HETEROCYCLIC AROMATIC COMPOUDS

Heterocyclic Aromatic Compounds

A heterocyclic compound is an organic compound in which one or more of the carbon atoms in the backbone of the molecule has been replaced by an atom other than carbon. Typical hetero atoms include nitrogen, oxygen, and sulfur.

Pyridine (C ₅H ₅N), pyrrole (C ₄H ₅N), furan (C ₄H ₄O), and thiophene (C ₄H ₄S) are examples of heteroaromatic compounds.

                                                                                                                                            

Because these compounds are monocyclic aromatic compounds, they must obey Hückel's Rule. Hückel's Rule requires 4 n + 2 π electrons, so the simplest aromatic compound should contain 6 π electrons ( n = 1). Pyrrole, furan, and thiophene appear, however, to have only 4 π electrons (2 π bonds). In systems such as these, the extra electrons needed to produce an aromatic condition come from the unshared electron pairs in sp ² hybrid orbitals around the hetero atom.

SOME OTHER AROMATIC COMPOUNDS

Other Aromatic Compounds

Many other compounds also exhibit aromatic characteristics. Some of the most common have two or more benzene rings fused together. Such compounds are called polycyclic benzenoid aromatic compounds. A typical example of this type of molecule is naphthalene, C ₁₀H ₈. Structurally, naphthalene looks like this:

This structure is a hybrid of the following four resonance structures.

The resonance energy associated with naphthalene is 61 kcal/mole. Thus naphthalene is more stable than benzene. The benzenoid or benzene‐like ring should have 36 kcal per mole of resonance energy, and the nonbenzenoid ring should have 25 kcal per mole of resonance energy. Because there is total delocalization of the electrons in the system, the benzenoid and nonbenzenoid rings cannot be identified at any given time.

Other benzenoid structures include anthracene, phenanthrene, and pyrene.

HUCKEL'S RULE

Hückel's Rule

In 1931, Erich Hückel postulated that monocyclic (single ring) planar compounds that contained carbon atoms with unhybridized atomic p orbitals would possess a closed bond shell of delocalized π electrons if the number of π electrons in the molecule fit a value of 4 n + 2 where n equaled any whole number. Because a closed bond shell of π electrons defines an aromatic system, you can use Hückel's Rule to predict the aromaticity of a compound. For example, the benzene molecule, which has 3 π bonds or 6 π electrons, is aromatic.

• Number of π electrons = 4 n + 2
• 6 = 4 n + 2

• n = 1

However, 1,3,5,7‐cyclooctatetraene, which has 4 π bonds or 8 π electrons, is not only nonaromatic but is actually considered antiaromatic because it is even less stable than the open‐chain hexatriene.

• Number of π electrons = 4 n + 2

• 8 = 4 n + 2

• n = 1.5

Nomenclature

In IUPAC nomenclature, benzene is designated as a parent name. Other compounds that contain the benzene molecule may be considered as substituted benzenes. In the case of monosubstitution (the replacement of a single hydrogen), the prefix of the substituent is added to the name benzene.

In other cases, the substituent, along with the benzene ring, forms a new parent system.

When a benzene molecule is disubstituted (two hydrogens are replaced), two nomenclature methods exist. Either a number system or name system indicates the relative position of one substituent to the other. In the number system, one substituent is given the number one position and the second substituent is assigned the lower possible second number. The number position is given to the atom or group that has the higher priority as determined by the Cahn‐Ingold‐Prelog nomenclature system rules.

Notice that in the previous examples, the atom of the higher atomic weight is given the higher priority (Br = 79.1 versus Cl = 15.5, and I = 126.0 versus N = 14.0). These assignments are based on the priority rules of Cahn‐Ingold‐Prelog nomenclature.

In the name system, one carbon atom containing a substituent is considered to be the initial (locator) position. The carbon atom bonded to the other substituent is then located by the number of carbon atoms separating it from the locator position, as shown in Figure 1.

The ortho position is one removed from the initial substituent's position. The meta position is two removed, and the para is three removed.

Unlike the number system, you can assign an equally correct name with the names of the substituents reversed.

Benzene compounds that contain three or more substituents are always named by the number system. In this system, numbers are assigned to substituents so that the substituents have the lowest possible combination of numbers.

Reactions of benzene

Although the resonance structures of benzene show it as a cyclohexatriene, because of its fully delocalized π system and the closed shell nature of this π system, benzene does not undergo addition reactions like ordinary unsaturated compounds. The destruction of the π electron system during addition reactions would make the products less stable than the starting benzene molecule. However, benzene does undergo substitution reactions in which the fully delocalized closed π electron system remains intact. For example, benzene may be reacted with a halogen in the presence of a Lewis acid (a compound capable of accepting an electron pair) to form a molecule of halobenzene.

