Assistant Lecture
Tamara Ala'a
College of Dentistry
Organic Chemistry
Aromatic Hydrocarbons:In the early part of the nineteenth century chemists began to discover
organic compounds having chemical properties quite distinct from the
alkanes, alkenes, and alkynes. They called these substances aromatic
compounds because many of the first examples were isolated from the
pleasant-smelling resins of tropical trees. The carbon: hydrogen ratio of
these compounds suggested a very high degree of unsaturation, similar to
the alkenes and alkynes. Imagine, then, how puzzled these early organic
chemists must have been when they discovered that these compounds do
not undergo the kinds of addition reactions common for the alkenes and
alkynes.
We no longer define aromatic compounds as those having a pleasant
aroma; in fact, many do not. We now recognize aromatic hydrocarbons
as those that exhibit a much higher degree of chemical stability than their
chemical composition would predict. The most common group of aromatic
compounds is based on the six member aromatic ring, the benzene ring.
The structure of the benzene ring is represented in various ways in Figure
12.6.
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Assistant Lecture
Tamara Ala'a
College of Dentistry
Organic Chemistry
Structure and Properties:The benzene ring consists of six carbon atoms joined in a planar
hexagonal arrangement. Each carbon atom is bonded to one hydrogen
atom. Friedrich Kekulè proposed a model for the structure of benzene in
1865. He proposed that single and double bonds alternated around the ring
(a conjugated system of double bonds). To explain why benzene did not
decolorize bromine—in other words, didn’t react like an unsaturated
compound—he suggested that the double and single bonds shift positions
rapidly. We show this as a resonance model today.
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Assistant Lecture
Tamara Ala'a
College of Dentistry
Organic Chemistry
The current model of the structure of benzene is based on the idea of
overlapping orbitals. Each carbon is bonded to two others by sharing a pair
of electrons(σ_bonds). Each carbon atom also shares a pair of electrons
with a hydrogen atom. The remaining six electrons are located in p orbitals
that are perpendicular to the plane of the ring. These p orbitals overlap
laterally to form pi (╥) orbitals that form a cloud of electrons above and
below the ring. These ╥_ orbitals are shaped like doughnuts, as shown in
Figure 12.7. Two symbols are commonly used to represent the benzene
ring. The representation in Figure 12.6b is the structure proposed by
Kekulé. The structure in Figure 12.6d represents the╥ _ clouds. The equal
sharing of the six electrons of the p orbitals results in a rigid, flat ring
structure, in contrast to the relatively flexible, nonaromatic cyclohexane
ring. The model also explains the unusual chemical stability of benzene
and its resistance to addition reactions. The electrons of the╥ _ cloud are
said to be delocalized. That means they have much more space and
freedom of movement than they would have if they were restricted to
individual double bonds. Because electrons repel one another, the system
is more stable when the electrons have more space to occupy. As a result,
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Assistant Lecture
Tamara Ala'a
College of Dentistry
Organic Chemistry
benzene is unusually stable and resists addition reactions typical of
alkenes.
Benzene Derivatives:The nomenclature of substituted benzene ring compounds is less systematic
than that of the alkanes, alkenes and alkynes. A few mono-substituted compounds are
named by using a group name as a prefix to "benzene", as shown by the combined
names listed below. A majority of these compounds, however, are referred to by
singular names that are unique. There is no simple alternative to memorization in
mastering these names.
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Assistant Lecture
Tamara Ala'a
College of Dentistry
Organic Chemistry
Two commonly encountered substituent groups that incorporate a benzene ring are
phenyl, abbreviated Ph-, and benzyl, abbreviated Bn-. These are shown here with
examples of their use. Be careful not to confuse a phenyl (pronounced fenyl) group
with the compound phenol (pronounced feenol). A general and useful generic
notation that complements the use of R- for an alkyl group is Ar- for an aryl group
(any aromatic ring).
