This is a list of topics covered on this page:

1. Huckel Rule
2. Classifications of Aromatic Compounds
3. Properties of Aromatic Compounds
4. Nomenclature of Aromatic Compounds
5. Electrophilic Aromatic Substitutions of Benzene
6. Substituent Effects on Polysubstitution
7. Other Reactions Involving Alkyl Benzenes

An aromatic hydrocarbon is a cyclic compound where all the atoms in the ring(s) are sp2 having a free "p" orbital to provide a pipeline or "conduit" in which Pi electrons can travel through being distributed throughout the molecule. This is referred to as a delocalized Pi electron system. A man by the name of Huckel came up with a mathematical rule which assumed a monocyclic planar ring system having 4n + 2 Pi electrons within the cyclic system where n = any integer beginning with the integer zero.

n                       4n + 2 Pi electrons

0                       4(0) + 2 = 2
1                        4(1) + 2 = 6
2                        4(2) + 2 = 10
3                        4(3) + 2 = 14
4                         4(4) + 2 = 18

etc

1. The ring must be planar (ie: all the atoms in the ring must be sp2 hybridized)
2. There must be 4n + 2 Pi electrons in the ring

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There are several classes of aromatic hydrocarbons.

1. Benzenoid aromatics (Fig 2)- cyclic compounds made up of one or more benzene like rings in which the Pi electrons are conjugated. Examples are benzene, napthalene, phenanthrene, and Anthracene.

   

2. Non-Benzenoid Aromatics-compounds that have ring systems that are larger or smaller than benzene rings. An example is the cycloheptatriene Cation, C₇H₇⁺. Earlier we had considered the cycloheptatriene molecule(Fig 1) as having the proper number of Pi electrons but wasn't aromatic because one of the carbon atoms within the ring was sp3 and not sp2. If one uses a mild reducing agent like Lithium on cycloheptatriene, a hydride ion can easily be removed from the sp3 carbon to create a carbocation whose positively charged carbon becomes sp2 making the ring planar. This is called the cycloheptatriene Cation(Fig 1). It still has six Pi electrons acceptable according to the Huckel Rule to be aromatic. Another example is the cyclopentadiene anion (Fig 1). The neutral molecule cyclopentadiene (Fig 1) has only four Pi electrons and therefore does not conform to the Huckel Rule. However if you removed a proton using a mild base you add two more electrons to the Pi system giving a total of six acceptable to the Huckel Rule. All the carbons become sp2 in the ring and the anion is aromatic. Another example is the aromatic annulenes except for the [6]Annulene(Benzene). Aromatic annulenes are monocyclic compounds where all the Pi electrons are conjugated. Not all annulenes are aromatic.[12] Annulene and [16] annulene are examples of non-aromatics because they have the wrong number of Pi electrons, 12 and 16. [6] annulene would not be counted as a non-benzenoid since this is Benzene itself and would be classified as a Benzanoid.

   

   The resonance stabilized conjugated structure of Benzene was first proposed by Kekule who, legend has it, had a dream vision of snakes chasing their tails around a circle. When he woke up he proposed the Benzene structure consisting of six carbons in a closed ring with alternating double and single bonds between its carbons. This makes all the carbons sp2 hybridized and the geometry around each carbon trigonal planar. Therefore, the benzene ring is flat or planar.

   

   Kekule did, however, explain the resonance by suggesting two non-equivalent structures that were in equilibium with one another. There has never been any experimental evidence to suggest such an equilibrium mixture exists. A more modern picture based on the more modern molecular orbital approach to bonding and Resonance Theory suggests that there are two equivalent structures that are resonce hybrids of one another. The composite structure that would be closer to reality is a single molecule that has attributes of both resonance structures. All carbon-carbon bond lengths would be equal and lie between what a single bond between two carbons and a double bond between two carbons would be.

   A brief biography of Friedrich August Kekulé

3. Heterocyclic aromatics- These are not actually pure hydrocarbon but ring systems that have at least one atom in the ring that is not carbon. Not all heterocyclic compounds are aromatic only those that follow the Huckel conditions above.

   Examples of heterocyclic aromatics include :

   • Furan (five membered ring with one atom Oxygen)(Fig 3). Furan has two double bonds within the five membered ring system with the Oxygen atom lending one of its lone pair to supply the necessary six Pi electrons for aromaticity. All the atoms within the ring are sp2. Furan has a second lone pair that makes it very basic.

   • Pyridine (six membered ring system with one atom a nitrogen.)(Fig 3). Pyridine is a six membered ring that resembles a Benzene ring except that one of the atoms is a Nitrogen atom. The ring has three double bonds giving it the necessary six Pi electrons. The Nitrogen atom has an additional lone pair of electrons which gives Pyridine a decidely basic character.

