Organic Chemistry I & II
Organic Chemistry I & II

Organic Chemistry I & II

Lead Author(s): Steven Forsey

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Steven Forsey, “Organic Chemistry”, Only one edition needed

Up to 40-60% more affordable

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Carey & Giuliano, “Organic Chemistry”, 10th Edition

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Always up-to-date content, constantly revised by community of professors

Constantly revised and updated by a community of professors with the latest content

Top Hat

Steven Forsey, “Organic Chemistry”, Only one edition needed

McGraw-Hill

Carey & Giuliano, “Organic Chemistry”, 10th Edition

Wiley

Solomons et al., “Organic Chemistry”, 12th Edition

Wiley

David R. Klein, “Organic Chemistry”, 3rd Edition

In-book Interactivity

Includes embedded multi-media files and integrated software to enhance visual presentation of concepts directly in textbook

Top Hat

Steven Forsey, “Organic Chemistry”, Only one edition needed

McGraw-Hill

Carey & Giuliano, “Organic Chemistry”, 10th Edition

Wiley

Solomons et al., “Organic Chemistry”, 12th Edition

Wiley

David R. Klein, “Organic Chemistry”, 3rd Edition

Customizable

Ability to revise, adjust and adapt content to meet needs of course and instructor

Top Hat

Steven Forsey, “Organic Chemistry”, Only one edition needed

McGraw-Hill

Carey & Giuliano, “Organic Chemistry”, 10th Edition

Wiley

Solomons et al., “Organic Chemistry”, 12th Edition

Wiley

David R. Klein, “Organic Chemistry”, 3rd Edition

All-in-one Platform

Access to additional questions, test banks, and slides available within one platform

Top Hat

Steven Forsey, “Organic Chemistry”, Only one edition needed

McGraw-Hill

Carey & Giuliano, “Organic Chemistry”, 10th Edition

Wiley

Solomons et al., “Organic Chemistry”, 12th Edition

Wiley

David R. Klein, “Organic Chemistry”, 3rd Edition

About this textbook

Lead Authors

Dr. Steven Forsey, Ph.D.University of Waterloo

Steven Forsey is currently a Professor at University of Waterloo, teaching a variety of organic chemistry courses to Chemistry, Science, Chemical Engineering, Nanotechnology and distance education students. He received his Ph.D. (2004) for Synthetic Organic Chemistry from University of Waterloo, Ontario. He is a recipient of the Excellence of Science Teaching Award and has acted as the Teaching Fellow for the Department of Chemistry since 2016.

Contributing Authors

Felix NgassaGrand Valley State University

Neil GargUCLA

Jennifer ChaytorSaginaw Valley State University

Greg DomskiAugustana College

Christian E. MaduCollin Community College

Christopher NicholsonUniversity of West Florida

Franklin OwEast Los Angeles College, UCLA

Robert S. PhillipsUniversity of Georgia

Grigoriy SeredaUniversity of South Dakota

Simon E. LopezUniversity of Florida

Brannon McCulloughNorthern Arizona University

Jason JonesKennesaw State University

José BoquinAugustana College

Stephanie BrouetSaginaw Valley State University

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Chapter 20: Aromatic Compounds

The blue color of many species of mushroom is due to azulene, a naturally occurring aromatic compound. [1]​

Contents

Learning Objectives

  • Recognize common monosubstituted aromatic molecules by name and structure.
  • Name disubstituted aromatic molecules with both ortho-, meta- and para- nomenclature as well as IUPAC numerical nomenclature.
  • Name polysubstituted aromatic molecules using IUPAC nomenclature.
  • Draw structures when given the common or IUPAC name.
  • Evaluate the physical properties of a series of aromatic molecules.
  • Use Huckel’s Rules to analyse a structure for aromatic properties.
  • Define Aromatic and Antiaromatic in terms of stability and Huckel’s Rules.
  • Evaluate rings of various sizes for aromatic properties.
  • Explain the impact of charge on the aromaticity of a ring.
  • Discuss the contribution of non-bonded electrons to an aromatic system in the case of nitrogen, oxygen and sulfur atoms.
  • Describe the basicity of nitrogen atoms in different heteroaromatic molecules.

