ABSTRACT

Aromaticity has been deeply rooted in chemical literature, even before the structural and bonding principles have been clearly established [1-6]. It has been one of the most ubiquitous concepts and its origin dates back to the isolation of benzene by Faraday in 1825 illuminating the gas from whale oil [1]. Historically, the molecule benzene is closely associated with aromaticity. The rst known use of the word “aromatic” as a chemical term is by Hofmann in 1855 [2]. However, even before that in 1833, Mitscherlich distilled benzene from benzoic acid and lime [3]. The cyclohexatriene structure for benzene was rst proposed by Kekulé in 1865 [4] while the cyclic nature of benzene was nally conrmed by the crystallographer Kathleen Lonsdale [5]. The term aromaticity owes its name to the pleasant aroma that some members of this class have and later on it was denoting exceptional stability that this family of compounds exhibit. An explanation for the exceptional stability of benzene is attributed to Sir Robert Robinson, who coined the term aromatic sextet

2.1 Introduction .................................................................................................... 31 2.2 On the Nature of π Electrons of Benzene ....................................................... 33 2.3 Measures of Aromaticity ................................................................................ 35

2.3.1 Structural ............................................................................................ 35 2.3.2 Energetic ............................................................................................. 37 2.3.3 Magnetic ............................................................................................. 41 2.3.4 Reactivity ............................................................................................ 42 2.3.5 Other Measures ................................................................................... 43

2.3.5.1 Atoms in Molecules Theory ................................................ 43 2.3.5.2 Generalized Polansky Index ................................................44 2.3.5.3 Polarizability ........................................................................ 45

2.4 Bottlenecks in Quantifying Aromaticity ........................................................46 2.5 Summary ........................................................................................................ 47 Acknowledgments ....................................................................................................48 References ................................................................................................................48