LECTURE NOTE SK025 (ORGANIC) KMKt

Introduction

COMPLETELY CONJUGATED

Aromatic compounds must have a p orbital on
each atom.

Introduction

SATISFY HUCKEL’S RULE

• 6 π electrons • 6 π electrons
• 4n + 2 = 6 • 4n + 2 = 4
• Where n = 1 • Where n = ½
• Aromatic • Not aromatic

Aromatic compounds must contain [4n+2]π electrons
(n = 0,1,2, and so forth).

Introduction

Examples:
Identify the aromaticity of the compounds below:

NOT NOT
Completely obeys
conjugated HÜckel’s
Rule

NOT
Planar

Introduction

• Aromatic compound is a cyclic conjugated unsaturated
molecule or ion that is stabilised by π electron
delocalisation.

• Benzene is aromatic because contain 6π electrons
(obey HÜckel’s Rule), cyclic, planar and has a
completely conjugated ring.

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Introduction

KEKULÉ STRUCTURE OF BENZENE

• August Kekulé (1872) proposed a cyclic
structure for benzene.

• The Kekulé structure of benzene can be written
as a six-membered ring of carbon atoms with
alternating single and double bonds.

• One hydrogen atom is attached to each carbon
atom.

14

Introduction

KEKULÉ’S STRUCTURE FOR BENZENE

The benzene ring is planar and all the carbon-
carbon bonds are the same length.

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Introduction

RESONANCE STRUCTURE

• Each carbon atom is sp2 hybridized, thus has pure 2p
orbital with single electron in it.

• The six 2p orbitals overlap to form three delocalised π
bonds.

• As a result, the resonance structure of benzene is a
hybrid resonance from two Kekulé structure as shown
below:

≡

• The six p electrons completely delocalized around the
ring

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6.2 NOMENCLATURE

LEARNING OUTCOMES

i.
ii.
iii.

17

Nomenclature

MONO SUBSTITUTED BENZENES

• To name a benzene ring with one substituent, name
the substituent and add the word benzene.

• Carbon substituents are named as alkyl groups.

Example…

CH2CH3 CH3CHCH3 CH3CHCH2CH3

ethylbenzene isopropylbenzene Sec-butylbenzene

Nomenclature

MONO SUBSTITUTED BENZENES
Halogen substituents are named as a halo, change
the –ine ending the name of the halogen to the
suffix -o. Eg : chlorine → chloro
Example…

Cl Br NO2

chlorobenzene bromobenzene nitrobenzene

Nomenclature

MONO SUBSTITUTED BENZENE

IUPAC rules allow some of the common names to be retained.

CH3 OH COOH

methylbenzene hydroxybenzene benzenecarboxylic acid
(toluene) (phenol) (benzoic acid)

CH3 H NH2

I I

C=O C=O

phenylethanone Phenylmethanal aminobenzene 20
(acetophenone) (benzaldehyde) (aniline)

Nomenclature

DISUBSTITUTED BENZENE

Relative position of substituents are indicated by

prefixes ortho, meta, and para ( o–, m–, and p–) or

by the use of number. Br

Br Br 12

1 2 Br 12 3
4
3
Br
Br

1,2–dibromobenzene 1,3–dibromobenzene 1,4–dibromobenzene

or or or
o–dibromobenzene m–dibromobenzene p–dibromobenzene

21

Nomenclature

DISUBSTITUTED BENZENE

NO2 NO2
1 2 NO2 1

2

1,2–dinitrobenzene CH3 3
or NO2
12
o–dinitrobenzene 1,3–dinitrobenzene
or

m–dinitrobenzene

3
4

CH3
1,4–dimethylbenzene

or
p–dimethylbenzene

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Nomenclature

DISUBSTITUTED BENZENE Priority Increase COOH

If two different substituents SO3H
present, CHO

• select one of the substituent that CN
gives new parent name.
C =O
• the highest priority substituent will
be the parent chain & numbered OH
as C1.
NH2
OR

CC

C =C

R (alkyl group)

NO2
X (Halogen)

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Nomenclature

DISUBSTITUTED BENZENE
Example…

highest priority : be a parent chain

COOH OH COOH
1 2 NO2 12
1
2–nitrobenzoic acid 2 3
or 4
3 OH
o–nitrobenzoic acid
NO2 4–hydroxybenzoic acid
or
3-nitrophenol
or p–hydroxybenzoic acid

m-nitrophenol

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Nomenclature

DISUBSTITUTED BENZENE

OH COOH

1 1
2 2

3 Cl 3
4
NO2 4
3 COOH
3-nitrophenol
or 1,4-benzenedicarboxylic acid

m-nitrophenol 2
1

Br

1-bromo-4-chlorobenzene

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Nomenclature

TRI & TETRASUBSTITUTED BENZENE

• Position of substituents must be indicated by numbers.
• The substituents are listed alphabetically when writing

the name.
• C atom bearing the substituent that define the new

parent's name is numbered as C1.

