Chapter 17 2
Although benzene’s pi electrons are in a stable aromatic
system, they are available to attack a strong electrophile to give
This resonance-stabilized carbocation is called a sigma
complex because the electrophile is joined to the benzene ring
by a new sigma bond.
Aromaticity is regained by loss of a proton.
Chapter 17 3
Mechanism of Electrophilic
Chapter 17 5
Mechanism for the
Bromination of Benzene: Step
Before the electrophilic aromatic substitution can take
place, the electrophile must be activated.
A strong Lewis acid catalyst, such as FeBr3, should
Br Br FeBr3 Br Br FeBr3
(stronger electrophile than Br2)
Chapter 17 6
Br Br FeBr3
+ FeBr3 + HBr
Step 2: Electrophilic attack and formation of the sigma complex.
Step 3: Loss of a proton to give the products.
Mechanism for the
Bromination of Benzene: Steps
2 and 3
Chapter 17 8
Chlorination and Iodination
Chlorination is similar to bromination.
AlCl3 is most often used as catalyst, but
FeCl3 will also work.
Iodination requires an acidic oxidizing
agent, like nitric acid, to produce iodide
+ HNO3 + ½ I2 I+
+ NO2 + H2O
Chapter 17 9
Predict the major product(s) of bromination of p-chloroacetanilide.
The amide group (–NHCOCH3) is a strong activating and directing group because the nitrogen atom
with its nonbonding pair of electrons is bonded to the aromatic ring. The amide group is a stronger
director than the chlorine atom, and substitution occurs mostly at the positions ortho to the amide. Like
an alkoxyl group, the amide is a particularly strong activating group, and the reaction gives some of the
Solved Problem 1
Chapter 17 10
Nitration of Benzene
Sulfuric acid acts as a catalyst, allowing the reaction
to be faster and at lower temperatures.
HNO3 and H2SO4 react together to form the
electrophile of the reaction: nitronium ion (NO2
Chapter 17 11
Mechanism for the Nitration of
Chapter 17 12
Reduction of the Nitro Group
Zn, Sn, or Fe
Treatment with zinc, tin, or iron in dilute acid
will reduce the nitro to an amino group.
This is the best method for adding an amino
group to the ring.
Chapter 17 13
Sulfonation of Benzene
Sulfur trioxide (SO3) is the electrophile in the reaction.
A 7% mixture of SO3 and H2SO4 is commonly referred
to as “fuming sulfuric acid”.
The —SO3H groups is called a sulfonic acid.
Chapter 17 14
Mechanism of Sulfonation
Benzene attacks sulfur trioxide, forming a sigma
Loss of a proton on the tetrahedral carbon and
reprotonation of oxygen gives benzenesulfonic acid.
Chapter 17 15
Sulfonation is reversible.
The sulfonic acid group may be removed
from an aromatic ring by heating in dilute
Chapter 17 16
Mechanism of Desulfonation
In the desulfonation reaction, a proton adds
to the ring (the electrophile) and loss of sulfur
trioxide gives back benzene.
Chapter 17 17
Nitration of Toluene
Toluene reacts 25 times faster than benzene.
The methyl group is an activator.
The product mix contains mostly ortho and
para substituted molecules.
Chapter 17 18
Ortho and Para Substitution
Ortho and para attacks are preferred because their
resonance structures include one tertiary carbocation.
Chapter 17 20
When substitution occurs at the meta position, the
positive charge is not delocalized onto the tertiary
carbon, and the methyl groups has a smaller effect
on the stability of the sigma complex.
Chapter 17 21
Alkyl Group Stabilization
Alkyl groups are activating substituents and ortho,
This effect is called the inductive effect because
alkyl groups can donate electron density to the ring
through the sigma bond, making them more active.
Chapter 17 22
Substituents with Nonbonding
Resonance stabilization is provided by a pi bond between
the —OCH3 substituent and the ring.
Chapter 17 23
Meta Attack on Anisole
Resonance forms show that the methoxy
group cannot stabilize the sigma complex in
the meta substitution.
Chapter 17 24
Bromination of Anisole
A methoxy group is so strongly activating that
anisole is quickly tribrominated without a
Chapter 17 25
The Amino Group
Aniline reacts with bromine water (without a
catalyst) to yield the tribromoaniline.
Sodium bicarbonate is added to neutralize
the HBr that is also formed.
Chapter 17 27
Activators and Deactivators
If the substituent on the ring is electron donating, the
ortho and para positions will be activated.
If the group is electron withdrawing, the ortho and
para positions will be deactivated.
Chapter 17 28
Nitration of Nitrobenzene
Electrophilic substitution reactions for nitrobenzene
are 100,000 times slower than for benzene.
The product mix contains mostly the meta isomer,
only small amounts of the ortho and para isomers.
Chapter 17 29
Ortho Substitution on
The nitro group is a strongly deactivating group when
considering its resonance forms. The nitrogen
always has a formal positive charge.
Ortho or para addition will create an especially
Chapter 17 30
Meta Substitution on
Meta substitution will not put the positive
charge on the same carbon that bears the
Chapter 17 32
Deactivators and Meta-
Most electron withdrawing groups are
deactivators and meta-directors.
