Organic Chemistry: Concepts and Applications - Headley Allan D. 2020
Aromaticity and Aromatic Substitution Reactions
17.2 Structure and Properties of Benzene
It was shown that the chemical formula of benzene is C6H6. Based on our knowledge of the number of hydrogens that are bonded to the carbons of saturated hydrocarbons, it is obvious that benzene is not a saturated hydrocarbon. In 1866, Friedrich Kekulé proposed that benzene had a cyclic structure with three alternating double bonds, as shown in Figure 17.1.
If this representation of benzene were correct, there should be another isomer in which the double bonds are in different locations. Such possible isomers of benzene are more obvious for 1,2-dichlorobenzene, which are shown below.
In the first structure, there is a double bond between the carbons that have the chlorine atoms, and in the other structure, there is a single bond between the same two carbons. It was known then that 1,2-dichlorobenzene has no isomers. To explain this discrepancy, Kekulé incorrectly proposed that a very rapid and undetectable equilibrium exists between both isomers, as shown in Reaction (17-1).
Figure 17.1 Kekulé's structure of benzene.
(17-1)
It was later shown by X-ray spectroscopy and other experimental analysis that all the carbon─carbon bond lengths of benzene are equal and that they are 1.387 Å. This bond length of 1.387 Å is between the lengths of a single carbon─carbon bond (1.48 Å) and an isolated carbon─carbon double bond (1.34 Å). Thus, the structures proposed by Kekulé for benzene, and shown in Figure 17.1 in which there is a rapid equilibrium between isomers were incorrect.
The model that is presently used to represent benzene is one that describes benzene as a flat molecule and one that has six carbon atoms, which are all sp2 hybridized, and in each of the unhybridized p orbitals, there is one electron, and each carbon atom is bonded to one hydrogen atom. This representation is shown in Figure 17.2.
The six electrons are delocalized throughout the entire framework of the six carbon atoms, and based on this model, all the carbon─carbon bond lengths are equal. Thus, it is technically incorrect to designate any one carbon─carbon bond of benzene as a double bond or as a single bond. However, it is difficult to represent the carbon─carbon bond as a bond and a half, so throughout the years, it has become acceptable to draw benzene as either of the structures shown in Figure 17.3.
Note that the first two structures are exactly the same as those proposed by Kekulé; however, either one is used in today's scientific literature to represent benzene only because they are less cumbersome representations of the very complex picture of the benzene molecule shown in Figure 17.2. Figure 17.4 shows the structures of some common aromatic compounds.
Other compounds that contain the benzene ring system are shown in Figure 17.5. Norepinephrine is used to treat low blood pressure; aspirin (acetylsalicylic acid) is an analgesic; vitamin E is an antioxidant that occurs in foods such as leafy green vegetables, nuts and seeds; atorvastatin is the active ingredients in the lipid-lowering drug, lipotor; and citalopram is a commonly used antidepressant.
Figure 17.2 Model of benzene showing p orbitals and the nature of the bonding involving the pi (π) electrons and all sp2 hybridized carbons.
Figure 17.3 Common and acceptable representations of benzene.
Figure 17.4 Familiar pleasant-smelling aromatic compounds, which all contain the benzene ring system.
Figure 17.5 Commonly used compounds that contain the benzene ring system.
Problem 17.1
Identify the benzene ring aromatic system in the examples given in Figures 17.4 and 17.5.