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CHE 203
FUNCTIONAL GROUP CHEMISTRY
(CLASS FIVE)

BENZENE AND OTHER AROMATIC COMPOUNDS
INTRODUCTION
Benzene (C6H6) is the simplest aromatic hydrocarbon (or arene)
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Benzene has four degrees of unsaturation, making it a highly
unsaturated hydrocarbon
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For example, bromine adds to ethylene
to form a dibromide, but benzene is inert under similar conditions
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Thus, any structure proposed for benzene must account for its high degree of unsaturation and
its lack of reactivity towards electrophilic addition
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In the Kekulé model, benzene was thought to be a rapidly
equilibrating mixture of two compounds, each containing a six-membered ring with three
alternating π bonds
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In the Kekulé

description, the bond between any two carbon atoms is sometimes a single bond and sometimes
a double bond
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Instead,
current descriptions of benzene are based on resonance and electron delocalization due to orbital
overlap
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Because these compounds possessed strong and characteristic
odours, they were called aromatic compounds
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• Aromatic compounds resemble benzene - they are unsaturated compounds that do not undergo
the addition reactions characteristic of alkenes
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This is a particular messy process
in the laboratory, and requires a lengthy business of separating the products from one another
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There is strong demand for
coke, which is produced by heating coal in the absence of air
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This is an oily liquid, which contains a variety of products
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Methylbenzene, naphthalene, and anthracene are also obtained in smaller quantities
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• It is planar
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Although the Kekulé structures satisfy the first two criteria, they break down with the third,
because having three alternating π bonds means that benzene should have three short double
bonds alternating with three longer single bonds
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The
resonance description of benzene consists of two equivalent Lewis structures, each with three
double bonds that alternate with three single bonds
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The two Kekulé representations are not in equilibrium with each other
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The resonance hybrid of benzene explains why all C – C bond lengths are the same
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The C–C bonds in benzene are equal and intermediate in length
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Carbon substituents are named as alkyl groups
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(b) Disubstituted Benzenes
There are three different ways that two groups can be attached to a benzene ring, so a prefix
- ortho, meta, or para - can be used to designate the relative position of the two substituents
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If the two groups on the benzene ring are different, alphabetize the names of the substituents
preceding the word benzene
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(c) Polysubstituted Benzenes
For three or more substituents on a benzene ring:
• Number to give the lowest possible numbers around the ring
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• When substituents are part of common roots, name the molecule as a derivative of that
monosubstituted benzene
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Examples of naming polysubstituted benzenes

4-chloro-1-ethyl-2-propylbenzene
• Assign the lowest set of numbers
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• Designate the position of the NH2 group as “1” and then assign the lowest possible set of
numbers to the other substituents
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• A phenyl group (C6H5–) is formed by removing one hydrogen from benzene (C6H6)
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The benzyl group, another common substituent that contains a benzene ring, differs from a
phenyl group
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Examples of aryl groups:

REACTIONS OF AROMATIC COMPOUNDS
Electrophilic Aromatic Substitution
Benzene has six π electrons delocalized in six p orbitals that overlap above and below the plane
of the ring
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Because benzene’s six π electrons satisfy Huckel’s rule, benzene is especially stable
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As a result, the characteristic
reaction of benzene is electrophilic aromatic substitution - a hydrogen atom is replaced by an
electrophile
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Substitution of hydrogen, on the other hand, keeps
the aromatic ring intact
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Mechanism
General Mechanism of Electrophilic Aromatic Substitution
Step 1: Addition of the electrophile (E+) to form a carbocation
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This carbocation intermediate is not aromatic, but it is resonance
stabilized - three resonance structures can be drawn
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Step 2: Loss of a proton to re-form the aromatic ring
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This step is fast because the aromaticity of the benzene ring is restored
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The choice of resonance structure affects how curved arrows are drawn, but not the identity
of the product
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This two-step mechanism for electrophilic aromatic substitution applies to all electrophiles
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NB:

The general mechanism outlined above can now be applied to each of the five specific
examples of electrophilic aromatic substitution
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This step is different with each electrophile
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These
two steps are the same for all the five reactions
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General Features

In Friedel - Crafts alkylation, treatment of benzene with an alkyl halide and a Lewis acid
(AlCl3) forms an alkyl benzene
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Examples of Friedel-Crafts alkylation

In Friedel - Crafts acylation, a benzene ring is treated with an acid chloride (RCOCl) and AlCl3
to form a ketone
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The acid chlorides are also called acyl chlorides
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1
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Because the carbocations
derived from vinyl halides and aryl halides are highly unstable and do not readily form, these
organic halides do not undergo Friedel–Crafts alkylation
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Rearrangements can occur
The Friedel - Crafts reaction can yield products having rearranged carbon skeletons when 1°
and 2° alkyl halides are used as starting materials, as shown in Equations (1) and (2)
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Intramolecular Friedel–Crafts Reactions
All of the Friedel–Crafts reactions discussed thus far have resulted from intermolecular
reaction of a benzene ring with an electrophile
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For example, treatment of
compound A, which contains both a benzene ring and an acid chloride, with AlCl3, forms αtetralone by an intramolecular Friedel - Crafts acylation reaction
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Common
substituents include halogens, OH, NH2, alkyl, and many functional groups that contain a
carbonyl
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What makes a substituent on a benzene ring electron donating or electron withdrawing? The
answer is inductive effects and resonance effects, both of which can add or remove electron
density
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• Atoms more electronegative than carbon - including N, O, and X- pull electron density away from
carbon and thus exhibit an electron withdrawing inductive effect
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Considering inductive effects only, an NH2 group withdraws electron density and CH3 donates
electron density
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•

N inductively withdraws electron density
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(ii) Resonance Effects
Resonance effects can either donate or withdraw electron density, depending on whether they
place a positive or negative charge on the benzene ring
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• A resonance effect is electron withdrawing when resonance structures place a positive charge on
carbons of the benzene ring
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Common examples of Z
include N, O, and halogen
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Because three of them place a negative charge on a carbon atom of the benzene ring, an
NH2 group donates electron density to a benzene ring by a resonance effect
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Electrophilic Aromatic Substitution of Substituted Benzenes
Electrophilic aromatic substitution is a general reaction of all aromatic compounds, including
polycyclic aromatic hydrocarbons, heterocycles, and substituted benzene derivatives
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• The orientation: The new group is located either ortho, meta, or para to the existing substituent
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Toluene (C6H5CH3) and nitrobenzene (C6H5NO2) illustrate two possible outcomes
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Toluene
Toluene reacts faster than benzene in all substitution reactions
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Although three products are possible,
compounds with the new group ortho or para to the CH3 group predominate
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2
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Thus, its electron
withdrawing NO2 group deactivates the benzene ring to electrophilic attack
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The NO2 group is called a meta director
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All substituents can be divided into three general
types
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• Substituents that deactivate a benzene ring and direct substitution ortho and para
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• All meta directors deactivate the ring
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Deactivated rings: the substituents on the ring are groups that withdraw electrons
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There are two general types of ortho, para
directors and one general type of meta director:
• All ortho, para directors are R groups or have a nonbonded electron pair on the atom bonded to the
benzene ring
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