20.1 A Brief History of Benzene

Benzene is one of the oldest known organic molecules and has a long and storied history both in chemistry and also in various industries. It has been known for centuries not for its chemical properties but for its sweet smell, different from many other hydrocarbons, which tend to be odorless. Benzene was used as a perfuming agent for centuries before more detail about its structure and chemical properties were determined.

The molecular formula of benzene, along with its ring structure, was determined in the last half of the 19th century and led to widespread industrial use of benzene and its derivatives in products from fuels to solvents to building blocks for polymers. Among its many different uses, benzene was used in the early 20th century to extract caffeine from coffee. Into the late 20th century, benzene was a common industrial solvent as well as a component of many household cleaning products.

In the 1970s, benzene was linked to cases of leukemia and its widespread industrial use was discontinued. It remains an important organic molecule for study in chemistry both as a solvent and a reagent. Benzene and other aromatic molecules serve as a scaffold for numerous commercial and medicinal compounds, including drugs such as aspirin and plastics like polystyrene.

20.2 Benzenes 

The molecular formula of benzene was determined in the middle of the 19th century to be C6H6. The molecular structure of such a simple molecule was widely debated for many years, with numerous prominent scientists hypothesizing different arrangements of the six carbons and the six hydrogens. Friedrich Kekulé proposed the ultimate structure in 1865, formed of six carbons in a ring with alternating single and double bonds. This molecular structure presents many challenges, including strategies for naming the derivatives of benzene, explaining its physical properties, and also its unusual stability. 

Figure 20.2. Hypothesized structures of benzene


Q20.1 - Level 1

Based on your knowledge of molecular geometry, which hypothesized structure of Benzene has bond angles strained well beyond known tolerances?

question description
A

a

B

b

C

c

D

d


20.2.1 Nomenclature

Monosubstituted benzene rings are named using benzene as the root, and the substituent as a prefix. Common examples of this convention are halogenated benzene rings such as fluorobenzene, nitrobenzene, and ethylbenzene.

Figure 20.3. Substituted benzene nomenclature

There are also many monosubstituted benzenes with common names. These compounds include the alcohol of benzene (phenol), the amine of benzene (aniline), and methyl-substituted benzene (toluene). Other common name benzenes are also shown. These names often come from the organic matter from which the compound was first isolated.

Figure 20.4. Common names of substituted benzenes​

When two groups are attached to the benzene ring, one of two naming methods may be used. The ring may be numbered with the highest priority substituent being at the first carbon of the ring, and a numerical location given to the second group. A more common relative naming system for disubstituted benzenes uses the prefixes ortho-, meta-, and para- (abbreviated o-, m-, and p-). Ortho groups are 1,2 relative to each other, meta groups are 1,3 relative to each other, and para groups are 1,4 relative to each other.

Figure 20.5. Systematic nomenclature of disubstituted benzenes

When more than two groups are present on a benzene ring, numbering must be used. In the case of three or more substituents, the ring is numbered to provide the lowest overall set of numbers. Common monosubstituted benzene rings, such as toluene, can be used as root names for both relative and numbered benzene rings. 

A benzene ring may also be a substituent on a chain or other ring, the benzene ring as a substituent is called a phenyl group and may be abbreviated as either C6H5- or Ph-. A benzene ring with one carbon attached to it is also commonly named as a substituent called a benzyl group.

Figure 20.6. Polysubstituted benzenes​


Q20.2 - Level 2

Match the proper structure and name.

question description
Premise
Response
1

I

A

2-chloro-5hydroxybenzenesulfonic acid

2

II

B

3-methyltoluene

3

III

C

o-hydroxybenzoic acid

4

IV

D

p-bromonitrobenzene


Q20.3 - Level 2

Which pair of names are equivalent?

A

m\it m -fluorotoluene / 4-fluorotoluene

B

o\it o -bromophenol / 2-bromophenol

C

p\it p -nitrobenzaldehyde / 3-nitrobenzaldehyde

D

m\it m -chloroaniline / 1-chloroaniline


20.2.2 Physical Properties

Benzene is a non-polar hydrocarbon with a relatively low molecular weight, 78.11 g/mol. Benzene has only Van der Waals forces and thus is readily soluble in non-polar solvents, such as hexane. It is miscible in weakly polar solvents such as chloroform or ethers. Benzene is almost totally insoluble in water.