Br Cl
1 2 Br
1
3
4 2 Br

Br 3I

1,2,4–tribromobenzene 2–bromo–1–chloro–3–iodobenzene 26

Nomenclature

TRI & TETRASUBSTITUTED BENZENE

NH2 CH3
1 2 NO2 1 2 NO2

3 3 27
4 4

Cl NO2
4-chloro-2-nitroaniline 2,4-dinitrotoluene

OH COOH
1 2 NO2
1
3 2
4
5
NO2
2,4–dinitrophenol HO 4 3 OH

3,5–dihydroxybenzoic acid

Nomenclature

TRI & TETRASUBSTITUTED BENZENE

12 3-amino-5-chloro-2-methylbenzoic acid
53
2-bromo-5-chloro-1,3-dimethylbenzene
4
28
5
4
31

2

KEEP IN MIND !

Br 4 3 2 CH

3

1 CH

3

4–bromo–1,2–dimethylbenzene

4–bromo–o–dimethylbenzene

o–, m– and p– naming system is used for arenes with 2
substituents only!

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Nomenclature

BENZENE AS A SUBSTITUENT - PHENYL

• If alkyl substituent is larger than the ring (more
than 6 C),the compound is named as phenyl-
substituted alkane.

• Benzene ring as substituent.
R
• Phenyl group :

*phenyl, Ph = C6H5–

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Nomenclature

BENZENE AS A SUBSTITUENT - PHENYL

1 CH

345 67

2CH2CH2CH2CH2CH2CH3

2–phenylheptane

1 2 3 45 6 7 8

CH3CH2CH2CHCH2CH2CH2CH3

4–phenyloctane

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Nomenclature

• If the chain is unsaturated (have C═C or C≡C) or
contains important functional group, the benzene
ring is considered as phenyl substituent.

1 2 34 OH 21

CH2C CCH3 1 23 CH2CH2OH

CH2CCH2

1–phenyl–2–butene 2–phenylethanol

2–phenyl-2-propanol

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Nomenclature

BENZENE AS A SUBSTITUENT - BENZYL

• Benzyl - the benzene ring that attach to the –CH2
group.

• Benzyl group :

CH2-

*benzyl = C6H5 CH2 –

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Nomenclature

BENZENE AS A SUBSTITUENT - BENZYL

CH2Br CH2OH

Benzylbromide Benzylalcohol
or or

(bromomethyl)benzene Phenyl methanol

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Nomenclature

4 3 21 6 5 43 21

CH3CH3CHCH2OH CH3CH3CHCH CHCH3
CH2
CH2

2-benzyl-1-butanol 4–benzyl-2-hexene

35

Nomenclature

Try this …

Write the structural formulae of
a. 2-chloromethylbenzne
b. isopropylbenzene
c. 2,4,6-trinitromethylbenzene
d. 2-ethylphenol

36

6.3
CHEMICAL
PROPERTIES

38

Learning Outcomes

(a) Explain the electrophilic aromatic

substitution reactions of benzene:

i. Nitration
ii. Halogenation
iii.Friedel-Crafts alkylation; and
iv.Friedel-Crafts acylation

(b) Illustrate the mechanism for the electrophilic
aromatic substitution of benzene. *6.3 (a) i.ii
& iii.

(c) Explain the influence of ortho-para and meta
directing substituents towards electrophilic
aromatic substitution reaction.

(d) Predict the product of electrophilic aromatic
substitution of monosubstituted benzene.

(e) Explain the following reactions of alkyl
benzene:

i. oxidation with hot acidified KMnO4 or

K2Cr2O7

ii. halogenation (free radical substitution)

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Chemical Properties of Benzene

Benzene ....unlike alkenes,

Br2 / CH2Cl2 no addition of Br2
(no decolorization)

KMnO4 / H+ no oxidation
∆ (no decolorization)

H2 / Ni slow addition at high
temperature and pressure

• The stability of π electron system will be lost if
benzene undergo addition reactions.

• Hence, benzene and its derivative undergo
electrophilic aromatic substitution reactions.

Chemical Properties of Benzene

Electrophilic Aromatic Substitution Reaction

H E
+ E+ + H+

electrophile

A H atom is replaced by an electrophile.