     

   • Pyrrole is a five membered ring with a nitrogen in the ring (Fig 3). The ring system has two double bonds with the Nitrogen lending its lone pair of electrons as the necessary fifth and sixth Pi electron in the ring. Because the Nitrogen atom is bonded to two sp2 carbons, its orbitals must also be sp2 in order to have maximum overlap for maximum stability. The lone pair occupy the "p" orbital necessary to complete the "p" orbital conduit spoken of before. Pyrrole, unlike Pyridine is not basic at all but surprisingly acidic. The lone pair is locked into the ring structure to maintain the more stable aromatic configuration.

   • Thiophene is a five membered ring system with a sulfur in the ring(Fig 3). Thiophene like Furan has two double bonds and a Sulfur that lends a lone pair of electrons to establish aromatic character to the ring system. Thiophene, like Furan, has an extra lone pair of electrons on the Sulfur atom to give the molecule a basic character. Indeed, Thiophene is more basic than Furan because Sulfur is a larger atom and the electronegativity of Sulfur is less than that of Oxygen. This allows the lone pair to be donated more easily than Oxygen.

     

   • Fullerenes (Fig 4) are an exciting new classification of aromatic organic molecules that consist of pure carbon. The formula for Fullerenes are either C₆₀ or C₇₀. The shape of these remarkable allotropic forms of pure carbon are shaped like soccer balls or icosahedrons. They are also called Buckministerfullerenes named after an architect, Buckminister Fuller, who developed the geodesic dome like structures as a viable architectural structure. This structure is a network of hexagons and pentagon. Regardless of the number of carbon atoms in the Fullerene molecule, there must always be 12 five membered faces with a varied number of six membered faces. Each carbon atom in the molecule is sp2 hybridized. Since each carbon has four valence electrons, three of them are in the three sp2 hybrid orbitals with the fourth electron in the unhybridized "p" orbital and delocalized. The Fullerenes possess facinating properties because of their unusually high electron affinity. They readily accept electrons from metals such as Potassium to form fulleride salts like K₃C₆₀. These salts have the Potassium cations attached to the exterior of the Buckey Balls, but recent research has shown that the metal cations can be encapsulated within the geodesic structure. Apparently, these salts become superconductors below 18 Kelvin.

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     Properties of Aromatic Hydrocarbons

     All aromatic hydrocarbons both benzenoid and non-benzenoid are resonance stabilized. They are water insoluble. They are also carcinogenic (cancer forming) if exposed with sufficient concentration and exposure time. They all undergo electrophilic aromatic substitutions. Most have a sweet aroma hence the reason for the name "aromatic".

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     Nomenclature of Aromatic Hydrocarbons

     1. Nomenclature of Monosubstituted Benzenes.

     2. Nomenclature Of Aromatic Di-substituted benzene derivatives

     3. Nomenclature of Polysubstituted Benzene Derivatives

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     Electrophilic Aromatic Substitutions of Benzene And Its Derivatives

     Benzene and its derivatives undergo substitution reactions where a hydrogen attached to a carbon atom in the ring is replaced by an electrophile. This can happen as many as six times on the Benzene ring. The general mechanism for such a substitution consists of a two step process plus the steps required to generate the electrophile. These steps depend upon the specific reaction involved.

     There are several specific reactions involving electrophilic substitutions:

     1. Halogenation

     2. Nitration

     3. Sulfonation

     4. Freidel Crafts Alkylation

     5. Freidel Crafts Acylation

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     Polysubstitution: The Effect of Substituents On The Benzene Ring

     In discussing electrophilic aromatic substitutions on Benzene, it was apparent that all six carbon atoms were equivalent in their ability to be the center of the substitution since the electron density exhibited by the Pi electron system was evenly distributed over the entire ring. When the electrophile decided to attach itself to one of the ring carbons with the help of a pair of Pi electrons from the ring, the electrophile was equally attracted to each carbon. If we should continue to substitute on a monosubstuted ring, the electrophiles are not equally attracted to the remaining five positions in the ring. It turns out that the group already attached to the ring will exert electronic influence on the remaining five positions affecting the kinetics and directing the next electrophile to positions that are more attractive than other positions in the same ring. This kinetic and directive effects differ for different groups.

     Activating Groups:Ortho-Para Directors

     There are some groups which when attached to one of the ring carbons of Benzene or some other Benzenoid will activate the ring toward further substitution. They do this by increasing the negative electron density in the ring thus making the ring more attractive to the approaching electron seeking electrophile. There are two ways in which the electron density can be affected:

     1. Inductive Electronic effects- This effect relies upon the electronegativity of the atom directly attached to the ring carbon. If the atoms within the group attract the Pi electrons less than the ring that will have an activating effect. Usually this represents the weakest of the two effects.
     2. Resonance Electronic Effect- This effect relies upon electrons in "p" orbitals being in conjugation with the Pi electrons of the ring. This allows electrons to flow into the ring through the "p" orbital conduit established by the conjugation. This effect is the strongest of the two effects and accounts for some groups being powerful activators. Generally speaking any atom having at least one lone pair that is directly connected to a ring carbon will increase the electron density of the ring by lending that lone pair of electrons to the Pi electron system making the ring more attractive to the electrophile. The most powerful and strongest activators are shown in Fig 5 below:

        

        Notice that each of the strong activators have a lone pair attached to the atom bonded to a ring carbon. Through resonance this lone pair increases the electron density making the ring more active.

