Transcript 5. Benzene and Aromaticity

5. Benzene and
Aromaticity
Aromatic Compounds
 The term “Aromatic” is used to refer to the
class of compounds structurally related to
Benzene.
 The first discovered of these compounds
were fragrant substances but the term
aromatic, though still used, is not applicable
to the vast majority of these compounds

The common
names of
some
substituted
aromatics are
so firmly
entrenched in
the literature
that they
must be
memorized
=>
15.2 Naming Aromatic Compounds
 Monosubstituted benzenes are named by first
naming the substituent and following this with
the word benzene
Naming Alkyl Substituted
Benzenes
 Alkyl benzenes are named in one of two different ways:
 If the alkyl group contains 6 or fewer carbons, then the
compound is named as an alkyl substituted benzene
 If the alkyl group contains more than 6 carbons then the
compound is named as phenyl substituted alkane
Naming Benzenes With More Than
Two Substituents
 Choose numbers to get lowest possible values
 List substituents alphabetically with hyphenated
numbers
 Common names, such as “toluene” can serve as root
name
Naming Disubstituted Benzenes
 Relative positions on a benzene ring are indicated by
the following prefixes
 ortho- (o) on adjacent carbons (1,2)
 meta- (m) separated by one carbon (1,3)
 para- (p) separated by two carbons (1,4)
 Also used to describe reaction patterns (“reaction
occurs at the para position”)
Complete the Following Examples
Structure of Benzene
 The actual structure of benzene lies
somewhere between the two
resonance forms pictured below
Experimental Observations That Lead To
This Resonance Picture of Benzene
 All its C-C bonds are the same length: 139 pm —
between single (154 pm) and double (134 pm)
bonds
 Electron density in all six C-C bonds is identical
 Structure is planar, hexagonal
Molecular Orbital Description of
the Resonance in Benzene
 Each C is sp2 hybridized and has a p orbital
perpendicular to the plane of the sixmembered ring. Each p orbital has one
electron in it. This makes it impossible to
identify 3 localized double bonds in benzene
.
=>
Consequence of Resonance Stability
 The resonance stability of benzene is so very
substantial that benzene shows none of the
characteristic chemical behavior of other alkenes


Alkene + Br2/CCl4  dibromide (addition product)
Benzene + Br2/CCl4  no reaction.
Alkene + HBr  Bromoalkane (addition product)
Benzene + HBr  no reaction.
 The reason that benzene does not take part in any
electrophilic addition rxns. is that, to do so, would
destroy it’s stable conjugated system. An
energetically unfavorable situation.
Please Recall the General Mechanism for
Aromatic Substition
+
Br -
Heats of Hydrogenation as Indicators
of Resonance Stability of Benzene
 The addition of H2 to C=C normally gives off about
118 kJ/mol ; 3 double bonds should give off
356kJ/mol
 Benzene has 3 double bonds but gives off only 206
kJ/mol on reacting with 3 H2 molecules
 Therefore it is about 150 kJ more “stability” than a
regular alkene having s set of three double bonds
Reactions of Aromatic Compounds
 Electrophilic addition reactions,
 so common amongst normal alkenes, do not occur
with aromatics, in spite of the fact that each aromatic
ring contains three double bonds.
 The reason for this is that these reactions break the
double bond and this would mean that the very stable
aromatic system would be disrupted.
 Instead, the characteristic reactions of aromatics are
electrophilic substitution reactions rather that
addition because these retain the very stable cyclic
aromatic system
Electrophilic Addition and
Electrophilic Substitution
E++ base-
ElectrophilicAddition
Electrophilic Substitution
base:-
Aromatic Addition Compared to
Aromatic Substition
Br+ Br-
All Electrophilic Aromatic Substitution Reactions
take place by the same General Mechanism.
 Aromatics (benzene) are less reactive towards electrophiles
then are normal alkenes.
 Consequently, a catalyst is usually needed to convert the
“electrophile containing reagent” into a stronger electrophile.
 The catalyst needed to react molecular bromine (Br2) with
benzene is ferric bromide. FeBr3 basically turns the weaker
electrophile, Br2, into the stronger electrophile, Br+
 FeBr3 is a Lewis Acid and accepts an electron pair from Br2
and thereby puts a strong positive charge on the end
Bromine atom.
FeBr4- + Br+
A stronger electrophile than
Br2
Generalized Mechanism for Electrophilic
Aromatic Substitution cont.
 Once generated. the stronger electrophile
gets attacked by the pi electrons of the
aromatic system, forming an intermediate,
resonance stabilized, carbocation.
 Finally, the carbocation stabilizes itself by
loosing a ring H+ and regenerating the stable
cyclic conjugated system, with the
electrophile on the ring where the H+ used to
be. See next slide.
+
FeBr4- + Br+
A stronger
Electrophile than Br2
+
Br +
-
+
FeBr4- + Br+
A stronger
Electrophile than Br2
+
Br +
-
Aromatic Chlorination
 Chlorine and iodine (but not fluorine, which is too
reactive) can substitute on an aromatic ring. Each
requires a special catalyst or promoter to generate a
sufficiently strong electrophile
 Chlorination follows the same mechanism as
bromination and requires FeCl3 catalyst
Aromatic Iodination
 Iodine (I2) must be oxidized with Cu+2 or
peroxide to form the more powerful
electrophile, I+
Aromatic Nitration
 The combination of nitric acid and sulfuric acid
produces the electrophile NO2+ (nitronium ion)
 It reacts with benzene to produce nitrobenzene
HNO3
Nitroaromatics are Important
for Two Reasons
 Nitroaromatics are important in themselves and also the nitro
group can be converted into other functional groups that
couldn’t be placed on the aromatic ring directly
 For example, reduction of the nitro group by stannous
chloride yields the corresponding amine
Aromatic Sulfonation
The combination of sulfuric acid and sulfur
trioxide (SO3) produces the electrophile
HSO3+
 Its reaction with benzene produces
benzenesulfonic acid

