Chapter 15 - Be neand Arom nze aticity 1825 Faraday discove...

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Unformatted text preview: Be neand Arom nze aticity 1825 Faraday discove be neby pyrolysis of whaleoil: rs nze 1825 C olorle liquid bp ~80°C Ve unre ss . ry unre active . C: H 1 :1 S ynthe Mitsche sis: rlich by che ical de m gradation of be nzoic acid (1833): (1833): Analysis: C OOH Michae Faraday l Michae 1791-1867 1791-1867 C H6 6 C yclic structure Ke : kulé 1865 1865 Nomenclature Nomenclature Many com on nam s be e causem be nede any nze rivative occur s Many m in nature Functional groups havepriority, but othe are . rs substitue he alkylbe ne halobe ne nitrobe ne nts, nce nze , nze , nze . substitue Disubstitute d: ortho m ta e para Functional groups t akeove r: Functional I UPAC acce d pte acce Ge ral te for be necontaining com ne rm nze pounds: Are . ne Are C H5- is phe ; ge ral aryl. 6 nyl ne aryl. phe ge C H C - is phe e or be . H is phe nylm thyl or be nzyl Be causem be nede any nze rivative e s xhibit a nicesm ll, com e pounds containing the be nering we calle historically “arom . nze re d atic” “arom S tructure Re : gular He xagon Both theπ and σ f ram sym e e m trizethestructure Both Be neis unusually unre nze active Doe this m an that it is also . s e e cially stablethe odynam spe rm ically? Look at ΔH° of hydroge nation: hydroge RE = (2x28.6) – 54.9 = 2.3 Product is thesam for all: cyclohe e xane Calculated ΔH°(H ) = Calculated (3x28.6) –6.9 = 78.9 (3x28.6) S cial stability is now calle arom pe stability d aticity. All cyclic 6earrange e arearom m nts atic, including transition state s. including Aromatic Transition States Aromatic All 6 eTS s π Molecular Orbitals . p Spectra Spectra UV-Visible C ple se of pe at 250–290 nm : om x t aks , which shift to visiblerangewith re sonating substitue dye nts: s. substitue λmax (ε ) = 289 (18,600) nm suntan lotions. : I R: Mass spe ctrum : Re lative stableM ly 1 H NMR Be ne δ = 7.27 (s) ppm Low fie (“to theleft”) nze : : ld! (“to Be “Arom re atic gion” in 1H Arom NMR: δ ~ 6.5– NMR: 8.5 ppm . Why? Why? Ring curre nt! Ring We ns H ake We S nghte H tre ns H ∝ν Substitued Benzenes Can Show Complex NMR Patterns Complex S trong push-pull e cts e cially on ortho-H: ffe spe Pull Jortho = 9 Hz Push Alde s hyde de lde shie d 13CNMR: Alke -like ne 13 Be neδ = 128.7 ppm nze Be Polycyclic Benzenoid Hydrocarbons Hydrocarbons Fusion: Line and angular ar Naphthale : Arom ne atic C lar’s rule Full e ctron se ts are“good” : le xte UV UV Fullerenes: “Curved Graphite” Fullerenes: Harry Kroto, Robe C Richard S alle rt url, my NP 1996 Conjugated Cyclopolyenes: Annulenes Annulenes Hücke rule l’s : C yclically de localize polye s (“annule s”) with [4n +2] π d ne ne +2 e ctrons arearom (stabilize re le atic d lativeto acyclic re re ), fe nce arom thosewith [4n] π e ctrons areantiarom (de le atic stabilize ); n = d); t hose antiarom 1, 2, 3, e tc. 1, I nte rruption of cyclic conjugation: Nonarom . atic Nonarom Erich Hücke l 1896-1980 De ction by 1H NMR: Arom syste s show de lde oute te atic m shie d oute r NMR: Arom de Hs (shie d inne Hs). Antiarom syste s show there rse lde inne r atic m ve : Hs Antiarom shie d oute , de lde inne Hs. lde r shie d inne r shie oute de Re call: Re S lding hie zone zone De l-ding shie De zone zone S lding hie zone zone De l-ding shie De zone zone Exam s of cyclic, de ple localize polye s: d ne Cyclobutadiene ([4]Annulene) Cyclobutadiene Antiaromatic 1 Unstable ve re , ls-Alde dim rization at re Unstable ry active(Die 35K), re ctangular (not square S ). tabilize by bulky d substitue nts. substitue H NMR of tri(te -butyl)cyclobutadie : δ = 5.38 ppm High fie Re rt -butyl)cyclobutadie ne : ld! lativeto arom syte s and 1,3-butadie (δ = 6.27 ppm atic m ne ). arom Te te -butyl)cyclobutadie X-ray structure tra( rt ne : Cyclooctatetraene ([8]Annulene) Cyclooctatetraene Nonplanar, which inhibits de localization: Nonplanar, Nonarom (likea polye ). atic ne Nonarom Barrie to ring flip 11 kcal m -1 r ol [10]Annulene: Aromatic? Aromatic? H H trans,trans I som r e Planar structuretoo crowde by theinside d