When more than one substituent is present on a benzene ring, the relative locations of
the substituents must be designated by numbering the ring carbons or by some other
notation. In the case of disubstituted benzenes, the prefixes ortho, meta & para are
commonly used to indicate a 1,2- or 1,3- or 1,4- relationship respectively. In the
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Assistant Lecture
Tamara Ala'a
College of Dentistry
Organic Chemistry
following examples, the first row of compounds show this usage in red. Some
disubstituted toluenes have singular names (e.g. xylene, cresol & toluidine) and their
isomers are normally designated by the ortho, meta or para prefix. A few
disubstituted benzenes have singular names given to specific isomers (e.g. salicylic
acid & resorcinol). Finally, if there are three or more substituent groups, the ring is
numbered in such a way as to assign the substituents the lowest possible numbers, as
illustrated by the last row of examples. The substituents are listed alphabetically in
the final name. If the substitution is symmetrical (third example from the left) the
numbering corresponds to the alphabetical order.
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Assistant Lecture
Tamara Ala'a
College of Dentistry
Organic Chemistry
Reaction of Benzene:1- Electrophilic Aromatic Halogenation:One of the substitutions that can occur with Benzene or one of its derivatives is
to replace a Hydrogen atom on the ring with a halogen.
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Assistant Lecture
Tamara Ala'a
College of Dentistry
Organic Chemistry
Figure 1 above shows the net reaction for halogenation of a Benzene ring. The AlX3
acts as a Lewis acid catalyst. Chlorination and Bromination is possible using such a
catalyst. Iodination requires the use of I-Cl as the Iodination reagent. Flourination is
much too explosive to be a practical synthesis.
The reaction mechanism (Fig 2) shows initially in the first step the generation of the
Cl+ electrophile.
The catalyst is required in step 1 because the halogen molecule is non-polar. The
AlCl3 forces the bonding electrons in the Cl-Cl molecule to one end so that an
induced polarity is effected. This weakens the co-valent bond between the two
halogen atoms so that the bond breaks heterolytically generating the Cl + electrophile
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Assistant Lecture
Tamara Ala'a
College of Dentistry
Organic Chemistry
2- Electrophilic Aromatic Nitration Of Benzene
Nitration of Benzene (Fig 1) or one of its derivatives involves a rather unusual
nitrating mixture. Sulfuric Acid and Nitric Acid are mixed where the stroger Sulfuric
Acid forces a proton on Nitric Acid.
The first two steps in the Reaction Mechanism (Fig 2) shows how the nitro
electrophile was generated.
Notice that the Sulfuric Acid is regenerated and therefore serves as a catalyst in this
reaction. The generation of a positively charged Oxygen atom (oxonium ion) in step
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Assistant Lecture
Tamara Ala'a
College of Dentistry
Organic Chemistry
1 produces an unstable intermediate which spontaneously decomposes losing water
and generating the electrophile in step 2.
3- Electrophilic Aromatic Sulfonation of Benzene
Aromatic Sulfonation places a Sulfonyl group (-SO3H) onto the Benzene ring. It accomplishes
this by the use of Sulfur Trioxide in the presence of Sulfuric Acid. (Fig 1-a)
Sulfur Trioxide gas is dissolved into concentrated Sulfuric Acid at 20% to give what
is called "fuming Sulfuric Acid" which earns its name by billowing out white plumes
of smokey SO3 fumes when the bottle is opened. The Sulfur Trioxide (SO 3) serves
two purposes. It acts first as the sulfonating agent, and it prevents the unfavorable
reversable of this equilibrium by reacting with and effectively removing the water
product thus driving this equilibrium to the right.
SO3 + H2O ---> H2SO4
The Sulfonation can be accomplished without the Sulfur Trioxide by using excess
Sulfuric Acid (Fig 1-b). The extra Sulfuric Acid will act as a dehydrating agent
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Assistant Lecture
Tamara Ala'a
College of Dentistry
Organic Chemistry
absorbing the water and preventing the reversal of the equilibrium. This is not as
efficient as the Sulfur Trioxide so the reaction is much slower.
