5 CHAPTER Concepts of Aromaticity, Benzene and its Derivatives ~ A-Concepts of Aromaticity — 9A.1_ INTRODUCTION In earlier days of organic chemistry, the term ‘Aromatic’ was used for compounds associated ‘with certain aroma or fragrance. Benzene is the representative member of the class of aromatic organic compounds which was isolated in 1825 by Michael Faraday. The term aromatic has been widely used for benzene and its derivatives since many of them are associated with a distinct odour. However, there are other organic compounds which are known to be associated with some odour but are not classified as aromatic. Thus, the classification of organic compounds as aromatic’is not only based on aroma but on structure and reactivity of the compounds. To study the, characteristics of aromatic compounds and the criteria for aromaticity, let us:first study the structure of benzene. 9A.2 STRUCTURE OF BENZENE Benzene has a molecular formula CH that suggests a high degree of unsaturation, However, it does not undergo the usual reactions of unsaturated compounds such as addition, oxidation, and reduction. It does not decolourize bromine water.or potassium permanganate solution, which is the characteristic ‘of unsaturated compounds. In fact, benzene undergoes substitutions reactions. Reaction of benzene with bromine in presence of ferric bromide catalyst results in the formation of only one compound i.e. CeHsBr. This suggests that-all hydrogens in benzene are equivalent. This unusual behaviour of benzene is attributed to its structure. 294Pee eee Concepts of Aromaticity, gA.2.1. Kekule Structure In 1865, Kekule proposed a six membered ring structure for benzene where each carbon is bitched to one hydrogen atom. To satisfy the {etra-valency of carbon, he further Proposed the presence of three alternate double bonds in benzene ting, Presence’ a ds i which change their postion rapi i two forms of benzene exist in equilibrium and canner es ilies Meee ee Benzene and its Derivatives 295 q H H H H H = H H H H H H a a Kekule structures of benzene + ((U) and (1) are in equilibrium) Although Kekule structure satisfied the structural features of benzene and also explained the equivalent nature of hydrogen, it was not able to account for the unusual behaviour of benzene. However, the Kekule structure of benzene was a giant step forward and for this reason, the structure is still used but at present, the explanation for the structure and reactivity of benzene is given in an entirely different manner, 9A.2.2 Resonance Structure The resonance theory gave the correct description for the, structure of benzene. It states that Whenever a molecule is represented by two or more Lewis strictures (known as contributing structures) any one of the structure alone is not able to explain the characteristics of the molecule on the whole. The actual structure in such cases is a hybrid of all the contributing structures. These contributing structures (or resonance forms) are hypothetical and cannot be isolated, These are Tepresented by placing a double-headed arrow between them and all such structures are said to be in resonance with each other (Note that contributing structures are never in equilibrium as these are not real structures). Thus, for benzené major contributing structures are written as follows: —- oO Contributing structures for benzene The actual hybrid structure has lower energy compared to either of the contributing structures alone. This decrease in energy of the hybrid structure stabilizes the molecule and is known as resonance energy. The resonance energy is also termed as delocalization energy (see Notabilia 2). Benzene has high resonance energy, In general, all aromatic compounds show high resonance energy. 9A.2.3 Orbital Picture of Benzene Experimental evidence indicate the molecule of benzene to be Planar with all the six carbon-carbon bond lengths being same. The bond length value in benzene is in between that of a single and the296 Organic Chemistry : jour of benzene in an a orbital picture of benzene explains the behaviour ee A eatieale two of ae The ac carbons in bonaene are op hybridized. Each carbon bas three hyjbrid vol hybrid orbinct aoe vse in the formation of enrbon-carbon o bonds on either sides and one of th © hybrid oat oii the foraton of a carbon ycrogen © bond. This i A erleadh taibca inane aiacked ta cach other through o bonds in a cyclic maner and furth ach catbon is atached ate a Bey carbon also has a pure p orbital with so pisanmea: there are six p orbitals (six unpaired electrons) preset in te same plane ee eee abe ng : ix p orbit lap sideways to foi ' ay Da ureee the six electrons are shared equally among all the six carbons and thig is known as delocalization of m electrons. Lower lobe ) Fig. 9.1, Molecular orbital model of bonding in benzene. (a) C-C sigma bonds are a result of. sp’-sp? orbital overlap, C-H sigma bonds are a result of sp?