AROMATIC HYDROCARBONS: Part I DR SUNIL K. SINGH Assistant Professor in Chemistry SK SINGH, KMC Concept of Aromaticity Aromatic compounds apparently contain alternate double and single bonds in a cyclic structure and resemble benzene in chemical behaviour. They undergo substitution rather than addition reactions. This characteristic behaviour is known as aromaticity. Addition reaction: Substitution reaction: Br Br 2 H E CCl + 4 Br E + H+ Br2 No Reaction SK SINGH, KMC CCl4 Following are the main criteria of aromaticity: Chemical behaviour: electrophilic aromatic substitution. Structural: bond length equalization due to cyclic delocalization of electrons. C-C bond length in benzene id 139 pm. SK SINGH, KMC Contd……………….. Enhanced stability (large resonance energy). Heat of hydrogenation of Benzene is 49.8 Kcal/mole. Heat of hydrogenation of 1,3,5- hexatriene is 85.8 Kcal/ mole. Smaller the heat of hydrogenation Magnetic: "ring current" effects. more is the stability. Benzene is ~36 Kcal/mole more stable than the 1,3,5- • anomalous chemical shifts in NMR. hexatriene. • large magnetic anisotropies • high diamagnetic susceptibility. SK SINGH, KMC Huckel’s Rule and aromaticity A molecule is aromatic if all the following conditions are fulfilled: It is cyclic, planar and it has continuous delocalization of p electrons (electrons in p orbitals) with or without the participation of lone pair(s)/ -ve charge/ +ve charge. Huckel’s rule: The delocalised p-electron system must contain a total of (4n+2)p electrons, where n is a whole number (i.e., n = 0,1,2,3,………). Note that the p-orbitals in which the electrons are delocalizing must be parallel to each other so that a continuous overlap of electrons is possible along the ring. SK SINGH, KMC Contd…………………………….. So whenever a cyclic, planar and (4n+2)p electrons are continuously delocalized, it leads to the extra stability of the molecule referred to as aromatic compound. Whenever a cyclic, planar and (4n)p electrons are continuously delocalized, it leads to the extra destability of the molecule referred to as anti-aromatic compound. When either the molecule is not cyclic or is not planar or the continuous delocalization of p electrons is not there, the molecule is aliphatic or non- aromatic compound. Stability Order: Aromatic > NonaromaticSK SINGH, KMC / Aliphatic > Antiaromatic In other words Condition for Aromatic Anti- Non- Aromaticity Compounds aromatic aromatic Compounds Compounds 1. Cyclic, planar Fulfill the first Fulfill the first Not fulfilled and continuous condition condition delocalization of p electrons 2. (4n+2)p (4n+2)p (4n)p No need to electrons electrons electrons look for this condition if 1st (Huckel’s rule) [(4n+2)p electrons condition is condition not not fulfilled fulfilled] SK SINGH, KMC Construction of MO diagram for conjugated cyclic polyenes Frost Diagram (Polygon Method): A circle is inscribed with a polygon with one vertex down; The Vertex represent the energy levels (p molecular orbital levels); The semicircle line shows the nonbonding level. p * * * 4 p3 p2 n n ……………………………………..p2 ……………………………………..p3 …………………………………….. p 1 p1 …………………………………….. …………………………………….. …………………………………….. SK SINGH, KMC p-Molecular orbital level diagram of benzene: Points to remember: Aromaticity is observed when all bonding MO’s are filled and nonbonding MOn’s, if present, are empty of completely filled. Antiaromaticity is observed if it has a electron in antibonding molecular orbital or it has half-filled bonding molecular orbital(s) / nonbonding molecular orbital(s), provided it satisfies the 1st condition of aromaticity. SK SINGH, KMC Examples: Three membered cyclic species Practice Problems Nonaromatic Aromatic Antiaromatic Antiaromatic (Unstable) Cyclopropene, Cyclopropenyl cation, Cyclopropenyl anion Nonaromatic More acidic proton Pracice Problems: O O O Cyclopropenone is relatively more stable than anticipated. Explain. Cyclopropenone has larger dipole moment than cyclopropanone. Why ? I II Resonance Hybrid (II contribute more to the resonance hybrid) as cyclopropenyl cation is aromatic in SK SINGH,nat uKMCre, which provide it extra stability. Explanation: Aromatic Antiaromatic SK SINGH, KMC Four membered cyclic species Antiaromatic Aromatic Aromatic Cyclobutadiene, Cyclobutadiene dication, Cyclobutadiene dianion 2+2+ 2-2+ 2+ 2- C4H4 C4H4 SK SINGH, KMC Explanation: SK SINGH, KMC AROMATIC HYDROCARBONS: Part II DR SUNIL K. SINGH Assistant Professor in Chemistry SK SINGH, KMC Revision………………………………………. Condition for Aromatic Anti- Non- Aromaticity Compounds aromatic aromatic Compounds Compounds 1. Cyclic, planar Fulfill the first Fulfill the first Not fulfilled and continuous condition condition delocalization of p electrons 2. (4n+2)p (4n+2)p (4n)p No need to electrons electrons electrons look for this condition if 1st (Hückel’s rule) [(4n+2)p electrons condition is condition not not fulfilled fulfilled] SK SINGH, KMC Revision………………………………………. Points to remember: Aromaticity is observed when all bonding MO’s are filled and nonbonding MOn’s, if present, are empty of completely filled. Antiaromaticity is observed if it has a electron in antibonding molecular orbital or it has half-filled bonding molecular orbital(s) / nonbonding molecular orbital(s), provided it satisfies the 1st condition of aromaticity. SK SINGH, KMC Five membered cyclic compounds / species Cyclopentadiene Cyclopentadienyl cation Cyclopentadienyl anion (Nonaromatic) (Antiaromatic) (Aromatic) Practice Problems: 1. Cyclopentadienyl anion is much more stable than allyl anion. 