Benzene also has very low boiling and freezing points, freezing at 5.5°C and boiling at 80.1°C. These values are quite similar to cyclohexane, and a common trend is seen across aromatic and saturated rings with various substituents. The relationship can be best seen in the table below.

Table 20.1. Physical Properties of Cyclohexane and Aromatic Molecules​


Q20.4 - Level 1

What part of 3,5-dimethylbenzoic acid is polar?


Q20.5 - Level 2

Organize the molecules from least polar to most polar.

question description
A

Molecule C

B

Molecule D

C

Molecule B

D

Molecule A


20.3 Aromaticity

From its discovery as a cycloalkene, one of the unique aspects of benzene was its relative unreactivity compared to other alkenes and cycloalkenes. As an example, the decolorization of bromine, a common test for unsaturation, yields no reaction with benzene in spite of the four degrees of unsaturation present in benzene. To understand this unusual stability, it is necessary to consider the structure of benzene and the molecular orbitals that arise from the combination of the six adjacent p orbitals.


Q20.7 - Level 1

Choose the answer that properly describes the types of unsaturation of benzene.

A

Three rings and one double bond

B

Two double bonds and two rings

C

Three double bonds and one ring

D

Seven double bonds and one ring



The Smell of Bacon

“Aromatic” compounds in organic chemistry are cyclic molecules with unsaturated bonds in conjugation (e.g., consecutive pi bonds). The term ‘aromatic’ was first introduced into chemistry by Hoffman in 1855. For centuries earlier, the term had been used to describe fragrances and aromas, which is still the case today.

Why does bacon smell so good? When cooked, it releases a number of beautifully scented volatile organic compounds. Coincidentally, some of these, such as 2,5-dimethylpyrazine and 2-ethyl-3,5-dimethylpyrazine are actually ‘aromatic’ compounds. 

[2]​



20.3.1 Structure

Benzene is represented in a Lewis structure by six carbon atoms in a ring with alternating single and double bonds. Benzene and other aromatic molecules are unique because a resonance structure can be drawn where the double bonds are shifted, but no atoms end up with charges. This can also be drawn with a circle inside a six-member ring indicating the resonance “flow” of electrons around all six atoms. 

Figure 20.7. Resonance structures of benzene

Because all six atoms have an unhybridized p orbital, each has a planar structure, making the ring itself planar. Because the p orbitals are orthogonal to the three hybridized sp2 orbitals, the edge-to-edge overlap of the π orbitals makes a circle of π bonds. Thus the overlapping of the p orbitals around the ring results in molecular orbitals encompassing the top and bottom faces of the ring. This distributes the π electron density equally throughout the top and bottom of the carbocyclic ring as seen in the electrostatic potential map. 

Figure 20.8. Orbital depiction and electrostatic potential energy map of benzene.

The resonance structures given in Figure 20.7 imply that the bond order between each adjacent carbon is 1.5. Thus, you would expect the bond length to be between a single bond and a double bond. Experimentally, this is what researchers find. Additionally, all six carbon-carbon bonds are the same length because each is a 1.5 bond order. In a nonaromatic molecule, the bonds are either single or double. Single bonds and double bonds have different bond lengths (Figure 20.9), so the ring would very different if there were no resonance.

Figure 20.9. Bond lengths and angles in aromatic and nonaromatic alkenes​


Q20.8 - Level 1

Which hybridization is correct for the carbon atoms in benzene?

A

spsp3^3dd

B

spsp

C

spsp3^3

D

spsp2^2


20.3.2 Stability

The stability of benzene and other aromatic rings can be qualitatively observed by a simple comparative test. When cyclohexene and elemental bromine are combined in a test tube, the color of the elemental bromine disappears as the bromine is added to the double bond. This reaction is covered in Chapter 8. However, when benzene and elemental bromine are combined, no color change is observed. That means the elemental bromine is still present, and the π electrons are failing to react as they did in the case of cyclohexene. 

