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Aromaticity and Conditions. Neutral and Charged Homo- and Hetero Aromatic Systems

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Aromaticity and Conditions. Neutral and Charged Homo- and Hetero Aromatic Systems Aromaticity and Conditions. Neutral and charged homo- and hetero aromatic systems. Electrophilic aromatic substitution reaction its mechanism and basic cases. Substituent effects in electrophilic substitution reactions to the rate (reactivity), directing rules (regioselectivity). Electrophilic substitution reactions of five- and six-membered heteoaromatic compounds. Addition reactions. Reaction of aromatic hydrocarbons containing alkyl side chain, the benzyl type reactive intermediates. Some polycyclic aromatic hydrocarbons. Aromatic compounds and their classification Formally, cyclic compounds containing conjugated double bonds (and non-bonding electron pairs) - BUT itself is not a sufficient condition for aromaticity Aromatic compounds - characteristic chemical properties (in spite of unsaturation no addition reaction, BUT substitution reactions) + Anomalous NMR spectroscopy behaviour Classification 1. Homo aromatic compounds - carbocyclic (carbon atoms only!) 1.1. Monocyclic homo aromatic compounds - substituted benzene derivatives Mostly trivial or semi-trivial names (BUT the suffix is often misleading) benzene toluene xylene styrene phenol anisole aniline xylidine (o-, m-, p-) (o-, m-, p-) Some important groups 1.2. Polycyclic homo aromatic compounds 1 1.2.1. Isolated polycycles Ar-(C)n-Ar n = 0 Biphenyl and its derivatives E conjugation interaction between electron system of the two rings: coplanar nature? No! In gas and liquid phases decreasing overlap due to the van der Waals repulsion 90o 180o 270o o 360 between o, o'-hydrogens n ≥ 1 Aryl-substituted alkanes Consequence: Atrop isomerism Trityl cation (radical, anion) very stable. Reason: electron delocalization on 19 C Resonance structures of trityl cation 1.2.1. PAH: Polycyclic aromatic hydrocarbons: Condensed polycyclic - rings connected through two points (anellation points) Linearly condensed Angularly condensed naphthalene anthracene phenanthrene coronene Trivial names: special numbering: reason: highlighted position with different reactivity 2. Heteroaromatic compounds ≥ 1 hetero atom (mostly O,S,N) in the ring – BUT! not all heterocyclic or unsaturated heterocyclic compound is heteroaromatic! Classification according to : • ring size (most important: 5 and 6-membered) • type and numbers of heteroatoms (most important: systems with 1 and 2 heteroatoms) Nevezéktan – itt is sok triviális név furan thiophene pyrrole pyrasole imidazole isoxazole oxazole pyridine pyrimidine Systematic nomenclature of heterocyclic compounds Hantzsch-Widman (it can be used for each heterocycle ) Element oxygen sulfur nitrogen priority order increasing from right to left Prefix Oxa Thia Aza Ring Size 3 4 5 6 7 8 9 10 Suffix: Unsaturated irene ete ole ine epine ocine onine ecine Saturated irane etane olane inane epane ocane onane ecane • The terminal "e" in the suffix is optional though recommended. • Saturated 3, 4 & 5-membered nitrogen heterocycles should use respectively the traditional "iridine", "etidine" & "olidine" suffix. • Unsaturated nitrogen 3-membered heterocycles may use the traditional "irine" suffix. • Consistent use of "etine" and "oline" as a suffix for 4 & 5-membered unsaturated heterocycles is prevented by their former use for similar sized nitrogen heterocycles. • Established use of oxine, azine and silane for other compounds or functions prohibits their use for pyran, pyridine and silacyclohexane respectively. Aromaticity and its requirements - starting point: benzene Discovery: Faraday (1825) - whale oil pyrolysis, naming: Liebig, molecular formula (C6H6): Mitscherlich Special Features: • There is no detectable unsaturation • Mono-substituted single all hydrogen equivalent • one 1,2 - (or 1,3 - or 1,4 -) disubstituted product all carbon equivalent (There are only three di-substituted products); ideas for structure: Exists! Spectroscopic studies: plane six-membered ring, Exists! prismane symmetrical structure with equivalent C-C and C-H Benzene: dC-C = 0.140 nm, dC-H = 0.108 nm bonds, C-C bond order between 1 and 2! dC-C = 0.154 nm, dC=C = 0.134 nm Requirements for aromaticity: E The simplest model: 6 + bonds, ‚continuous ’ e-flow in the, 6 system, 6-centered MO Not a simple conjugation, totally symmetric electron distribution! 