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 C olorle liquid bp ~80C Ve unre ss . ry active . C: H 1 :1 S ynthe Mitsche sis: rlich by che ical de m gradation of be nzoic acid (1833): Analysis: C OOH Michae Faraday l 1791-1867 C H6 6 C yclic structure Ke : kul 1865 Nomenclature Many com on nam s be m e causem be nede any nze rivative occur s in nature Functional groups havepriority, but othe are . rs substitue he alkylbe ne halobe ne nitrobe ne nts, nce nze , nze , nze . Disubstitute d: ortho m ta e para Functional groups takeove r: I UPAC acce d pte Ge ral te for be necontaining com ne rm nze pounds: Are . ne C H5- is phe ; ge ral aryl. 6 nyl ne C H C - is phe e or be . H nylm thyl 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" S tructure Re : gular He xagon Both the and fram sym e e m trizethestructure 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 H of hydroge nation: RE = (2x28.6) 54.9 = 2.3 Product is thesam for all: cyclohe e xane Calculated H(H ) = (3x28.6) 6.9 = 78.9 S cial stability is now calle arom pe d aticity. All cyclic 6earrange e arearom m nts atic, including transition state s. Aromatic Transition States All 6 eTS s Molecular Orbitals . p Spectra UV-Visible C ple se of pe at 250290 nm : om x t aks , which shift to visiblerangewith re sonating substitue dye nts: s. max ( ) = 289 (18,600) nm suntan lotions. : I R: Mass spe ctrum : Re lative stableM ly 1 H NMR Be ne = 7.27 (s) ppm Low fie ("to theleft") nze : : ld! "Arom re atic gion" in 1H NMR: ~ 6.5 8.5 ppm . Why? Ring curre nt! We ns H ake S nghte H tre ns H Substitued Benzenes Can Show Complex NMR Patterns 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 Be ne = 128.7 ppm nze Polycyclic Benzenoid Hydrocarbons Fusion: Line and angular ar Naphthale : Arom ne atic C lar's rule Full e ctron se ts are"good" : le xte UV Fullerenes: "Curved Graphite" Harry Kroto, Robe C Richard S alle rt url, m y NP 1996 Conjugated Cyclopolyenes: Annulenes Hcke rule l's : C yclically de localize polye s ("annule s") with [4n +2] d ne ne e ctrons arearom (stabilize re le atic d lativeto acyclic re re ), fe nce thosewith [4n] e ctrons areantiarom (de le atic stabilize ); n = d 1, 2, 3, e tc. I nte rruption of cyclic conjugation: Nonarom . atic Erich Hcke l 1896-1980 De ction by 1H NMR: Arom syste s show de lde oute te atic m shie d r Hs (shie d inne Hs). Antiarom syste s show there rse lde r atic m ve : shie d oute , de lde inne Hs. lde r shie d r Re call: S lding hie zone De l-ding shie zone S lding hie zone De l-ding shie zone Exam s of cyclic, de ple localize polye s: d ne Cyclobutadiene ([4]Annulene) Antiaromatic 1 Unstable ve re , ry active(Die ls-Alde dim rization at r e 35K), re ctangular (not square S ). tabilize by bulky d substitue nts. H NMR of tri(te -butyl)cyclobutadie : = 5.38 ppm High fie Re rt ne : ld! lativeto arom syte s and 1,3-butadie ( = 6.27 ppm atic m ne ). Te te -butyl)cyclobutadie X-ray structure tra( rt ne : Cyclooctatetraene ([8]Annulene) Nonplanar, which inhibits de localization: Nonarom (likea polye ). atic ne Barrie to ring flip 11 kcal m -1 r ol [10]Annulene: Aromatic? H H trans,trans I som r e Planar structuretoo crowde by theinside d hydroge . Othe isom rs suffe bond anglestrain: ns r e r Unde e ctrocyclic ring closure ! rgo le s H H 154 H h con H , 25 C H dis , 10 C H all-cis I som r e H H S olution to this proble : Bridge m d annule s. ne - 0.50 ppm S lde by ring hie d curre nt 7.10 ppm [18]Annulene Franz S ondhe e im r 1926-1981 Aromatic -2.99 ppm shie d lde 9.28 ppm de lde shie d [16]Annulene: Antiaromatic 10.43 ppm 5.40 ppm TheAnnule s ne Cyclobut adiene: planar, ant iaromat ic Benzene: planar, aromat ic Cyclooct at et raene: nonplanar = nonaromat ic [10]Annulene: nonplanar = nonaromat ic [12]Annulene: planar = ant iaromat ic [14]Annulene: planar = aromat ic [16]Annulene: planar = ant iaromat ic [18]Annulene: planar = aromat ic 1, 3- Cyclopent adiene: noncyclically delocalized = nonaromat ic 1, 4- Cyclohexaadiene: noncyclically delocalized = nonaromat ic Charged Annulenes 5.57 ppm Whe corre d for . n cte charge ~7.5 ppm : (prope = 40) ne S e ix But, thecyclope ntadie cation is antiarom nyl atic, ve unstable m worsethan allyl ry , uch cation: 4 e ctrons! le 9.17 ppm Whe corre d for . n cte charge ~7.5 ppm : S e ix Brom ocyclohe ptatrie spontane ne ously dissociate S e But, thecyclohe s: ix . ptatrie anion nyl is unstable pKa of cyclohe : ptatrie ~ 39!, 8 e ctrons. ne le Hckel'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 +2], including charge TS . d s Hydrideshift in carbocations: 2 eTS [4n +2], n = 0. Cycloprenium ion: planar, aromat ic Hckel's Rule in Polycycles Good approxim ation: Count pe riphe e ctrons ral le Arom atic I solate d Pe riphe arom ry atic: 14 e 3 Be ne and pe nze s riphe 18 e ry But: Antiarom atic 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) (Not addition) Me chanism : The Intermediate Cation in EAS - . . E=H X-ray structureof [C H ] + + 1. Haloge nation: F2 viole C 2, Br2 ne d catalyst ; nt; l e I2 e ndothe ic rm Ele ctron poor -HBr 3 Resonanceform s S tops. Br is e -withdrawing, de activate ring s -10 kcal m ol Othe catalytic Le acids: BF3, AlCl3, e r wis tc. 2. Nitration with nitric acid HNO = HO--NO2 O N+ HNO3 , 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 O O : :O: Valence :O S O or shell S O 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 SO O3 3 Applications of sulfonic acids: Be ne nze sulfonyl chloride Re ROH RC with PC 5 (C call l l hapte 9) r O S O NHR O S RNH 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 (acylation) A. Alkylation 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. m -pushing alkyl group, m aking thebe ne nze m re ore active causing ove , ralkylation; 2. Le acid activation m s carbocationic alkyl, which wis ake is proneto re arrange e and polym rization. m nts e B. Alkanoylation (acylation) - se ctive le Me chanism : Le acid ne de in e wis e d quim am olar ounts, be causeit binds to 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 functions. ...
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