From a quantitative perspective, the reaction of π electrons can be considered by using enthalpy of reaction data for the addition of hydrogen to π bonds in cyclic systems. When hydrogen is added to cyclohexene, the enthalpy of reaction (ΔHrxn) is 120 kJ/mol. Multiple double bonds will have a more negative enthalpy, twice the value of an individual double bond as seen in 1,4-cyclohexadiene. 

 When the double bonds are in resonance, however, the enthalpy of reaction for the addition of hydrogen of cyclohexa-1,3-diene is 232 kJ/mol, so the additional alkene is slightly more stable than two isolated double bonds. The 8 kJ/mol difference can be attributed to resonance stabilization. In the case of three double bonds then, with no resonance, the enthalpy is expected to be 360 kJ/mol. The observed enthalpy of reaction is 208 kJ/mol, which is a 152 kJ/mol more stable than the predicted three double bond system. This stability is attributed to the measurable concepts of resonance and aromaticity. 

Figure 20.10. Enthalpy of hydrogenation of some cycloalkenes and benzene

Because all these reactions result in cyclohexane as a product, we can compare the enthalpy values to each other. This difference in reactivity causes benzene to remain unreacted under the reaction conditions that cause less stable alkenes to react.

Figure 20.11. Reaction of bromine in carbon tetrachloride with different alkenes


Q20.9 - Level 1

The enthalpy of hydrogenation of cyclohexa-1,3-diene is about 1.6 kJ/mol less than for cyclohexa-1,4-diene. What property is most likely responsible for this difference?


Q20.10 - Level 2

What product would be expected when cyclohexa-1,4-diene reacts with excess elemental bromine? (For help, look at Alkene Section 12.7)

question description
A

a

B

b

C

c

D

d


20.3.3 Molecular Orbitals

The reason for the unusual stability in benzene and other aromatic molecules lies in the molecular orbital (MO) diagram of benzene. When nodes are introduced into hexa-1,3,5-triene, the first node divides the molecule between the third and fourth carbons. In benzene, however, the node cuts the ring in half, either through the bonds or through the atoms. These two different single node MOs are degenerate, or of equal energy. 

 The energy gap between the degenerate bonding MOs and anti-bonding MOs is greater than the energy gap between the bonding and anti-bonding MOs in hexa-1,3,5-triene. Additionally, the total stabilizing energy of the six π electrons in benzene is more negative than the total stabilizing energy of the six π electrons in hexa-1,3,5-triene.  

Figure 20.12. MO diagrams of benzene and hexa-1,3,5-triene


Q20.11 - Level 1

How many nodes are in the highest energy molecular orbital of benzene?


20.3.4 Huckel’s Rules

Not all conjugated cyclic systems are aromatic. While the C6H6 molecule, benzene, is incredibly stable and unreactive, both C4H4 and C8H8 exhibit no such stability. In fact, C4H4 is so reactive that it is difficult to isolate and C8H8 is approximately as reactive as a normal cycloalkene. If a simple ability to arrange unhybridized p orbitals in a ring leads to stability, then both C4H4 and C8H8 should be roughly as stable as benzene, so there must be some other influence at work. The true rationale lies in the construction of the molecular orbital diagram of these molecules, but it was also summarized in a set of rules by Erich Huckel now known as Huckel’s rules.

Huckel’s rules stipulate that a compound will be aromatic if the following conditions are met:

  •  The structure is cyclic.
  •  Each atom of the ring has an unhybridized p orbital.
  •  The unhybridized p orbitals form a continuous cycle around the ring, so the ring must be planar.
  •  The number of electrons residing in the π electron cloud (either the π bonds they form or solely on an atom) is equal to 4n+2, where n is an integer (n=0,1,2…).

Aromatic molecules are defined by following all four of Huckel’s rules. Adherence to all four rules results in the significant stabilization benefit seen in the resonance energy and unreactivity discussed in 20.3.2. Note that there is no specific requirement that there only be six atoms as in benzene, nor is there a requirement that the atoms all be carbon. We will address both of these points in future sections. If the definition of aromatic is following Huckel’s rules and benefiting from the resulting stability, what happens if Huckel’s rules are changed or not adhered to?

