UEC Chapter 30
Aromatic Compounds
Chemis y
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JLWong
Arenes
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aromatic hydrocarbons. The simplest and most important
Arenes are __________
benzene
aromatic hydrocarbon is ___________,
C6H6.
planar
non-polar
symmetrical and _____________.
Benzene molecule is __________,
______________
π electrons. Each
Benzene is electron rich due to the delocalised ____
π bond. The π bonds do
carbon atom contributes one electron to a ____
not lie between the two adjacent carbon atoms, but is spread over the
delocalized
six carbon atoms. Hence, they are said to be ______________.
Due to its electron richness, benzene is often reactive with electrophile
___________,
electrophilic substitution
commonly undergoing ____________________________.
Benzene can be drawn a number of different ways, as the delocalized
electrons move from one carbon to the next.
Resonance Theory, it is suggested that the structure
According to the ____________
Kekulé forms,
of benzene is a hybrid of 2 ___________
The hybrid structure is often represented by a hexagon containing an
delocalized electrons
inscribed circle, where the circle represents the _____________
of the benzene ring.
https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Map%3A_Organic_Chemistry_(McMurry)/
15%3A_Benzene_and_Aromaticity/15.02%3A_Structure_and_Stability_of_Benzene
https://www.britannica.com/science/theory-of-resonance
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Kekulé structure
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Molecular Orbital Theory, all 6 carbons in the ring
According to the ___________________
2
sp
use ____ hybrid for bond formation.
The sp2 orbitals are involved in forming 3 localized sigma
______ bonds (2 C-C
singly occupied p orbital on each carbon atom.
and 1 C-H), leaving a ________
The p orbital overlaps with the adjacent orbital on either side of it to form
π electrons above and below the plane of the
a ring of delocalised ___
π bond.
ring, that is a ___
Thus, a very stable structure is
obtained. The 6 delocalised electrons
are free to move over the entire ring
and any substituent attached to it.
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Naming Aromatic Compounds
Monosubstituted benzene ring:
• In general, for monosubstituted benzenes, benzene is the parent name
and the substituent is indicated by a pre x.
bromobenzene
chlorobenzene nitrobenzene
methylbenzene
ethylbenzene
(C6H5Br)
(C6H5Cl)
(C6H5NO(toluene,
C6H5CH
(C36)H5CH2CH3)
2)
When the hydrocarbon chain attached to the benzene ring is small, the
hydrocarbon is named as a benzene derivative.
benzoic acid
benzenesulphonic acid benzenonitrile
(C6H5COOH)
(C6H5SO3H)
(C6H5CN)
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benzaldehyde
(C6H5CHO)
In some instances, the name is based on the pre x phenyl. Phenyl
group has the formula C6H5.
phenol
(C6H5OH)
phenylamine
(C6H5NH2)
phenylethanoate
(CH3COOC6H5)
The pre x “phenyl” is also used in cases where the group substituted is
more complex (e.g. containing other functional group).
phenylmethanol
(C6H5CH2OH)
phenylethene
(C6H5CH=CH2)
2-phenylheptane
(CH3CH(C6H5)CH2CH2CH2
CH2CH3)
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phenylethanone
(C6H5COCH3)
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The group C6H5CH2 — is called the benzyl group.
benzylchloride
benzylamine
benzylalcohol
(C6H5CH2Cl)
(C6H5CH2NH2)
(C6H5CH2OH)
Multiple substituted benzene ring:
• When two or more substituents are present, their relative positions are
indicated by numbering carbon atoms of the benzene nucleus. For
example, the 3 isomers of dichlorobenzenes are named as follow:
1,4-dichlorobenzene
1,2-dichlorobenzene1,3-dichlorobenzene
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When two substituents attached to a benzene ring are different, the
parent name of the benzene
numbering system is based on the ______________
derivative, the carbon atom attached to the functional group is
designated ____.
C1
lowest possible
The numbering is then carried out such that the _________
alphabetically
numbers are used and the substituents are listed _________________
when writing the name.