The reaction mechanism in the presence of Sulfur Trioxide differs by that of excess
concentrated Sulfuric Acid in that step 1 in the mechanism shown in Fig 2 below is
not necessary, and the mechanism begins essentially with step 2. In both
environments the SO3 is apparently the sulfonating agent (step 2 in Fig 2).
This problem with the water product in this reaction can be used to our advantage if we wish
to remove a sulfonyl group by using dilute Sulfuric Acid. The excess water in the diluted acid
will drive this equilibrium to the left (Fig 1) according to Le Chatlier's Principle.
4- Friedel-Crafts Alkylation of Benzene and Its Derivatives
Franco-American cooperation was in effect with the alkylation reaction as Professor
Friedel, a French chemist, collaborated with Professor Crafts, an American chemist,
to develop the Friedel Crafts Alkylation. This is an electrophilic aromatic substitution
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Assistant Lecture
Tamara Ala'a
College of Dentistry
Organic Chemistry
whereby a carbocation is generated as the electrophile. There are several ways this
can be done as shown in Fig 1 below.
Once the carbocation has been generated then the reaction proceeds to the alkylated
Benzene Ring (Fig 2)
The reaction mechanism for this alkylation is shown in Fig 3 below.
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Assistant Lecture
Tamara Ala'a
College of Dentistry
Organic Chemistry
There are a number of problems dealing with the alkylation reaction that lead to the
development of a similar reaction, Friedel-Crafts Aycylation. The limitations to this
alkylation reaction include:
1. Polysubstitution- Since the alkyl group that has been placed on the Benzene ring
activates the ring toward further substitution, and each subsequent alkylation
increases this activation of the ring, this leads to alkylation of the ring in several
positions. This is called "polysubstitution" and leads to extremely low yields of
the monosubstituted product.
2. Possible rearrangement of the initial carbocation- Carbocations will undergo
molecular rearrangement where a Hydrogen will move over to an adjacent carbon
with the bonding electrons (hydride shift) or a methyl group will move over to the
adjacent carbon with the bonding electrons(methide shift). This molecular
rearrangement within a carbocation will only occur if it results in a more stable
carbocation. We know that the relative stability of carbocations runs:
C6H5-CH2+ = CH3-CH=CH2+ > tertiary > secondary > primary > CH3+
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Assistant Lecture
Tamara Ala'a
College of Dentistry
Organic Chemistry
So if a hydride or methide shift results in a primary carbocation becoming a tertiary
or secondary carbocation then the molecular rearrangement will occur at least
partially resulting in possible multiple products for alkylation. Molecular
rearrangement will not occur if it would result in a tertiary carbocation becoming a
secondary or primary carbocation.
3. Benzene derivatives where the substituents deactivate the ring give very poor
yields in all Friedel-Crafts reactions
4. Halobenzenes or vinylic halides cannot be used as the alkylating agent because
they do not form carbocations readily. This is true for both Friedel-Crafts
reactions.
5- Friedel-Crafts Acylation of Benzene and Its Derivatives
An acyl group is an alkyl group attached to a carbonyl carbon:
R-C=O
Acylation involves the generation of the acyl group as an electrophile and
substituting it upon a Benzene ring.(Fig 1)
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Assistant Lecture
Tamara Ala'a
College of Dentistry
Organic Chemistry
Basic reactions
Aromatic nitrations to form nitro compounds take place by generating a nitronium
ion from nitric acid and sulfuric acid.
Aromatic sulfonation of benzene with fuming sulfuric acid gives benzenesulfonic
acid.
Aromatic halogenation of benzene with bromine, chlorine or iodine gives the
corresponding aryl halogen compounds catalyzed by iron tribromide.
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Assistant Lecture
Tamara Ala'a
College of Dentistry
Organic Chemistry
The Friedel-Crafts reaction exists as an acylation and an alkylation with as
reactants acyl halides or alkyl halides.
The catalyst is always aluminum chloride.
For example :
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