Figure 20.13. Aromatic molecules

If the last of Huckel’s rules are changed to 4n π electrons rather than 4n+2, we get situations such as cyclo-1,3-butadiene. Cyclobuta-1,3-diene follows the first three rules but has only four electrons in the π cloud. As mentioned previously, cyclobuta-1,3-diene is so unstable that it is difficult to isolate. This condition is known as antiaromaticity. Antiaromaticity is a very specific case, as is aromatic, in that a structure must adhere to all four of Huckel’s rules with the fourth rule modified. While aromatic molecules are unusually stable, antiaromatic molecules are unusually unstable, and because of that, there are very few examples of antiaromatic compounds.

Figure 20.14. Antiaromatic molecules

All cases that fail to adhere to Huckel’s rules, whether acyclic, non-planar or without a continuous cycle of p orbitals, are nonaromatic. Nonaromatic molecules exhibit similar stability and reactivity as normal alkenes, whether conjugated or not. In general, nonaromatic molecules follow all the reactivity trends seen in Chapters 12 and 19. So is C8H8 aromatic, antiaromatic, or nonaromatic? 

A quick check of the π electron count yields eight electrons, which satisfies the 4n formula when n=2. So C8H8 can’t be aromatic, but is it antiaromatic or nonaromatic? Because antiaromatic is highly unstable, large rings will bend to break the continuous cycle of orbitals to yield an unconjugated nonaromatic molecule. This also explains why C8H8 reacts very similarly to normal cyclic alkenes. A physical model of cyclooctatetraene may be helpful to see how the ring can bend to prevent the ring from having an antiaromatic cycle of p orbitals.

Figure 20.15. Cyclooctatetraene, a non-aromatic molecule ​


Figure 20.16. Nonaromatic molecules

Cyclohepta-1,3,5-triene has six π electrons but because there is a sp3 carbon separating the double bonds, a closed-cycle, aromatic π system cannot form. All atoms in the ring must possess a p orbital. 5,5-dimethylcyclopentadiene has four π electrons, so you may think that it is anti-aromatic. However, the π bonds are also separated by a sp3 carbon, so this molecule is also nonaromatic.

Click on the following videos to help you visualize through molecular orbitals why the Huckel’s rule works.




Q20.12 - Level 2

Match the structure with the correct description based on Huckel’s rules.

question description
Premise
Response
1

a)

A

Antiaromatic

2

b)

B

Nonaromatic

3

c)

C

Aromatic


20.3.5 Annulenes

As we have seen Huckel’s rules applied to a few small rings, it seems reasonable to ask how far this set of rules extends. Aromatic properties such as stability and predicted NMR values have been observed in a wide array of rings called annulenes. Annulene is a term for a single hydrocarbon ring with a conjugated set of p orbitals regardless of the size. The size of the ring is indicated in front of the word annulene, so benzene is also [6]-annulene because it has six carbons. 

 Annulenes that follow the 4n+2 rule such as [14]-annulene and [18]-annulene exhibit aromatic properties like shifted proton NMR chemical shifts. [10]-annulene is an unusual exception because of steric or strain energies. In the case of the all cis [10]-annulene, versions, the angle strain is significant, and in the case of the trans, cis, trans, cis, cis-[10]-annulene d, there is a disfavored steric penalty between hydrogens on the alkenes of the ring.

Figure 20.17. Aromatic annulenes​


Figure 20.18. Problems with [10]-annulene

Annulenes such as [8]-annulene, [12]-annulene and [16]-annulene that would satisfy a rule of 4n π electrons rather than the 4n+2 rule are nonaromatic rather than antiaromatic because the rings are large enough to bend out of a single plane. Large nonaromatic annulenes exhibit no NMR properties like the aromatic annulenes, and nonaromatic annulenes react as though they were conjugated alkenes, but not aromatic systems.

Figure 20.19. Nonaromatic annulenes

Click on the video to see the three-dimensional structure of [18]-annulene and its molecular orbitals.

​Note: This video does not feature audio​


Q20.13 - Level 2

Which of the following annulenes is aromatic according to Huckel’s rules?

question description
A

a

B

b

C

c


Q20.14 - Level 1

Match the number of π electrons to the correct annulene.

question description
Premise
Response
1

a)

A

18

2

b)

B

14

3

c)

C

8

4

d)

D

22


20.3.6 Summary of Aromaticity

Aromatic compounds have 4n+2 π electrons and are more stable than their open-chained counterparts. Thus delocalization of the π electrons makes them more stable and less reactive than the compounds whose double bonds are isolated.

