RGS Academic Journal 2022 - 2023

020 021 Hückel’s Rule and the Peculiar Stability of Aromatic Compounds Tom O’Keeffe In the early days of organic chemistry, the surprising stability of benzene and similar compounds was a compelling issue. Hückel’s rule is a rule which predicts which organic compound will display aromatic properties. The rule is named after the German chemist and physicist Erich Hückel, who developed the rule during the period 1931- 1938. The displayed formula of benzene is shown in figure 1; however, it is more commonly represented by its skeletal formula depicted in figure 2. Although it was initially assumed that benzene had a structure as above (what would be called cyclohexa-1,3,5-triene), experimental data did not support this structure. For example, the enthalpy of hydrogenation for benzene was -208 kJ mol-1, which did not fit the predicted value of -360 kJ mol-1. Furthermore, the bond lengths between carbons in benzene are identical at approximately 139pm, roughly halfway between the C-C bond (154pm) and C=C (133pm). These disparities are due to the system of delocalised pi electrons in benzene’s structure. When benzene forms, one of the 2s2 electrons is promoted into the empty 2pz orbital (fig. 3). Each carbon is required to bond to three other atoms, so the 2s, 2px and 2py orbitals are hybridised to form sp2 hybrids (fig. 4). The 2pz electron is unaffected. Each carbon atom uses the three sp2 hybrids to form three sigma bonds with two other carbons and one hydrogen atom. The remaining p electrons on each carbon overlap, producing a system of pi bonds; these electrons are spread out over the whole ring. This delocalised system means that there are not in fact alternating single and double bonds in the benzene ring (unlike the expected cyclohexa-1,3,5-triene or Kekule structure) and it is this spread of electrons which is responsible for benzene’s surprising stability. Hückel’s rule can be used to determine which compounds will show similar aromatic properties (notably relative stability) and applies to fully conjugated, monocyclic, planar molecules. A fully conjugated compound is a compound in which there is a continuous array of p orbitals along the whole system, which can align to form a system of overlapping pi bonds. A planar monocyclic compound is a molecule comprising of a single ring of atoms, in which the p orbitals are roughly parallel (although parallel p orbitals is already a requirement for conjugation). Hückel’s rule states: if a compound meets the aforementioned criteria and contains 4n+2 pi electrons, it will have aromatic properties. Therefore fully conjugated, monocyclic, planar compounds with 6 pi electrons, such as benzene, will demonstrate aromatic properties. Some ions, such as the cyclopentadienyl anion (fig. 5), and compounds with elements other than carbon in the ring such as furan (fig. 6), can also be aromatic. Figure 3 Figure 5 Figure 6 Figure 4 RGSHW 2022/23 Academic Journal Hück l’s Rule and the Peculiar Stability of Aromatic Compounds - Tom O’Keeffe In the early days of organic chemistry, the surprising stability of benzene and similar compounds was a compelling issue. Hückel’s rule is a rule which predicts which organic compound will display aromatic properties. The rule is named after the German chemist and physicist Erich Hückel, who developed the rule during the period 1931-1938. The displayed formula of benzene is shown in figure 1; however, it is more commonly represented by its skelet l formula depicted in figure 2. Figure 1 Figure 2 Although it was initially assumed that benzene had a structure as above (what would be call d cyclohexa-1,3,5-triene), experimental data did not support this structure. For example, the enthalpy of hydrogenation for benzene was -208 kJ mol -1 , which did not fit the predicted value of -360 kJ mol -1 . Furthermore, the bond lengths between carbons in benzene are identical at approximately 139pm, roughly halfway between the C-C bond (154pm) and C=C (133pm). These disparities are due to the system of delocalised pi el ctrons in b nzen ’s struc ure. When benzene forms, one of the 2s 2 electrons is promoted into the empty 2p z orbital (fig. 3). Each carbon is required to bond to three other atoms, so the 2s, 2p x and 2p y orbitals are hybridised to form sp 2 hybrids (fig. 4). The 2p z electron is unaffected. Each carbon atom uses the three sp 2 hybrids to form three sigma bonds with two other carbons and one hydrogen atom. The remaining p electrons n each carbon overlap, producing a syst m of pi bonds; thes el ctrons are spread out over the whole ring. Figure 3 Figure 4 C C C C C C H H H H H H Figure 1 Figure 2