Organic Chemistry: Concepts and Applications - Headley Allan D. 2020
Aromaticity and Aromatic Substitution Reactions
17.5 Characteristics of Aromatic Compounds
As mentioned earlier, benzene is just one of the many compounds of this very large category of compounds that are called aromatic compounds. Based on our knowledge of benzene, it is possible to compare certain characteristics of benzene with other molecules in order to determine if other molecules are aromatic or not. Let us now carry out a more detailed examination of benzene. An important observation about benzene is that it is cyclic and planar, i.e. flat. Thus, all aromatic compounds should be cyclic and flat. Since all the carbons that make up the aromatic system are sp2 hybridized, and we know that sp2 hybridized carbons are trigonal planar, the benzene molecule has to be flat. This concept is very important because the electrons in the unhybridized p orbitals can delocalize about the plane perpendicular to the atoms of the molecule.
Another very important characteristic of benzene and hence other aromatic molecules is the number of electrons that actually are in conjugation with each other. Note that if the electrons are not in conjugation, the system cannot be aromatic. For benzene, there are six electrons, and a very important question should come to mind: is it possible to have aromatic compounds with more or less than six electrons? This question was answered by Erich Hückel, a theoretical physicist, in that he examined numerous compounds in the early 1930s and showed that those molecules that have similar properties as benzene and hence aromatic compounds (neutral molecules as well as ions) all have 4n + 2 pi (π) electrons, where n is an integer, such as 0,1,2,3,4, etc. Thus, for benzene which has 6 pi (π) electrons, n is one. This method of determining if a system is aromatic or not based on the number of electrons of the aromatic system is known as the Hückel’s rule.
In summary, all aromatic compounds must meet the following criteria in order to be considered aromatic:
1. Must follow the Hückel’s Rule (4n + 2) electrons that are in conjugation. These are typically pi (π) electrons, but could include electrons from an adjacent heteroatom
2. The structure must be a cyclic conjugated system
3. Each carbon atom must have at least one unhybridized p orbital
4. There must be overlap of the “p” orbitals so that there is a continuous ring of electrons, i.e. conjugated.
5. The energy of the system must be lower than the hypothetical system in which the electrons are localized and not in resonance.
17.5.1 Carbocyclic Compounds and Ions
In this section, we will apply the observations and criteria listed above to different molecules and ions to determine if they are aromatic or not. Let us apply these requirements to the ions shown in Figure 17.13.
1,3,5 Cycloheptatriene cation has six pi (π) electrons, which are all in conjugation with each other. Note that all the carbons of the ring are sp2 hybridized and the carbon of the carbocation has no electrons, whereas the other orbitals each contain one electron in each p orbital. Thus, this cation contains six pi (π) electrons, which meets Hückel's rule and also meets the other characteristics outlined above for aromaticity. Thus, 1,3,5 cycloheptatriene cation is aromatic. The next ion shown in Figure 17.13 is the cyclopentadiene anion. For this anion, the carbons are also all sp2, and the caboanion carbon contains two electrons in the p orbital. Thus, the total pi (π) electrons are six, which again meet Hückel's rule. As a result, this anion is also aromatic. The next ion shown in Figure 17.13 is the cyclopentadiene cation. For this cation, the carbons are also all sp2, and the carbocation carbon contains no electrons in the p orbital. Thus, the total pi (π) electrons are four, which does not meet Hückel's rule requirement, since n equals half (½). As a result, this cation is not aromatic. Regarding the last ion is the cyclopropene cation, note that for this ion, the carbocation carbon has no electrons in the p orbital. Hence, the total number of pi (π) electrons is two (from the double bond). Two electrons also satisfy the Hückel's rule since the integer n for the Hückel equation is zero (0).
Figure 17.13 Carbocyclic-conjugated ions.
Problem 17.4
Determine which of the following molecules or ions is(are) aromatic? Briefly explain your answer.