Figure 20.20. Comparison of benzene and 1,3,5-hexatriene

Antiaromatic compounds have 4n π electrons and have coninuous p orbitals, but delocalization of π electrons increases their energy, making them less stable. Thus they are destabilized by conjugation.

Figure 20.21. Comparison of cyclobutadiene and 1,3-butadiene​​

Nonaromatic conjugated rings include all compounds that do not satisfy all of Huckel’s rules. This may be due to them being non-planar, having an incorrect number of electrons, or not having a full cycle of p orbitals. Anything that prevents all of Huckel’s rules from being satisfied results in nonaromaticity.

Figure 20.22. Comparison of 1,3-cyclohexadiene and 2,4-hexadiene


20.4 Other Aromatic Compounds

While the majority of aromatic molecules are neutral hydrocarbons or substituted hydrocarbons, there are several other types of aromatic molecules that possess either charge or or heteroatom which may provide the necessary number of electrons to satisfy the Huckel's rule.

20.4.1 Ions 

According to Huckel’s rules, it is not necessary for all atoms in the aromatic system to be neutral. Both cations and anions can also be part of aromatic systems as long as the rules are satisfied. The most common aromatic ion is generated by deprotonation of cyclopenta-1,3-diene to form an anion. Typically the alkane pKa of C-H (alkane sp3 C-H) is ~50, but in the case of cyclopenta-1,3-diene, the sp3 C-H has a pKa of 15, and can be removed by a base like KOH. The reason is that the resulting carbanion will have a pair of electrons in an unhybridized p orbital which can participate in the ring aromaticity. In that case, all of Huckel’s rules are satisfied and the molecule has aromatic stability that the unstabilized anion does not have.  

The process is not limited to anions. Removing a leaving group from the 7 position of a molecule like cyclohepta-1,3,5-triene results in a cation, which is an unhybridized p orbital in a cycle of existing p orbitals. Because electrons leave with the leaving group, the π electron count is six electrons, which satisfies Huckel’s rules. This ion, called the tropylium ion, is one example of an aromatic cation.

Figure 20.23. a) Formation of cyclopentadienyl anion. b) Formation of cycloheptatrienyl cation. Also called a tropylium ion.

Watch the video to help you visualize the aromaticity of the cyclopentadienyl anion and the antiaromatic nature of the cation.


Q20.15 - Level 1

The reaction below would yield an aromatic ion. True or false?

question description
A

True

B

False


Q20.16 - Level 2

Which one of the following compounds is most acidic?

question description
A

1)

B

2)

C

3)

D

4)


20.4.2 Heterocycles

A number of cyclic structures also exist that demonstrate aromatic stability but incorporate non-carbon atoms or heteroatoms. Rings that incorporate oxygen, nitrogen or sulfur, but demonstrate aromatic stability, are often referred to as heterocyclic aromatic molecules or heterocycles.

Oxygen, nitrogen, and sulfur all differ from carbon structurally through the existence of lone pairs of electrons. These electrons may contribute to the delocalized π electrons or may remain localized on an atom, often in a hybrid orbital rather than an atomic p orbital. The most common heterocycles are either five- or six-member rings containing only one heteroatom. Common five-membered rings are furan, pyrrole, and thiophene (Figure 20.24).

Figure 20.24. Heteroaromatic molecules

If we revisit Huckel’s rules, the molecules are all cyclic. The carbons all have an unhybridized p orbital because they are all sp2 hybridized, but when it comes to the non-carbon atoms, the lone pairs can occupy either a hybrid orbital, sp2 or sp3, or an unhybridized p orbital. In order to satisfy the second of Huckel’s rules, the non-carbon atom cannot be sp3 hybridized, because that would consume the p orbital necessary for Huckel’s rules. Interestingly, in these molecules, the non-carbon atom also cannot usually be sp hybridized because the geometry of the ring does not allow for a linear atom. 