2 (i vagy i ) „ Benzene is not cyclohexa-1,3,5-triene …” Extra stabilization energy (empirical resonance energy) = Ecyclohexa-1,3,5-triene - Ebenzol = 147 kJ/mol (but this value depending on model!!) Robinson-ring (1925) - only good for benzene, for polycyclic aromatic compounds it is misleading Evidence - hydrogenation Heat of hydrogenation: difference is 149 kJ / mol far exceeding a simple conjugation stabilization energy gain! Something more… Representation of extra stability associated with aromaticity with resonance structures Note: resonance forms very often are fictitious / not representing the current structure forms Requirements for aromaticity 1. The molecule is cyclic 2. The molecule is planar (all atoms in the ring lie in the same plane, sp2 hybridised C) 3. The molecule is fully conjugated (pz orbitals at every atom in the ring) 4. The molecule has 4n+2 π electrons (n=0 or any positive integer, Hückel’s rule) Other versions for formation of aromatic systems 1.1. Charged homoaromatic systems (Hückel’s rule, n = 1) sp3 → sp2 rehybridisation: aromatic system Cyclopentadienyl anion Resonance structures – show a completely symmetrical, smooth charge distribution Evidence: unusually strong CH acidity of cyclopentadiene Realization in case of a cation - cycloheptatrienylium (tropylium) ion Note: the number of atoms in the ring has no significance in terms of the aromaticity! The electron distribution is completely Empty p z smooth on carbon atoms of the ring orbital Resonance structures: 1.2. Polycyclic homoaromatic hydrocarbons (Hückel’s rule, n > 1) PAH („polycyclic aromatic hydrocarbons”) naphthalene phenanthrene anthracene chrysene tripnenylene n = 2 n = 3 n = 3 n = 4 n = 4 But! -electron cloud is not symmetric, different bond lengths, different electron density decreasing resonance energy (stabilization energy) Resonance E: 152 255 385 351 (kJ/mol) Per ring 152 127.5 128.3 117 2. Aromaticity of heteroaromatic compounds 2.1. Five-membered heteroaromatic compounds Source: March, Advanced Organic Chemistry The simplest case: ✓One heteroatom (O, N, S):a Formal e- densities common characteristic: non bonding e-pairs ✓sp2 hetero atoms which participates in 6 system with one nonbonding e-pair by resonance structures: ✓Non-symmetric charge distribution, reduced aromatic character - the electron density is decreased on the heteroatom, while on the C-s it is increased „electron rich” heteroaromatic system Resonance E: 149 68 122 90 (kJ/mol) The less aromatic five-membered system! Insertion of further heteroatoms into the ring: the preservation of aromaticity is possible only with the insertion of "pyridine-like" nitrogen! Nitrogen: two types sp2 hybrid state 1 1 1 2 1 1 2 1 „pyrrol-like”: h1 h2 h3 pz „pyridine-like”: h1 h2 h3 pz 2.2. Six-membered heteroaromatic compounds Pyridine: analogous structure to benzene, the nitrogen heteroatom gives only one pz electron into 6 system nonbonding e-pair on hybrid orbital Bond length Due to differences in EN the electron distribution is distorted, (nm) but in this case the electron density is increased on the heteroatom and the electron density is decreased on C-s "Electron-poor" heteroaromatic system Benzene Pyridine Energies: MO's are similar to one of benzene, but! the equivalence of doubly degenerated orbitals disappears. Highly aromatic system, resonance energy: 117 kJ / mol Resonance structures of pyridine: Further "pyridine-like" nitrogen can be inserted into the six-membered heteroaromatic ring. Oxygen and sulphur heteroatoms can exists only as positively charged in the six-membered heteroaromatic ring! 1. Acid-base properties 2 Acidity: weak CH acidity (pKa 36), reason: sp carbon has higher EN order of acidity: R-H < Ar-H < RC≡C-H after deprotonation nonbonding e-pair (negative charge) remains on sp2 hybrid orbital no interaction with the -electrons, there is no stabilization effect so less favoured Basicity: due to the presence of -electron cloud weak -donor bases so Lewis acids can attack it 2. Electrophilic aromatic substitution (SE) Starting point: Due to its -donor base character (similar to alkenes) it attacks electron-deficient particles, primarily electrophiles. First step: formation of -complex, E is not yet linked to any carbon atom only interaction with the -cloud (some debates this idea…) Step Two: disorganisation of 6 system, formation of -complex (Whelan complex); the electrophile is linked to a carbon atom But! By the formation of complex the -sextet is disrupted, the stabilization energy arising from aromaticity is lost large G‡ necessary to apply strong electrophiles and / or the catalyst Alternatives for the second step: addition or substitution Re-aromatisation is favoured: substitution occurs Summarizing… Usually the formation of complex is the rate-determining step The most important electrophilic aromatic substitution reactions 2.1. Halogenation The formal electrophile: Hlg Function of catalyst: generation of Hlg Hlg: usually Cl, Br Lewis acid: AlHlg , FeHlg , BF , etc. 3 3 3 „push-pull” mechanism 2.2. Friedel-Crafts alkylation – formal electrophile: alkyl cation (carbenium ion, R) „push-pull” mechanism Function of catalyst: generation of R Lewis acid: AlHlg3, BF3, SnCl4, SbF5, etc. Note: alkyl cation (carbenium ion) can be generated in some other way
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