E.g : H FeBr3 Br
+ Br2
bromobenzene
benzene

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6.3 (a) Chemical Properties of Benzene

conc.HNO3,conc.H2SO4 NO2
50-55oC + H2O nitration

X2, FeX3 X halogenation
(X = Cl or Br) + HX

RCl , AlCl3 R Friedel-Crafts
(R can rearrange) + HCl Alkylation

O O
RCCl , AlCl3
C–R Friedel-Crafts
+ HCl Acylation

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Chemical Properties of Benzene

Examples ... NO2

conc. HNO3, conc. H2SO4
50oC-55oC

Br2 , FeBr3 Br

CH3CH2Cl , AlCl3 CH2CH3

O O
CH3CH2-C-Cl , AlCl3 C-CH2CH3

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6.3 (b) Mechanism for Electrophilic
Aromatic Substitution of Benzene

Mechanism of benzene involves 3 steps of reaction:

Step 1: Formation of electrophile
Step 2: Formation oaf reniumion
Step 3: Deprotonation oaf reniumion (lost of H+)

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(i) Nitration of Benzene

• Benzene reacts rapidly with a mixture of
concentrated nitric acid and concentrated
sulphuric acid to give nitrobenzene.

Examples ...

conc. HNO3, conc. H2SO4 NO2
50oC-55oC + H2O

• Product : nitrobenzene
• Observation: yellow oil formed

45

A Mechanism For The Reaction

STEP 1 Formation of nitronium ion (NO2+)

H

H-O−NO2 + H−OSO3H....H-O..+-NO2 + HSO4–

H2O + NO2+

nitronium ion

46

A Mechanism For The Reaction

STEP 2 Formation of arenium ion

+ NO2+ arenium ion

STEP 3 Loss of H+

+ HSO4– + H2SO4

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(ii) Halogenation of Benzene

• Benzene reacts with bromine and chlorine in the
presence of a Lewis acid.

• The Lewis acids commonly used are FeCl3, AlCl3 for
chlorination and FeBr3 for bromination.

Examples ...

Br2 , FeBr3 Br
+ HBr

• Product : halobenzene

• Observation: reddish brown colour of bromine

decolourises 48

A Mechanism For The Reaction

STEP 1 Bromine combines with FeBr3 to form a complex

:Br—B+ r–F–eBr3

complex
:::Br–Br: + FeBr3
::

::
::
::
::
STEP 2 Formation of arenium ion

+ :Br—B+r–F– eBr3 arenium ion + FeBr4–
STEP 3 Loss of H+

+ FeBr4– + HBr + FeBr3

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(iii) Friedel-Crafts Alkylation

• Benzene reacts with haloalkane in the presence of
Lewis acid catalyst such as AlCl3 or FeCl3 to form
alkylbenzene.

• For CH3Cl and CH3CH2Cl, the Lewis acid-base
complex itself serves as the electrophile for
electrophilic aromatic substitution.

Examples 1:

CH3CH2Cl ,AlCl3 CH2CH3

+ HCl + AlCl3

Product : ethylbenzene

50

A Mechanism For The Reaction

STEP 1
Chloroethane combines with AlCl3 to form a
complex.

Electrophile
Lewis acid-base complex

51

A Mechanism For The Reaction

STEP 2 The complex react with benzene to
form an arenium ion

+ HH
+H

++

arenium ion + AlCl4-

52

A Mechanism For The Reaction

STEP 3
The arenium ion then loses a proton, H+ and
becomes ethylbenzene.

+H + AlC4l–

+ HCl + AlCl3

53

Friedel-Crafts Alkylation

• Most Friedel - Crafts alkylation reactions involve
carbocation electrophiles.

• The reaction can yield products having rearranged
carbon skeletons when 1o and 2o haloalkanes are
used as starting materials.

Examples 2:

H

CH3—CH2—CH—C+ H2 CH3—CH2—+CH—CH3

1o carbocation 2o carbocation

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Examples 2 Friedel-Crafts Alkylation

CH3
CHCH2CH3

CH3CH2CH2CH2Cl (65%)
AlCl3
+
CH2CH2CH2CH3

(35%)

Product : alkylbenzene

55

A Mechanism For The Reaction

STEP 1 Formation of carbocation

1o haloalkane

2o carbocation

56

A Mechanism For The Reaction

STEP 2 Formation of arenium ion

CH3
+ +CHCH2CH3

STEP 3 Loss of H+ arenium ion

CH3 CH3
CHCH2CH3
+ CHCH2CH3
H + AlCl4– + HCl + AlCl3

57

Friedel-Crafts Alkylation

Examples 3

(CH3)2CHCl
AlCl3

Reagent : 3o Haloalkane
Product : alkylbenzene

58

A Mechanism For The Reaction

STEP 1 Formation of carbocation

+-
(CH3)2CH–Cl–AlCl3
::
::
(CH3)2CHCl : + AlCl3

(CH3)2CH+ + AlCl4–
3⁰ carbocation

59

A Mechanism For The Reaction

STEP 2 Formation of arenium ion

+ +CH(CH3)2 arenium ion

STEP 3 Loss of H+

+ AlCl4– + HCl + AlCl3

60