If we consider pyrrole then, the nitrogen must be sp2 hybridized, where the hydrogen is attached to the nitrogen through a hybrid sp2 orbital and the unhybridized p orbital contains the lone pair of electrons. This brings us to the last of Huckel’s rules, the π-electron count must equal 4n+2. In a pyrrole, there are four electrons in the carbon atom p orbitals and the two electrons in the nitrogen p orbital contribute to the π electron cloud, bringing the total number of electrons to six, which satisfies 4n+2. 

Figure 20.25. Delocalization of the heterotom lone pairs as part of the five membered rings aromaticity.

In pyridine, however, the nitrogen is participating in a double bond with one of the adjacent carbons, so the lone pair of electrons cannot be involved in the π system and are in a sp2 orbital, which is co-planar with the aromatic ring and at 90° to the p orbitals of the π system. In pyridine, the available lone pair makes the nitrogen reasonably basic. In pyrrole, by contrast, the nitrogen lone pair is delocalized through resonance and for this reason is not very basic.

Figure 20.26. Delocalization of electrons in pyridine

Imidazole is an example of a heterocyclic aromatic molecule that has both a basic and non-basic nitrogen. The formula of imidazole is C3H4N2, in which one nitrogen is single bonded while the other is double bonded to one carbon. As a result, one nitrogen has a hydrogen and a non-bonded pair, where the non-bonded pair is delocalized in the ring. In all heteroaromatic rings, both the carbon and the heteroatoms must have unhybridized p orbitals to contribute to the cycle of p orbitals in Huckel’s rule. 

Figure 20.27. Imidazole

Click below for a video explanation of orbital occupancy and the non-bonded pair.


Q20.17 - Level 1

Which statement is true about thiazole?

question description
A

Both lone pairs on sulfur contribute to the π -electron clouds and the lone pair on nitrogen does not

B

One lone pair on sulfur contributes to the π -electron clouds and the lone pair on nitrogen does not

C

One lone pair on sulfur and one lone pair on nitrogen contribute to the π electron cloud

D

The lone pair on nitrogen contributes to the π electron cloud and neither lone pair on sulfur contributes to the π electron cloud


Q20.18 - Level 1

Which atom in oxazole is contributing a lone pair of electrons to the π electron cloud for aromaticity?


20.4.3 Benzenoids

The last general category of aromatic compounds are compounds called polycyclic aromatic hydrocarbons, PAH’s, or benzenoids that have multiple fused rings that satisfy Huckel rules. The simplest PAH is naphthalene, which consists of two six-member rings sharing two carbon atoms. There is a p orbital on each carbon atom, providing a total of 10 π electrons that provide a resonance stability for naphthalene that is greater than that of bezene but not quite double the stability of benzene.

Figure 20.28. Polynuclear aromatic hydrocarbons

Not all bonds in polycyclic aromatic hydrocarbons are involved in the resonance delocalization of electrons. Some bonds are exclusively single bonds that function to preserve the planar nature of the system but are never themselves double bonds. Consider the resonance forms of azulene: all the bonds around the perimeter of the molecule are either single or double bonds depending on the resonance form. The single bond dividing the five and seven-member rings, however, is a single bond in both forms. In both resonance forms, there are 10 π electrons in the delocalized system.

Figure 20.29. Resonance structures of azulene​
Figure 20.30. The Deepwater Horizon oil rig burns in the Gulf of Mexico, April 2010.[3]​

Polynuclear aromatic hydrocarbons can be many rings in size, with three, four and five ring systems occurring commonly through combustion and also in crude oil. PAH’s tend to be quite carcinogenic and highly unreactive, leading to their role as dangerous environmental pollutants. During the 2010 Deepwater Horizon oil spill in the Gulf of Mexico, one of the persistent environmental pollutants were PAH’s, and their impact on the Gulf ecosystem persists many years later.


Q20.19 - Level 2

Which of the molecules below is a resonance structure of anthracene?

question description
A

a

B

b

C

c


Aromaticity & Electronics

Do you associate aromaticity with modern electronic devices? You should!

An important class of materials is called conductive polymers. They serve as electrical conductors or semiconductors due to aromaticity and conjugation.