17.5.2 Polycyclic Compounds
In this section, we will examine molecules that have more than one ring fused to form polycyclic molecules. Let us examine the three molecules shown in Figure 17.14 and determine if they are aromatic or not.
Napthalene, which is used as an insecticide and pest repellent, is flat since all the carbons are sp2 hybridized. It contains ten (10) pi (π) electrons that are in conjugation with each other. Based on the characteristics outlined above for aromaticity, naphthalene is aromatic. The second molecule is anthracene, which is used in the preservation of wood and for coating various materials. A similar analysis reveals that it is also flat and the electrons are in conjugation with each other. Equally important, there are 14 pi (π) electrons, which meets Hückel's rule requirement. Hence, anthracene is aromatic. The last molecule shown is phenanthrene, which is used to make dyes, plastics, and pesticides and also used in the pharmaceutical industry to make various drugs. Even though the three rings of this molecule are fused differently than that of anthracene, it meets the requirements to be aromatic and is an aromatic molecule.
Figure 17.14 Examples of polycyclic molecules.
Figure 17.15 Examples of heterocyclic compounds.
17.5.3 Heterocyclic Compounds
Heterocyclic compounds are compounds that are cyclic and contain at least one heteroatom in the ring. Examples of heterocyclic compounds are shown in Figure 17.15.
The first molecule, pyridine, is used in the synthesis of a host of different products, including food flavorings, paints, dyes, rubber products, and adhesives. Let us concentrate first on the heteroatom in pyridine. First, it is sp2 hybridized and it has a lone pair of electrons in one of the sigma orbitals and it also contains one electron in the p orbital, which is in conjugation with the electrons of the conjugated system. Note that the pair of electrons in the sigma orbital are orthogonal to the six electrons of the conjugated system, and hence is not considered in the analysis of aromaticity as shown in Figure 17.16.
There are six electrons that are in conjugation, which meet the Hückel's requirement for aromaticity; hence, pyridine is an aromatic molecule. The second molecule shown in Figure 17.15 is pyrrole, which is used as an intermediate in the synthesis of a host of pharmaceutical products and agrochemicals. Initial inspection of the heteroatom, nitrogen, in pyrrole would indicate that it is sp3 hybridized, with a lone pair of elections in a sp3 hybridized orbital. Since these electrons would prefer to be in conjugation with the four electrons of the conjugated two double bonds resulting in essentially six electrons that are in conjugation in the cyclic system which would be aromatic. Hence, pyrrole is an aromatic molecule as illustrated in Figure 17.17. Note that in order to have complete conjugation with all six electrons, the heteroatom rehybridizes.
The next molecule shown in Figure 17.15 is thiophene, which is also widely used as an intermediate in the agrochemical and pharmaceutical industries. Initial inspection of the sulfur in thiophene would indicate that it is sp3 hybridized like that of nitrogen in pyrrole. Thiophene has two nonbonded pairs of electrons and you can imagine, there is only one pair that can be in conjugation with the other four elections of the conjugated two double bonds. In order to have complete conjugation with all six electrons, the heteroatom rehybridizes as shown in Figure 17.18.
Figure 17.16 Model of pyridine showing p orbitals and the nature of the bonding involving the pi (π) electrons; note that the carbons and nitrogen are all sp2 hybridized.
Figure 17.17 Delocalization of electrons about the plane of pyrrole.
Figure 17.18 Delocalization of pi (π) and sigma electrons about the plane of thiophene.
Figure 17.19 Imidazole, a heterocyclic aromatic molecule.
The last molecule is imidazole, which is used as an important intermediate for the synthesis of various pharmaceuticals and agrichemicals. It has two heteroatoms, which must be analyzed separately. A similar analysis as that carried out for pyrrole shows that in order to have complete conjugation with all six electrons, the heteroatom at the right of moelcule rehybridizes. The other nitrogen atom is sp2 hybridized similar to that of pyridine. As a result, imidazole is an aromatic molecule as illustrated in Figure 17.19.
Problem 17.5
Determine which of the following molecules is (are) aromatic, note that the unshared electrons are not shown?