2,4-dinitrobenzoic acid
4-amine-3,5-dinitrobenzoic acid
2,4-dichloro-3,5dinitrobenzoic acid
2-bromophenol
3,5-dinitrotoluene
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•
When there are multiple functional groups based upon which a parent
name can be written, note the priority order as follow:
OH
carboxyl
(–COOH)
H
>
aldehyde >
(–CHO)
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>
sulfonic acid
(SO3H)
R’
ketone
(R’COR)
>
ester
(–COOR)
alcohol
(-OH)
>
>
Cl
acyl
(–COCl)
thiol
(-SH)
>
>
amide >
(–CONH2)
(nitrile)
>
amine
(-NH2)
When no simple parent name is appropriate, the lowest possible
numbers are used and the substituents are listed alphabetically.
1-chloro-2,4-dinitrobenzene
NOT 4-chloro-1,3-dinitrobenze
1-bromo-4-chlorobenzene
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Ortho-, Meta-, Para- (OMP) Nomenclature for Disubstituted Benzenes
• Instead of using numbers to indicate substituents on a benzene ring,
ortho- (o-), _______
meta- (m-), or _______
para- (p-) can be used in place of
_______
positional markers when there are two substituents on the benzene
ring (disubstituted benzenes).
• They are de ned as follow:
1,2 - (next to each other in a benzene ring)
• ortho- (o-): ______
• meta- (m): ______
1,3 - (separated by one carbon in a benzene ring)
• para- (p): ______
1,4 - (across from each other in a benzene ring)
o-dinitrobenzene
m-dinitrobenzene
p-dinitrobenzene
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o-chloronitrobenzene
m-chloronitrobenzene
p-chloronitrobenzene
Physical Properties of Benzene
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Colourless liquid at room temperature, which gives out an aromatic
odour.
80.1 ºC
Boiling point: ________
5.5ºC
Melting point: ______
0.88g/ml
Density: __________
Immiscible with water but soluble in all organic solvents.
An organic solvent and can be used to dissolve non-polar inorganic
molecules like iodine and sulphur.
Highly ammable and burns with a yellow smoky ame.
Both the liquid and vapour of benzene are highly poisonous.
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Chemical Properties of Arenes
1. Halogenation:
Cl2 or ________
Br2
• Reagents: _______
halogen carrier (such
• Condition: in the presence of catalyst called _________________
Fe _______,
FeCl3 _______,
FeBr3 _______
AlCl3 or ______)
AlBr3 and at room temperature.
as ____,
halogenobenzene
• Product: ____________________.
• Example: Benzene reacts with Cl2 in the presence of AlCl3 to give
chlorobenzene and steamy fumes of HCl can be seen.
•
substitution
The halogenation of benzene proceeds via electrophilic
________________________
mechanism, involving the following steps:
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✴
Step 1: Production of electrophile Br+ from Br2, and a Lewis acid
(FeBr3). FeBr3 is used to polarize the Br-Br molecule.
+
+
✴
Step 2: Formation of a carbocation intermediate due to the attack of Br+
on the benzene ring.
✴
Step 3: Expulsion of H+ from the intermediate and reforming of
delocalized π electron clouds, i.e. benzene ring is reestablished.
+
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2. Nitration:
conc. H2SO4
conc. HNO3
• Reagents: ________________
and _________________
• Condition: re ux at about _______
50-60 ºC.
• Product: ____________________.
nitrobenzene
• Example: Benzene reacts with a mixture of conc. H2SO4 and conc.
HNO3 at 55ºC to give nitrobenzene, a yellow colour oil.
•
Nitration also occurs through electrophilic substitution mechanism:
✴ Step 1: Production of the electrophile, NO2+ (nitronium ion or nitryl
cation)
Note: HNO3 acts as a base in the presence of the stronger H2SO4. The
nitronium ion formed is a strong electrophile.
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✴
Step 2: Formation of a carbocation intermediate due to the attack of
NO2+ on the benzene ring.
✴
Step 3: Expulsion of H+ from the intermediate and reforming of
delocalized π electron clouds.
Note: If temperature rises above 100 ºC, nitrobenzene is formed rst and
then further substituted to form 1,3-dinitrobenzene.
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3. Alkylation of benzene ring (Friedel-Crafts reaction)
RX; R = alkyl, X = halogen
• Reagents: ____________________________
re ux in the presence of ________.
AlCl3
• Condition: heat under __________
alkylbenzene
• Product: __________________
• The electrophile, ____,
R+ is formed from the reaction of RX with AlCl3.