SO3
Importance of Aromatic Sulfonic
Acids
 Aromatic Sulfonic Acids are valuable intermediates in
the preparation of dyes and pharmaceuticals.
 Aromatic Sulfonic Acids are the precursors needed
for the synthesis of Sulfa Drugs such as
sulfanilamide.These were among the first useful
antibiotics known and credited with saving countless
lives during W.W.II
Aromatic Sulfonic Acids are also important for
the further chemistry that they can undergo
 When sulfonic acids are mixed with sodium
hydroxide at elevated temperatures a net
replacement of the sulfonic group by the hydroxyl
group results.
 This constitutes one of the few methods for
preparing phenols.
16.3 Alkylation of Aromatic Rings: The
Friedel–Crafts Reaction
 Aromatic substitution
of a R+ for an aromatic
proton (H+)
 Aluminum trichloride, a
Lewis Acid catalyst,
promotes the
formation of the (R+)
carbocation
Limitations of the Friedel-Crafts
Alkylation
 Only alkyl halides can be used (F, Cl, I, Br)
 Aryl halides and vinylic halides do not react (their
carbocations are too hard to form)
 This rxn will not work with rings containing an amino
group or a strongly electron-withdrawing deactivating
group
Control Problems with F/C Alkylations
 Unwanted multiple alkylations can occur because the first
alkylation is activating. That is to say, once the first alkyl
group substitutes on the ring; the monosubstituted
benzene is more reactive than benzene itself and
consequently more likely to be substituted with another
alkyl group
Carbocation Rearrangements During
Alkylation
 The last problem associated with F/C Alkylation is the
possible rearrangement of the intermediate
carbocation to a more stable carbocation
 These rearrangements usually involve hydride (H-) or
alkide (R-) shifts
16.4 Acylation of Aromatic Rings
 Reaction of an acid chloride (RCOCl) in the
presence of AlCl3 catalyst with an aromatic
ring substitutes an acyl group, COR , on to
the aromatic ring
 Benzene with acetyl chloride yields
acetophenone
Mechanism of Friedel-Crafts
Acylation
 Similar to alkylation
 Reactive electrophile: resonance-stabilized
acyl cation
 An acyl cation does not rearrange
Electrophilic Aromatic Substitution
of a Monosubstituted Benzene
 What effects does a substituent already present
on a benzene ring have on the electrophilic
substitution of a second group?