The atom attached to the aromatic ring has a
positive or partial positive charge.
Electron density is withdrawn inductively
along the sigma bond, so the ring has less
electron density than benzene and thus, it will
be slower to react.
Chapter 17 33
Ortho Attack of Acetophenone
In ortho and para substitution of acetophenone, one
of the carbon atoms bearing the positive charge is
the carbon attached to the partial positive carbonyl
Since like charges repel, this close proximity of the
two positive charges is especially unstable.
Chapter 17 34
Meta Attack on Acetophenone
The meta attack on acetophenone avoids
bearing the positive charge on the carbon
attached to the partial positive carbonyl.
Chapter 17 41
Effect of Multiple Substituents
The directing effect of the two (or more)
groups may reinforce each other.
Chapter 17 42
Effect of Multiple Substituents
The position in between two groups in
Positions 1 and 3 is hindered for substitution,
and it is less reactive.
Chapter 17 43
Effect of Multiple Substituents
If directing effects oppose each other, the
most powerful activating group has the
major products obtained
Chapter 17 44
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
Chapter 17 45
Mechanism of the Friedel–Crafts
Chapter 17 46
Protonation of Alkenes
An alkene can be protonated by HF.
This weak acid is preferred because the
fluoride ion is a weak nucleophile and will not
attack the carbocation.
Chapter 17 47
Alcohols and Lewis Acids
Alcohols can be treated with BF3 to form the
Chapter 17 48
Limitations of Friedel–Crafts
Reaction fails if benzene has a substituent
that is more deactivating than halogens.
Rearrangements are possible.
The alkylbenzene product is more reactive
than benzene, so polyalkylation occurs.
Chapter 17 50
Devise a synthesis of p-nitro-t-butylbenzene from benzene.
To make p-nitro-t-butylbenzene, we would first use a Friedel–Crafts reaction to make t-butylbenzene.
Nitration gives the correct product. If we were to make nitrobenzene first, the Friedel–Crafts reaction to
add the t-butyl group would fail.
Solved Problem 2
Chapter 17 51
Acyl chloride is used in place of alkyl chloride.
The product is a phenyl ketone that is less
reactive than benzene.
Chapter 17 52
Mechanism of Acylation
Step 1: Formation of the acylium ion.
Step 2: Electrophilic attack to form the sigma complex.
Chapter 17 53
The Clemmensen reduction is a way to
convert acylbenzenes to alkylbenzenes by
treatment with aqueous HCl and
Chapter 17 54
A nucleophile replaces a leaving group on the
This is an addition–elimination reaction.
Electron-withdrawing substituents activate the
ring for nucleophilic substitution.
Chapter 17 55
Mechanism of Nucleophilic
Step 1: Attack by hydroxide gives a resonance-stabilized complex.
Step 2: Loss of chloride gives the product. Step 3: Excess base deprotonates the product.
Chapter 17 56
Nitro groups ortho and para to the halogen
stabilize the intermediate (and the transition
state leading to it).
Electron-withdrawing groups are essential for
the reaction to occur.
Chapter 17 57
Benzyne Reaction: Elimination-
Reactant is halobenzene with no electron-
withdrawing groups on the ring.
Use a very strong base like NaNH2.
Chapter 17 58
Sodium amide abstract a proton.
The benzyne intermediate forms when the bromide is
expelled and the electrons on the sp2
to it overlap with the empty sp2
orbital of the carbon
that lost the bromide.
Benzynes are very reactive species due to the high
strain of the triple bond.
Chapter 17 59
Nucleophilic Substitution on the
Chapter 17 60
Chlorination of Benzene
Addition to the
benzene ring may
occur with excess of
chlorine under heat
The first Cl2 addition is
difficult, but the next
two moles add rapidly. An insecticide
Chapter 17 61
Elevated heat and pressure is required.
Possible catalysts: Pt, Pd, Ni, Ru, Rh.
Reduction cannot be stopped at an
Chapter 17 62
Na or Li
NH3 (l), ROH
This reaction reduces the aromatic ring to a
The reducing agent is sodium or lithium in a
mixture of liquid ammonia and alcohol.
Chapter 17 63
Mechanism of the Birch Reduction
Chapter 17 64
Limitations of the Birch Reduction
Chapter 17 65
Alkylbenzenes are oxidized to benzoic acid by
heating in basic KMnO4 or heating in Na2Cr2O7/H2SO4.
The benzylic carbon will be oxidized to the carboxylic
(or Na2Cr2O7, H2SO4 , heat)
Chapter 17 66
Br2 or NBS
The benzylic position is the most reactive.
Br2 reacts only at the benzylic position.
Cl2 is not as selective as bromination, so
results in mixtures.
Chapter 17 67
Mechanism of Side-Chain
Chapter 17 69
Benzylic halides are
100 times more
primary halides via
The transition state
is stabilized by a
Chapter 17 70
Oxidation of Phenols
Phenol will react with oxidizing agents to produce
Quinones are conjugated 1,4-diketones.
This can also happen (slowly) in the presence of air.