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Ch16 JL2008 Arom - Ch.16 Aromatic Ch.16 Aromatic Compounds...

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Unformatted text preview: Ch.16 Aromatic Ch.16 Aromatic Compounds Organic Chemistry I BCC, Central Campus Professor Joseph Lawry, Instructor © 2008 All rights reserved. Use of this material for any purpose other than education without prior consent is prohibited Resonance in Benzene Resonance in Benzene Benzene is actually a resonance hybrid between two Kekule structures. This representation implies the pi electrons are delocalized, with a bond order of 1.5 between adjacent carbon atoms. The carbon­carbon bond lengths in benzene are shorter than typical single­bond lengths, yet longer than typical double­bond lengths. Orbital Represintation of Benzene Orbital Represintation of Benzene Benzene: a flat ring sp2 hybrid carbon atoms & unhybridized p orbitals aligned & overlapping. carbon­ carbon bond lengths = 1.397 Å. bond angles = 120°. Con­ jugation and delocalization of e­’s confers greater stability than non­conjugated cyclics. The term aromatic describes a “cyclic compound with conjugated double bonds stabilized by resonance.” Aromatics don’t react as alkenes Aromatics don’t react as alkenes Alkene + Br2 / FeBr3 vic­dibromide, exhibiting anti­ stereochemistry via the bridged bromonium ion. Addition is not observed with aromatics… Instead a Br substitutes for an aromatic H; The aromatic stability must always be preserved Benzene does’nt have predicted heat of hydrogenation of ­85.8 Benzene does’nt have predicted heat of hydrogenation of ­85.8 kcal/mol. Observed H hydrogenation is ­49.8 kcal/mol, difference of 36 kcal. Difference between the predicted and the observed value is resonance energy. Hydrocarbons with alternating double and single bonds are annulenes. For a compound to be aromatic it must be 1. cyclic 2.conjugated double bonds 3. planar to allow the p orbitals to overlap 4. must have a Huckel (4N+2)* number of pi electrons. Cyclobutadiene and cyclooctatetraene are not aromatic. *N = 0, 1, 2, 3, 4, 5, … Cyclobutadiene and cyclooctatetraene have Cyclobutadiene and cyclooctatetraene have alternating double bonds, but aren’t aromatiac The molecular orbitals of benzene The molecular orbitals of benzene Six A.O.’s over to form aromatic π system, lowest is all bonding. π 2 &3 show 1 node. π ∗ 4 &5 show 2 nodes. π ∗ 6 is all antibonding. No. nodes increase with increasing energy. Notice M.O.’s are evenly divided & symmetrical π 1 is all bonding; all lobes show is all bonding; all lobes show same sign (not electric charge); low Ep π 2 &3 are degenerate­ one nodal plane π ∗ 6 is all antibonding; each “p” orbital is out of phase and destructively overlaps Energy Diagram for the M.O.’s of Energy Diagram for the M.O.’s of Benzene; all bonding M.O.’s filled, energetically very favorable Cyclobutadiene (antiaromatic) has Cyclobutadiene four π M.O.’s The electronic energy diagram of The electronic energy diagram of cyclobutadiene is “energetically unfavorable”; shows it to be “unstable” Polygon Rule: M.O. energy diagram of a Polygon Rule: regular, cyclic, conjugated compound has the same shape as the compound. Aromatic, Antiaromatic and Aromatic, Antiaromatic and Nonaromatic compounds Aromatic 1. [continuously]cyclic 2. planar 3. conjugated 4. Huckel number of π e­’s must be present: π e­’s must = 4N* + 2 { 2, 6, 10, 14, 18…} *N = 0, 1, 2, 3, 4… Antiaromatic Molecule meets the first three criteria, but is a 4N system. Eg. Cyclobutadiene, cyclooctateraene. Nonaromatic Does not meet one of the first three Aromatic requirements. eg. Cyclooctatetraene is not planar thus Cyclooctatetraene is not planar thus nonaromatic. Aromatic, Antiaromatic and Aromatic, Antiaromatic and Nonaromatic Large­Ring Annulenes 12 16 20 Large- ring Annulenes do not show aromaticity, they react as partially conjugated polyenes... 10 H H Non-fused = noncyclic, ...neither is the all cis- (4N +2) because of clashing H's non-aromatic cyclodecapentaene 14 ...as is the [14] and [18] annulenes with enough trans double bonds to achieve planarity. 18 Napthalene is aromatic... Is this cyclopropenyl cation aromatic? Molecular Orbital Derivation of Molecular Orbital Derivation of Huckel’s Rule Cyclopentadienyl cation and anion Cyclopentadienyl Cyclopentadienyl anion is fairly Cyclopentadienyl anion is fairly acidic As predicted by Huckel’s Rule, the creation of the Cyclopentadienyl anion is relatively easy since the product, cyclopentadienyl anion, with its six pi electrons is aromatic. Removal of an H+ creates a full p­orbital that Removal of an H is conjugated and completes the cyclic, planar, Huckel system Dehydration of 2,4­ Cyclopentadienol is not facile, since it creates an antiaromatic 4N π system Dehydration of the cyclic alcohol below Dehydration of the cyclic alcohol below creates the aromatic tropyllium ion Oddly enough, the cyclooctatetraenyl Oddly enough, the dianion is aromatic, stable. Heterocyclic Aromatic Compounds: pyridine­ Heterocyclic Aromatic Compounds: pyridine­ non­bonding e­ pairs don’t participate in aromatic pi system Pyridine is aromatic (Eresonance = Pyridine is aromatic (E 27kcal/mol), as is pyridinium ion non­bonding e­ pairs of pyrrole do participate non­bonding e in aromatic pi system… …thus pyrrole is not very basic, since the use of non­bonding e­ pairs destroys aromaticity Pyrrole, furan and thiophene Pyrrole, furan and thiophene Polynuclear Aromatic Hydrocarbons Polynuclear Aromatic Hydrocarbons (PAH’s) can be carcinogenic Napthalene two fused six memebered rings wityh 10 pi electrons are, planar, conjugated, cylcic and in this case, Huckel, thus are aromatic The three Kekule Structures of naphthalene are shown PAH’s anthracene and PAH’s anthracene and phenanthrene are found in diesel exhaust PAH epoxides can cause a “point PAH epoxides can cause a “point mutation” at cytidine Diamond and graphite Diamond and graphite Buckminsterfullerenes and carbon Buckminsterfullerenes and carbon nanotubes­ “hotly researched” Biologically fused heterocyclics Biologically fused heterocyclics Ortho­, meta­, para­ isomers Ortho­, meta­, para­ Mass spectrometry of aryl alkanes Mass spectrometry of aryl alkanes produce tropyllium UV­vis of benzene & styrene UV­ ...
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