Here are two examples and their applications in modern electronics. Conductive polymers can be synthesized through a few methods including cross-couplings, Wittig reactions, and condensation processes.

Figure 20.31. Photo of solar array [4], TV [5]

Below are the aromatic monomers of MEH-PPV, an organic photovoltaic material that can be synthesized through a Heck reaction. This catalytic process allows for the formation of a new C–C bond between the two fragments, with net loss of HBr.

The first step of the polymerization is shown below.

Figure 20.32


Keeping it Real Q20.1 - Level 1

Using the image above, predict the structure of conductive polymer MEH-PPV below (hint: look for conjugation!).

question description
A

a

B

b

C

c

20.5 Chapter Summary

Benzene is a cyclic conjugated aromatic compound, with a molecular formula of C6H6. It is made of six identical carbon-carbon bonds, each with a bond order of 1.5. Benzene is represented in a Lewis structure by six carbon atoms in a ring with alternating single and double bonds. Benzene and other aromatic molecules are unique because a resonance structure can be drawn where the double bonds are shifted, but none of the atoms end up with charges. Benzene can also be drawn with a circle inside  the six-member ring, indicating the resonance “flow” of electrons around all six atoms. 

20.07.png
Figure 20.33. Resonance structures of benzene

Monosubstituted benzene rings are benzene derivatives containing one group attached to the ring. These rings are named by using benzene as the root and the substituents as prefix.

20.03.png
Figure 20.34. Substituted benzene nomenclature

There are also many common name monosubstituted benzenes (Figure 20.4). These names often come from the organic matter from which the compound was isolated.

Disubstituted benzene rings are benzene derivatives that contain two groups directly bonded to the ring. These compounds are named by using either of these two methods: 

  1. The ring may be numbered with the highest priority substituent at the first carbon of the ring, and a numerical location given to the second group.
  2. Using the prefixes ortho (o - groups that are 1,2 relative to each other), meta (m - groups that are 1,3 relative to each other), and para (p - groups that are 1,4 relative to each other)
20.05.png
Figure 20.35. Systematic nomenclaure of disubstituted benzenes

A benzene ring may also be a substituent on a chain or bonded to another ring. The benzene ring is then treated as a substituent and is called a phenyl group,  which may be abbreviated as either, C6H5-or -Ph. A benzene ring with one carbon attached to it, is also commonly named as a substituent called a benzyl group (C6H5CH2-) .

Benzene is very stable: One of the unique aspects of benzene is its relative stability compared to other alkenes and cycloalkenes. For example, unlike an alkene, benzene  does not react with molecular bromine in addition reactions. 

Resonance energy: Benzene is 152 kJ/mol more stable than the hypothetical cyclohexa-1,3,5-triene molecule. This difference in energy is called resonance energy or delocalization energy. It is a measure of the extra stability due to conjugation, compared to the corresponding number of hypothetical isolated double bonds. (Figure 20.10)

The reason for the unusual stability  of benzene and other aromatic molecules  can be seen in the molecular orbital (MO) diagram of benzene and hexa-1,2,5-triene. All of the electrons in the occupied π bonding molecular orbitals are delocalized over several nuclei. This lowers the energy of the bonding molecular orbitals compared to the localized double bonds of the acyclic triene. This makes benzene more stable and less reactive.    

20.12.png
Figure 20.36. MO diagrams of benzene and hexa-1,3,5-triene

Huckle’s rule: Not all conjugated cyclic compounds can be classified as aromatic. For a compound to be classified as aromatic it must follow the following sets of rules devised by Erich Huckel, known as Huckle’s rules

  1. The structure must be cyclic
  2. Each atom of the ring has an unhybridized p orbital
  3. The unhybrid p orbitals form a continuous cycle around the ring, so the ring must be planar
  4. The number of electrons residing in the π electron cloud (either the π bonds they form or solely on an atom) is equal to 4n+2, where n is an integer (n=0,1,2…). 

If the last of Huckle’s rule is changed to 4n π electrons rather than 4n+2, the system is considered antiaromatic. Antiaromatic compounds are structures that adhere to all four of Huckle’s rules with the fourth rule modified to 4n π electrons, where n is an integer. Antiaromatic compounds are very unstable, and beca