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Example: Benzene reacts with CH3Cl to give methylbenzene. A
hydrogen atom is replaced by an alkyl group (CH3 –).
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The mechanism of the reaction is as follow:
✴ Step 1: Formation of the electrophile CH3+.
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✴
Step 2: Formation of a carbocation intermediate due to the attack of
CH3+ on the benzene ring.
+
✴
+
Step 3: Expulsion of H from the intermediate and reforming of
delocalized π electron clouds.
+
✴
+
Step 4: H reacts with
AlCl4
to give back AlCl3.
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Example: Benzene reacts with isopropyl chloride to give isopropyl
benzene.
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4. Acylation of benzene ring (Friedel-Crafts reaction)
• Reagents: __________________________
RX; R = acyl, X = halogen
• Condition: heat under __________
re ux in the presence of ________.
AlCl3
• Product: ____________________.
acylbenzene
• The electrophile, ____,
R+ is formed from the reaction of RX with AlCl3.
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Example: Benzene reacts with CH3COCl to give C6H5COCH3. A
hydrogen atom is replaced by an acyl group (– COCH3).
•
Friedel-Crafts reaction is used to substitute a hydrogen in the benzene
alkyl group ( – CH3 , – CH2CH3) or an _______
acyl group (–
ring for an _______
COCH3, – COCH2CH3 ).
side chain into a
Friedel-Crafts reactions result in the introduction of a ___________
benzene ring. They are also called alkylation or acylation reaction.
electrophilic substitution In
Friedel-Crafts reaction proceeds via __________________________.
R+ while in acylation it is an acylium ion.
alkylation, the electrophile is ___,
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5. Sulphonation of benzene ring
sulphuric acid
• Reagents: ________________
• Condition: ________
80ºC
benzenesulphonic acid
• Product: ________________________
SO3 formed from the dissociation of H2SO4.
• The electrophile is ______,
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Step 1: Production of electrophile.
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Step 2: Formation of carbocation.
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Step 3: Loss of a proton.
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Step 4: Reaction with hydronium ion.
General Mechanism of Electrophilic Substitution in Benzene
• Step 1: Formation of an electrophile.
• Step 2: Electrophilic attack on a pi bond of benzene by an electrophile to
form a carbocation.
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Step 3: Deprotonation of the carbocation to restore its aromaticity,
resulting in a substituted product.
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6. Addition Reaction of Benzene
Due to the stability of the aromatic ring, benzene typically undergoes
substitution reaction, but addition reaction can occur under suitable
conditions.
(a) Hydrogenation
excess hydrogen
• Reagents: ____________________
• Condition: ________
heat and ______________________
nickel or palladium as catalyst.
• Product: ____________________.
cyclohexane
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Example: Benzene is heated with excess hydrogen gas in the
presence of catalyst.
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heat
(b) Halogenation
Halogen
• Reagents: ________________
uv light
• Condition: ________________
cyclohalohexane
• Product: ____________________.
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Example: Benzene is allowed to react with chlorine under uv light.
Chemical Tests for Benzene
Br2 water and _____________
• ____________
H+/KMnO4 can be used to distinguish benzene
from unsaturated compounds such as alkenes, cycloalkenes or
decolourizes
alkynes. Alkenes, cycloalkenes or alkynes ________________
these
reagents but benzene gives no reaction.
• Benzenes can also be distinguished from alkanes or alkenes by
nitration reactions. Benzene, but not alkanes or alkenes, reacts
____________
nitrating mixture to produce nitrobenzene which is a yellow,
with a ___________
oily liquid with a bitter almond smell.
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Methylbenzene
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toluene
Methylbenene, also known as _________,
is found naturally
occurring at low level in crude oils.
It is a byproduct in the production of gasoline and may be
extracted from the byproducts of coal tar or crude oil.
Like benzene, methylbenzene is a colourless liquid immiscible
with water, and has a density lower than water.
Chemical Properties of Methylbenzene
• Methylbenzene comprises a benzene ring and a methyl side
chain, it will undergo reactions involving:
alkanes
(a)Typical reactions of __________.
Electrophilic substitution of benzene ring, e.g. nitration and
(b)__________________________
halogenation.
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1. Reactions of the –CH3 group
a) Free radical substitution by halogen
• Reagent: Cl2 or Br2
sunlight/uv light
• Condition: presence of __________________
halogenomethylbenzene
• Product: ____________________________
Example:
Note: A mixture of products are formed.