Reactivity: Some monosubstituted benzenes are
more reactive that benzene towards further
electrophilic aromatic substitution (activating
substituents); some monosubstituted benzenes are
less reactive (deactivating substituents)

Orientation: A substituent that is already on a
benzene ring directs the position of any incoming
groups
Reactivity: Activating Substituents
 Activating Substituents – these activate a benzene
ring towards further substitution by donating electron
density into the aromatic ring. Donating electon
density into the ring increases the reaction rate by
stabilizing the intermediate carbocation.
Reactivity: Deactivating Substituents
 Deactivating Substituents – these deactivate a benzene
ring towards further substitution by withdrawing electron
density from the aromatic ring. Withdrawing electon
density from the ring decreases the reaction rate by
destabilizing the intermediate carbocation
Orientation
 The second effect that the substituent of a
monosubstituted benzene can have on further
electrophilic aromatic substitution is to direct
incoming electrophiles to particular positions on
the aromatic ring. Substituents are either ortho –
para directors or they are meta directors.
Combining this information with the reactivity
characteristics of a substituent we find that all
substituents can be classified into one of three
groups;
 Ortho – Para Activators
 Meta Deactivators
 Ortho – Para Deactivators
Ortho-Para Activating Groups
 Please recall that activating groups increase the
e- density of the aromatic ring. These
substituents also direct incoming groups to the
ortho and para positions as only these positions
afford a resonance structure for the intermediate
carbocation in which the positive charge is on
the ring carbon to which the e- donating group is
bonded – a very stable situation. The increased
stability of this resonance structure favors
substitution in these positions. The electron
donating substituents may stabilize the positive
charge by the inductive effect or by resonance.
See Next Slide for Example
+
Meta Deactivators
 Recall that deactivating groups withdraw e-
density from the aromatic ring. All members of
this group except for the halogens direct
incoming groups to the meta position for it is
only in this position that resonance structures
for the intermediate carbocation do not place
the positive charge on the ring carbon to which
the e- withdrawing group is bonded (an unstable
situation). Avoiding this extremely unstable
situation is what makes the meta position the
most highly favored (most stable).
Ortho-Para Deactivating Groups
 Recall that halogens deactivate aromatic rings by
inductive withdrawal of e- density. In addition to
this ability, all halogens possess nonbonded e-’s
that can be used to resonance-stabilize a positive
charge on an adjacent carbon. It is this ability that
make halogens ortho-para directors. If the
incoming group attaches to either the ortho or para
position, one of the resonance structures for the
intermediate carbocation places the positive
charge on a ring carbon to which the halogen is
bonded. This allows the halogens to resonancestabilize the positive charge.
16.5 Substituent Effects in Aromatic
Rings: Summarized
 Substituents already present on an aromatic ring can
cause the aromatic compound to be (much) more or
(much) less reactive than benzene
 Substituents also direct the orientation of incoming groups
on to the aromatic ring
 ortho- and para-directing activators, ortho- and paradirecting deactivators, and meta-directing deactivators
16.7 Trisubstituted Benzenes:
Additivity of Effects
 How does one predict the orientation of a third group
coming in to a disubstituted benzene
 If the directing effects of the two groups are the
same, the result is additive
Substituents with Opposite Effects
 If the directing effects of two groups oppose
each other, the more powerful activating
group decides the principal outcome
Meta-Disubstituted Compounds
 Substitution between two groups in a meta-disubstituted
compound rarely occurs because the site is too sterically
hindered
 To make aromatic rings with three adjacent substituents,
it is best to start with an ortho-disubstituted compound
16.10 Oxidation of Aromatic
Compounds
 Alkyl side chains can be oxidized to CO2H by
strong reagents such as KMnO4 and Na2Cr2O7 if they
have a C-H next to the ring
 Converts an alkylbenzene into a benzoic acid, ArR
 ArCO2H
16.11 Reduction of Aromatic
Compounds
 Aromatic rings are inert to catalytic hydrogenation
under conditions that reduce alkene double bonds
 Can selectively reduce an alkene double bond in the
presence of an aromatic ring
 Reduction of an aromatic ring requires more powerful
reducing conditions (high pressure or rhodium
catalysts)
Reduction of Aryl Alkyl Ketones
 Aromatic ring activates neighboring carbonyl group
toward reduction
 Ketone is converted into an alkylbenzene by catalytic
hydrogenation over Pd catalyst
16.12 Synthesis Strategies
 These syntheses require planning and
consideration of alternative routes
 Work through the practice problems in this
section following the general guidelines for
synthesis