Planar hydroge . Othe isom rs suffe bond anglestrain: ns r e r bond hydroge Othe Unde e ctrocyclic ring closure ! rgo le s H H 154º H hυ con H ∆ , 25 °C H dis ∆ , 1 0 °C H all-cis I som r e H H S olution to this proble : Bridge m d annule s. ne annule - 0.50 ppm S lde by ring hie d curre nt curre 7.10 ppm 7.10 [18]Annulene Franz S im r Franz ondhe e 1926-1981 Aromatic -2.99 ppm -2.99 shie d lde shie 9.28 ppm de lde shie d de [16]Annulene: Antiaromatic Antiaromatic 10.43 10.43 ppm ppm 5.40 ppm TheAnnule s ne The Cyclobut adiene: planar , ant iar omat ic Benz ene: planar , ar omat ic Cyclooct at et r aene: nonplanar = nonar omat ic [ 1 0 ] Annulene: nonplanar = nonar omat ic [ 1 2 ] Annulene: planar = ant iar omat ic [ 1 4 ] Annulene: planar = ar omat ic [ 1 6 ] Annulene: planar = ant iar omat ic [ 1 8 ] Annulene: planar = ar omat ic 1 , 3 - Cyclopent adiene: noncyclically delocaliz ed = nonar omat ic 1 , 4 - Cyclohex aadiene: noncyclically delocaliz ed = nonar omat ic Charged Annulenes Charged 5.57 ppm Whe corre d for . n cte 5.57 charge ~7.5 ppm : charge (prope = 40) ne Se ix But, thecyclope ntadie cation is antiarom nyl cation atic, ve unstable m worsethan allyl ry , uch But, cation: 4 e ctrons! cation: le 9.17 ppm Whe corre d for . n cte 9.17 charge ~7.5 ppm : charge Se ix Brom ocyclohe ptatrie spontane ne ously dissociate S e But, thecyclohe s: ix . ptatrie anion nyl anion Brom is unstable pKa of cyclohe : ptatrie ~ 39!, 8 e ctrons. ne le is of Hückel’s Rule in Charged Transition States Wehavealre se n ne ady e utral e xte TS but this is ge ral for othe [4n -se t s, ne r We t his +2], including charge TS . ds +2], Hydrideshift in carbocations: 2 eTS Hydride [4n +2], n = 0. Cyclopr enium ion: planar , ar omat ic Hückel’s Rule in Polycycles Hückel’s Good approxim ation: Count pe riphe e ctrons ral le Good pe Arom atic I solate d Pe riphe arom ry atic: 14 e 3 Be ne and pe nze s riphe 18 e ry But: But: Antiarom atic Antiarom Base 10 e Arom atic like What arethe ? se Arom aticity is an im portant conce include he rocycle pt, s te s (C hapte 25). >300,000 pape since1981! r rs Electrophilic Aromatic Substitution (EAS) Substitution (Not addition) Me chanism : The Intermediate Cation in EAS The - . . E=H X-ray structureof [C H ] + + 1. Haloge nation: F2 viole C 2, Br2 ne d catalyst ; nt; l e catalyst 1. I2 e ndothe ic rm Ele ctron poor -HBr 3 Resonancef orm s Re S tops. Br is e -withdrawing, de activate ring s -10 kcal m ol Othe catalytic Le acids: BF3, AlCl3, e r tc. Othe catalytic wis 2. Nitration with nitric acid 2. HNO = HO—NO2 O N+ H N O3 , H 2 SO4 O– Me chanism : C olor: Nu, E, L 3. S ulfonation: Re rsible Re nt is fum sulfuric acid: ve . age ing H2S 4 + S 3 OO : : O: Val ence : O S O or shel l SO ex panded: O: : O: : : : : SO3 , H 2 SO4 : O S O OH Zwitte rion S ulfonation : (will be eim com portant in Chapte 16) r Drive by largehe of n at hydration of S 3 hydration O Applications of sulfonic acids: Be ne nze sulfonyl Be chloride chloride Re ROH RC with PC 5 (C call l l hapte 9) r Re O S O N HR O S RN H 2 O Cl O S ROH O O R S ulfonam s ide Re call: Nu: S ulfonic e rs ste S drugs: Antibacte (urinary, m ulfa rial alaria, le prosy) Malaria occurre nce 4. Frie l-C de rafts re actions: Alkylation and alkanoylation Alkylation (acylation) (acylation) A. Alkylation A. Le acid wis Me chanism : Charle Frie l s de 1832-1899 Jam s Craft e 1839-1917 Ofte low yie Proble s: 1. Product contains an e n lds. Proble m -pushing alkyl group, m aking t hebe ne nze m re ore active causing ove , causing ove ralkylation; 2. Le acid activation m s carbocationic alkyl, which ake carbocationic 2. wis is proneto re arrange e and polym rization. m nts e re polym B. Alkanoylation (acylation) - se ctive le B. Me chanism : Me Le acid ne de in e wis e d quim am olar ounts, be causeit binds to product: product: EAS I m e : m diatesynthe takehom le tic e ssons 1. Halobe ne m Grignards, lithiumre nts nze s ake age 2. Alkanoylbe ne havecarbonyl function nze s Weshall se ne (C e xt hapte 16) how to usethenitro and sulfonyl r We functions. f unctions. ...
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