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b) Oxidation of the methyl side chain
Acidi ed KMnO4 (H+/K2Cr2O7 is not strong enough)
• Reagent: ______________________
Re ux
• Condition: __________________
Benzoic acid
• Product: ____________________
Example: Oxidation of methyl side chain to give benzoic acid.
H+/KMnO4
Note: Any number of carbon side chain on the benzene ring can be
oxidised to —COOH by prolonged re ux with acidi ed KMnO4.
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In uence of Substitution Groups on Benzene Ring
• When substituted benzenes undergo further substitution, the substitution
group already present in the benzene derivative will in uence
reactivity and _____________.
orientation
electrophilic substitution in two ways: ____________
• The substitution group can be classi ed into two groups, namely
ring activating group and _______________________.
ring deactivating group
________________________
• Ring Activating Groups
✴ They are ____________________
electron-donating
groups, which intensify
_________ the electron
electrophilic
density in the ring, activating the ring towards ____________________
substitution
______________.
✴ They ____________
increase the rate of reaction.
✴ They are ____
ortho-para
C2 and ____
C4 directing, and hence are _____________
directors.
✴ Generally, electron donating groups have _______
lone pair of electrons.
✴ Examples of ring activating groups include:
alkyl group (e.g. –CH3, –CH2CH3 etc), amine group (–NH2),
hydroxyl group (–OH) and ether group (e.g. –OCH3)
❖
F, Cl, Br, I are electron-___________,
withdrawing
Exception: Halogen atoms (e.g. _________)
deactivating but are ____
C4 directing.
C2 and _____
and hence ring ____________,
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• Ring Deactivating Groups
electron-withdrawing groups, which _________
reduce electron
✴ They are _______________________
electrophilic
availability in the ring, deactivating the ring towards ________________
substitution
______________.
✴ They ____________
decrease the rate of reaction.
C3 directing, and hence are _________
meta directors.
✴ They are ____
✴ Examples of ring deactivating group include:
Halogens:
Cl
OH
amide
ester
carbonyl
acyl
carboxyl
(–CONH2) (–COOR) (–CO)
(–COCl) (–COOH)
✴
(nitrile)
(nitro)
Note that, except for the halogens, the above examples of ring
deactivating groups comprise at least one highly electronegative
_______________
not directly attached to the benzene ring
atoms (N or O), which are _____________
withdrawing property.
and which contributes to the electron-_____________
Think! Is a sulphonyl group (–SO3R) electron-donating or withdrawing?
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•
Substituent group effects on electrophilic aromatic substitution:
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the benzene ring
Product: A mixture of _______________________
more
electron-donating
carrier such as FeCl3 and AlCl3
2/4- halomethylbenzene
dark
Note: A steamy fume of ______ is produced.
2
HCl
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b) Nitration
• Reagent: ______________
conc. H2SO4 and _______________
conc. HNO3
30 ºC
Condition: ____
2/4-nitromethylbenzene
• Product: A mixture of __________________________
Example:
CH3
CH3
CH3
At higher temperature, 2 and then 3 —NO2 groups can be substituted in
the benzene ring.
2,4,6-trinitromethylbenzen
or 2,4,6-trinitrotoluene (TN
Note: TNT is a pale yellow solid used as explosives.
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Summary of Benzene Reaction
uv
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Summary of Methylbenzene
Reaction
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Ex 1: Arrange benzene, methylbenzene and nitrobenzene
order of reactivity towards electrophilic substitution.
in increasing
Ex 2: Simply outline a route to synthesize 4-nitrobenzoic acid from
methylbenzene.
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Ex 3: Simply outline a route to synthesize 3-nitrobenzoic acid from
methylbenzene.
Ex. 4: Compound A has molecular formula C9H8, and decolourizes
bromine water and acidi ed potassium permanganate rapidly. Under
certain conditions, it reacts with one molecule of hydrogen to produce
Compound B (C9H10) and under more vigorous conditions, it reacts with
hydrogen to produce Compound C (C9H16). In the presence of strong
oxidizing agent, A can be oxidised to produce phthalic acid (1,2benzenedioic acid). Identify the molecular structure of Compounds A, B
and C.
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Ex 5
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