-1s orbital overlap. The six 2p orbitals, each containing ‘one electron also combine (b) -cloud formation due to six 2p orbital overlap. It is evident that double bonds are not localized between any two. carbons rather, there is a Continuous delocalization of electrons. This delocalization causes all the six carbon-carbon bond lengths to be same (1.39 A). This value lies in between a pure C-C bond length 1.54 A and C-C bond length (1.33 A). Benzene is thus, represented as a hexagon with a circle inscribed, where the circle represents delocalized 7 electrons, Mig. 92 Structure of benzene and its representation,Concepts of Aromaticity, Benzene and its Derivatives 297 9A3 RESONANCE ENERGY: STABILITY OF BENZENE simentally, the heat of hydrogenation of benzene is found to be -208 kJ/mol. We can calculate X of hydrogenation of benzene from Kekule structure. This is done by taking into consideration heat eat of hydrogenation of cyclohexene. Table 9A.1 tabulates the actual and calculated values eter of hydrogenation of cyclohexene, cyclohexa-1,4-diene, and cyclohexa-1,3,5-triene. The © ‘ual heat of hydrogenation of cyclohexene is 119.6 kJ mol, for diene and triene system it is ejculated by following formula: Calculated Heat of hydrogenation = Number of z-bonds x Heat of hydrogenation of cyclohexene (-119.6 kJ mol) Table 9A.1 Calculating the heat of hydrogenation of benzene Mobeie Number of | Hydrogenation | Heat of hydrogenation [kJ moI"] bonds product Actual Calculated gO 1 CI -119.6 Cyclohexene Cyclohexane 3 2x (119.6) 7 noes =-239.2 Cyclohexa-1,4-diene oe 3x (-119.6) 8 =-358.8 Hypothetical Calculated for cyclohexa-1,3,5-triene for hypothetical Benzene | cyciohexa-1,3,S-triene For benzene, the calculated value is -358.8 kJ mol"! while the experimentally observed. value. is -208 kJ mol"!. Thus, benzene is stabilized due to lowering of energy by 150.8 KI mol (ic. 358.8-208). This difference of energy between the observed heat of hydrogenation for actual molecule and the one calculated from the hypothetical structure is known as resonance energy (or delocalization energy). Thus, benzene has a resonance energy of 150.8 kJ mol", Characteristics of Benzene The discussion above explains the unusual characteristics of benzene, which is also known as its aromatic character. Benzene is a cyclic planar molecule. All the carbons are sp* hybridized and the p orbital available on each carbon contributes towards continuous delocalization of 7 electrons. This Provides stability to benzene and imparts it high resonance energy. To retain the resonance energy and thereby the stability, benzene undergoes substitution reactions. As a result, benzene does not behave as an unsaturated system and does not undergo addition reactions,298 Organic Chemistry ICITY 94.4 HUCKEL’S RULE AND AROMAT! : yunds, which states that the cyclic co for monocyclic compo! : He mpo1 In 1931, Huckel gave ae electrons exhibit aromatic character bres % can be zerg me containing (4n + 2) delocalize the value of (47 + 2) is 2-an Eh the ns 2) 55 ¢ for n= 0 . ene at ut containing 2, 6, 10, 14, 18, 22, and so on, delocalized Telectrons cand so on, This show aromatic behaviour. ‘A compound is said to exhibit aromaticity if it satisfies all of the following conditions simultaneously: i) It is cyclic. fe : : a i : aie In general presence of sp” hybridized carbons in the system imparts planarity. : ‘ i (iii) It exhibits continuous delocalization of electrons (continuous delocalization is possible if p orbitals are available on each carbon for overlap). (iv) It should follow Huckel’s rule, also known as Huckel magic number, that is, it should have (4n + 2) number of delocalized x electrons where 'n is zero or a whole number. 9A.5 AROMATICITY IN BENZENE AND OTHER CYCLIC SYSTEMS Aromaticity is not restricted to benzene and its derivatives but is also extended to other cyclic systems, cyclic ions, fused cyclic systems, and heterocyclic compounds. Following examples discuss the concept of aromaticity in different systems. 9A.5.1 Aromaticity and the Three Membered Ring Systems Cyclopropene Characteristics of cyclopropene molecule are as follows: (Ibis cyclic Gi) Itis planar (ii) It does not exhibit continuous delocalization: One of the carbon in eycic system is sp hybridized and thus, p oxbital 22S . is pot pride on that carbon for delocalization, z fv) Obeys Huckel’s rule, ized 7 Oreys ©, as the number of delocalized 7 electrons Thus, cyclopropene is not aromatic as a . «fied, The condition of continuous delocalization eae are not simultaneously satisfied. wed.