

Monosubstituted Benzenes

Monosubstituted aromatics are named using -benzene as the parent name preceded by the substituent name (as a prefix; all one word):

F NO2 CH2CH3

fluorobenzene nitrobenzene ethylbenzene Chlorobenzene

Cl

2424

II. Nomenclature

A. Single group

OCH3O H O OH

NH2 SO3H

methoxybenzene(anisole)

benzenecarbaldehyde(benzaldehyde)

benzenecarboxylic acid(benzoic acid)

benzenamine(aniline)

benzenesulfonic acid

Common Names of Subs. Benzenes

CH3 OH

OCH3

NH2

CCH2

H

C

O

HC

O

OH

C N

C

O

CH3

CHCH3

CH3

toluene phenol aniline

anisole styrene

benzaldehyde benzoic acid acetophenone

benzonitrile cumene

B] Disubstituted benzene

:

NH2

orthoposition

orthoposition

NH2

meta position

meta position

NH2

paraposition

When two substituents are present on the When two substituents are present on the benzene ring, the prefixes ortho, meta and benzene ring, the prefixes ortho, meta and para are used.para are used.

NH2

Cl

m-chloroaniline

Cl

Cl

p-dichlorobenzenepara-dichlorobenzene1,4-dichlorobenzene

2828

II. Nomenclature

B. Two groupsX

Y

X

Y

X

Yortho(or 1,2) meta

(or 1,3) para (or 1,4)

Br

NO2

NO2

02/19/1302/19/13 Farshid ZandFarshid Zand 2929

Disubstituted BenzenesDisubstituted Benzenes

Disubstituted benzenes use prefixes Disubstituted benzenes use prefixes ortho-(o), meta(m), or para-(p). These ortho-(o), meta(m), or para-(p). These prefixes are useful when discussing prefixes are useful when discussing reactions.reactions.

Disubstituted Benzenes

ortho-= o-= 1,2-disubstituted

)two groups on adjacent carbons on the ring(

F

F

o-difluorobenzene or1,2-difluorobenzene

CH3

CH3

Cl

Cl

Cl

CH3

1,2-dimethylbenzene 1,3-dichlorobenzene 1-chloro-4-methylbenzene

(ortho-dimethylbenzene) (meta-dichlorobenzene) (para-chloromethylbenzene)

OH

Cl

CH3

NO2

COOH OH

NH2

Br

4-Aminophenolp-

2-Chlorophenol O-

4-Nitrotoluen p-

3-Bromobenzoic acid m -

A. Preparation of Benzene

1. Decarboxylation

COOH1. NaOH

2. NaOH / heat

OH

heat

Zn dust

2. Reduction of phenol

C.Chemical Properties

1. Electrophilic substitution reactiona. Nitration (Forming nitro-compound)b. Sulphonation (Forming sulphonic acid)c. Halogenation (Forming halobenzene)d. Friedal-Craft Alkylation (Forming

alkylbenzene)e. Friedal-Craft Acylation (Forming

carbonyl compound)

Reactions of Benzene

The most characteristic reaction of aromatic compounds is substitution at a ring carbon

H Cl2FeCl3 Cl HCl+ +

Chlorobenzene

Halogenation:

H HNO3H2SO4

NO2 H2O++

Nitrobenzene

Nitration:

Reactions of Benzene

H H2SO4 SO3H H2O+

Benzenesulfonic acid

Sulfonation:

+

H RXAlCl3

R HX++

An alkylbenzene

Alkylation:

H R-C-XO

AlCl3 CR

O

HX++

Acylation:

An acylbenzeneAn acylhalide

Friedel –crafts Alkylation

Friedel –crafts Acylation

C.Chemical Properties

1. Addition Reaction (NOT electrophilic)

a. Catalytic hydrogenation

+ H23

Pt / r.t.

Ni / 150 Co

Some Substitution Reactions of BenzeneSome Substitution Reactions of Benzene

FeCl3

CH3

Cl2

Cl

CH3ClAlCl3

RC

Cl

O AlCl3 C R

O

OH NO

O

H2SO4

N O

O

S

O

OH

OOH S

O

O

OHSO3

+

+

+

+

+

Halogenation

Friedel-CraftsAlkylation

Friedel-CraftsAcylation

Nitration

Sulfonation

+ +

-

-

Summary of

Reactions of TolueneI. At side chain

1. Oxidation of alkyl benzene

With MnO4-/H+

CH3 MnO4 / H+ COOH

CH2CH3 MnO4 / H+ COOH

(white solid)

(white solid)

I. At side chain

2.Substitution at side chain

CH3

Cl2+u.v.

CH2Cl

ClH+

Cl2

CHCl2

u.v.

OH2

CH

OH

OHCO H

OH2-

E.S. Reactions of Toluene

Nitration of toluene (O.S.)

Trinitrotoluene (TNT) is explosive substance.

CH3

conc.HNO3

conc. H2SO4

CH3

NO2

CH3

NO2

major products+

CH3

NO2

NO2

O2N

Trinitrotoluene (TNT)

TolueneCH3

CH3

CH3CH3

CH3

CH3CH3

CH3

CH3

Br

Br

NO2

NO2

SO3H

SO3H

HNO3

H2SO4

SO3

H2SO4

Br2, Fe

+

+

+

faster than the same

reactions with benzene

NitrobenzeneNO2

NO2

NO2

NO2

NO2

NO2

NO2

SO3H

Cl

HNO3

H2SO4

H2SO4

SO3

Cl2, Fe

slower than the same reactions with benzene


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