BENZENE

Benzene

In 1834, Eilhardt Mitscherlich conducted vapor density measurements on benzene. Based on data from these experiments, he determined the molecular formula of benzene to be C ₆H ₆. This formula suggested that the benzene molecule should possess four modes of unsaturation because the saturated alkane with six carbon atoms would have a formula of C ₆H ₁₄. These unsaturations could exist as double bonds, a ring formation, or a combination of both.

Structure of the benzene molecule

In 1866, August Kekulé used the principles of structural theory to postulate a structure for the benzene molecule. Kekulé based his postulation on the following premises:

• The molecular formula for benzene is C ₆H ₆.

• All the carbons have four bonds as predicted by structural theory.

• All the hydrogens are equivalent, meaning they are indistinguishable from each other.

Based on these assumptions, Kekulé postulated a structure that had six carbons forming a ring structure. The remaining three modes of unsaturation were the result of three double bonds alternating with three single bonds. This arrangement allowed all the carbon atoms to have four bonds as required by structural theory.

Scientists soon realized that if Kekulé's structure were correct, substituting substituent groups for hydrogens on the 1,2 positions would lead to a different compound than substitution on the 1,6 positions.

Because no such isomers could be produced experimentally, Kekulé was forced to modify his proposed structure. Kekulé theorized that two structures existed that differed only in the location of the double bonds. These two structures rapidly interconverted to each other by bond movement.

Although Kekulé's structure accounted for the modes of unsaturation in benzene, it did not account for benzene's reactivity.

Resonance

Modern instrumental studies confirm earlier experimental data that all the bonds in benzene are of equal length, approximately 1.40 pm. (A picometer equals 1 × 10 ⁻¹² meter.) This bond length falls exactly halfway between the length of a carbon‐carbon single bond (1.46 pm) and a carbon‐carbon double bond (1.34 pm). In addition, these studies confirm that all bond angles are equal (120°) and that the benzene molecule has a planar (flat) structure.

Modern descriptions of the benzene structure combine resonance theory with molecular orbital theory.

Resonance theory postulates that when more than one structure can be drawn for the same molecule, none of the drawn structures is the correct structure. The true structure is a hybrid of all the drawn structures and is more stable than any of them. The greater the number of structures that can be drawn for a molecule, the more stable the hybrid structure will be. The difference between the calculated energy for a drawn structure and the actual energy of the hybrid structure is called the resonance energy. The greater the resonance energy of a compound, the more stable the compound.

The two Kekulé structures that can be drawn for the benzene molecule are actually two resonance structures.

The hybrid of these structures would be drawn as

  

where the circle represents the movement of the electrons throughout the entire molecule. This delocalization of π electrons (electrons found in π molecular orbitals) is also found in conjugated diene systems. Like benzene, the conjugated diene systems show increased stability.

Because of resonance, the benzene molecule is more stable than its 1,3,5‐cyclohexatriene structure suggests. This extra stability (36 kcal/mole) is referred to as its resonance energy.

Orbital picture of benzene

Because experimental data shows that the benzene molecule is planar, that all carbon atoms bond to three other atoms, and that all bond angles are 120°, the benzene molecule must possess sp ² hybridization. With sp ² hybridization, each carbon atom has an unhybridized atomic p orbital associated with it. The overlap of the sp ² hybrid orbitals would create the σ bonds that hold the ring together, while the side‐to‐side overlap of the atomic p orbitals can occur in both directions, leading to complete delocalization in the π system. This complete delocalization adds great stability to the molecule. Figure 1 illustrates this idea.

Molecular orbital theory predicts that overlapping six atomic p orbitals will lead to the generation of six π molecular orbitals. Three of these π molecular orbitals will be bonding orbitals, while the other three will be antibonding orbitals, as shown in Figure 2

The three low‐energy orbitals, denoted π ₁, π ₂, and π ₃, are bonding combinations, and the three high‐energy orbitals, denoted π ₄ ^*, π ₅ ^*, and π ₆ ^*, are antibonding orbitals. Two of the bonding orbitals (π ₂ and π ₃) have the same energy, as do the antibonding orbitals π ₄ and π ₅. Such orbitals are said to be degenerate.

Because the electrons are all located in bonding orbitals, the molecule is very stable. Additional stability occurs because all the bonding orbitals are filled and all the π electrons have paired spins. Molecules that possess all these characteristics are said to have a closed bond shell of delocalized π electrons. Molecules such as benzene that possess a closed bond shell of delocalized π electrons are extremely stable and show great resonance energies.