2. (a). Use (i) resonance theory and (ii) MO theory to predict whether 5-bromo-1,3-cyclopentadiene readily undergo CH2 solvolysis in water. (b). Show that cyclopentadienyl cation is a diradical. Aromatic Compouns Nonaromatic Compound 3. Write the structure of the smallest aromatic compound. 1. Cyclic, Planar, delocalized p-electrons 2. Follow's Huckel's Rule SK SINGH, KMC MO Diagram: Practice problem: Cyclopentadiene is more acidic than + H+ ethane . H H pKa = 15 1. Cyclic, Planar, delocalized p-electrons 2. Follow's Huckel's Rule (Aromatic Compound) + H3C CH2 + H H3C CH3 Unstable pKa = 50 SK SINGH, KMC Six membered ring compound: Benzene or ([6]-annulene) 1. It is cyclic, planar and p-electons are deloclised. 2. It follow’s Hückel’s rule. 4n+2 = 6; n = 1 (an integer) SK SINGH, KMC Seven membered ring compounds / species Cycloheptatriene Cycloheptatrienyl cation Cycloheptatrienyl anion (Tropylium cation) (Nonaromatic) (Aromatic) (Antiaromatic) + fast Practice Problem: Solution: Br + Ag+ + AgBr (ppt) Tropylium bromide (7-bromocycloheptatriene) completely dissociates in water and give ppt of Tropylium cation Aromatic in nature (More Stable) AgBr instantaneously with AgNO3, unlike the open chain analog, 3-bromo-1,4-pentadiene. + slow Explain. Br + Ag + AgBr (ppt) SK SINGH, KMC Nonaromatic (Less stable) MO Diagram: Tropone Tropolone O O OH OH Tropylium cation Aromatic SK SINGH, KMC Aromatic Solution: Practice Problem: Which of the following compounds has the greater O O dipole moment ? O O C I II I More Stable Charge is separated in this resonating structure, three membered ring becomes aromatic. O O C C II Charge is separated in this resonating structure, but does not have any additional aromaticity. SK SINGH, KMC Heterocyclic compounds N H O S N Pyrrole Furan Thiophene Pyridine Aromatic Aromatic Aromatic Aromatic lone-pair of electron is in p- orbital, involved to complete the aromaticity sextet. - 6p e ’s Sp2-Hybridised 6p e-’s Not the part of p-cloud 4n+2 = 6; n = 1 SK SINGH, KMC Lone pair of e-’s is in 3p-orbital, part of the p-electron cloud. 6p e-’s SK SINGH, KMC SK SINGH, KMC Other Heterocycles N 1. 1. It is cyclic, planar and have an fully conjugated Resonating structures of Imidazole cyclic π electron cloud. N 2. It follows Hückel‘s rule. H Contain 6 p e’s. Imidazole 4n + 2 = 6; n =1 (integer) Aromatic Lone pair of electron is part of p-electron cloud Perpendicular to p-orbital SK SINGH, KMC Other Heterocycles 1. 1. It is cyclic, planar and have an fully conjugated cyclic π electron cloud. 2. It follows Hückels rule. Contain 6 p e’s. 4n+2 =6; n=1 Oxazole Aromatic Compound Practice Problem: Isoxazole (I) is aromatic or not ? Explain. I SK SINGH, KMC Cyclooctatetraene or [8]-Annulene: Predicted to be Antiaromatic compound. [8]-Annulene “Tub” shaped structure of Cyclooctatetraene Cyclooctatetraene is not planar, (Nonaromatic compound) (Not planar) SK SINGH, KMC Polycyclic aromatic compounds Hückel’s (4n+2 )p electron rule is strictly applicable to monocyclic compounds. However, it applies to a number of polycyclic compounds as well. 14 p-electrons: aromatic SK SINGH, KMC Azulene: An isomer of naphthalene 1. It is cyclic, planar have an fully conjugated cyclic π electron cloud. 2. It follows Hückels rule. Contain 10 p e’s. 4n+ 2 = 10; n = 2 I Cyclopentadiene II III Cycloheptatriene Tropylium cation Cyclopentadienyl anion Aromatic Aromatic 1. Contribution of the resonating structure III to the resonance hybrid is more. In this charged resonating structure there is seven membered aromatic ring tropylium cation and five membered aromatic ring cyclopentadienyl anion is present. 2. The main contributing structure to the hybrid show charge separation, which is responsible for the higher dipole moment (1.0 D). SK SINGH, KMC Cyclodecapentaene or ([10]-Annulene): Cyclodecapentaene seems to be aromatic as it is cyclic, seems to be planar, conjugated system and has 10p electrons. But the fact is that it is not aromatic. The reason is that if the molecule acquires planar geometry (I), there will be angular strain in the molecule as each angle will try to be 120o (due to sp2 hybridization) but cannot do so, also it will lead to five cis double bonds. In order to become stable, it tries to exist as (II) where there are three cis and two trans double bonds. Looking closely we find that the two H atoms on the two trans double bonds come very close to each other and thus the molecule actually exists in a manner where the ring becomes puckered or non-planar so that the two half-cyclic rings are tilted in order to make these two hydrogen atoms slightly away from each other (III).