        All activators, strong, moderate, or weak will direct the electrophile to the ortho or para positions relative to the first substituent. To understand why the ortho and para positions are the most attractive to the electrophile you have to see where the negative electron density accumulates within the ring when a group activates the ring. Let's take an example with the -NH2 group.(Fig 6) You can see that through the writing of resonance structures the extra negative electron density coming into the ring accumulates at the two ortho and one para positions. Therefore, when an electron seeking electrophile forms a sigma complex it does so more likely at the ortho and para positions. This is true about all activator groups. They will all direct Ortho and para.

        

        The distribution from a probalistic view is that we should expect twice as much ortho disubstituted product as para since there are twice as many ortho positions. So the O/P ratio should be 2/1 or 67% Ortho and 33% para. One thing that will alter this distribution is the steric factor. If either of the substituents are bulky this will discourage the attachment at the Ortho position and will increase the amount of para disubstituted product. This can be very dramatic if one of the substituents is a tertiary butyl group. The dramatic results in the Nitration of tertiary butyl benzene is virtually 100% para because of the bulky nature of the tertiary butyl and the nitro groups. They would get in each others way being attached to adjacent carbons.

        Deactivating Groups and meta Direction

        An interesting situation arises with the weakly deactivating groups which include the halogens (Fig 5). The halogen atoms because of their extreme electronegativity will pull electrons out of the ring leaving the ring more positive ,and therefore, less attractive to the incoming electrophile. Such rings are said to be deactivated toward electrophilic substitution. However, the three lone pairs of non-bonding electrons found on the halogen atom will provide increased electron density at the ortho and para positions. To understand this you have to think of a ring that is deactivated ,but a ring which the relative electron density is greater at the ortho and para positions due to the resonance effect of the lone pairs. So if the substitution occurs it will be sluggish but occur either at the ortho or para positions.

        The other two deactivator groupings (Fig 5) are said to be meta directors. Most of these groups either have atoms attached to a ring carbon with a Pi bond or highly electronegative atoms attached as in the case of -CF₃ and -CCl₃ groups which are strong deactivators (Fig 5). These deactivators reduce the negative electron density of the ring by sucking it out of the ring either by resonance (Those atoms directly attached to the ring that have at least one Pi bond) or inductive effect as in the case of the -CCl₃, CF₃, and -NR₃⁺ groups. All deactivators with the exception of the weakly deactivating groups spoken of earlier will be meta directors. This is because when the electron density is dimenished in the ring , it occurs more at the ortho and para positions leaving the meta position relatively more negative.(Fig 7)Therefore, if the electrophile does manage to be attracted to the deactivated ring, it will find the meta position that will be more negative and more attractive to the incoming electrophile.

        

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        Other Reactions With Benzene or Its Derivatives

        Free Radical Halogenation of Alkyl Benzenes

        Alkyl Benzenes will undergo free radical substitution on the alkyl portion of an Alkyl Benzene. For example, Ethyl Benzene will undergo Free Radical Bromination to form 1-Bromo-1-Phenylethane (Fig 8)

        

        The first step initiates the reaction by generating halogen free radicals using enough thermal or radiant energy to break the covalent bond between the two halogen atoms. Steps 2 and 3 are propagation steps. Propagation steps are steps that each produce an essential intermediate for the other step so, in essence, they "feed" each other and propagate the reaction. This is common in all Free Radical reactions. The question might arise as to why the hydrogen atom on the carbon directly attached to the benzene ring is particularly easy to abstract. The answer to that question lies in the stability of the carbon free radical (step 2 in Fig 8). This free radical is what is called a benzylic free radical. Benzylic free radicals are particulary stable even moreso than tertiary free radical carbons. This is because of the ability of the unpaired electron to be spread out over the ortho and para positions of the ring thereby stabilizing the free radical carbon. (Fig 9)

        

        This spreading out of the electron charge density of the unpaired electron onto four carbons(benzylic carbon, two ortho and one para carbons) stabilizes the free radical better than even a tertiary radical.

        R. H. Logan, Instructor of Chemistry, Dallas County Community College District, North Lake College.

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        Acknowledgements:

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        Acknowledgements

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        rhl7460@dcccd.edu

        All textual content copyrighted (c) 1997
        R.H. Logan, Instructor of Chemistry, DCCCD
        All Rights reserved
        
        

        Revised: 7/12/97

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