Chapter 14 - Peter Vollhardt University of California at...

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Unformatted text preview: Peter Vollhardt University of California at Berkeley I, too, star te d small........ Chapter 14 Delocalized Pi Systems Re thedoublebond call Trigonal The bond is e rich: E attack, add) Thelobe of thep-orbitals: s Pe ndicular to thesigm rpe a fram and paralle to e e l ach othe r. Ethe ne R 2-Propenyl (Allyl) H Que stion: What about adding a third p-orbital adjace to thedoublebond? nt I s the som thing spe re e cial? Or: Is the any spe re re cial activity at the carbons adjace to a doublebond? nt Re placing oneof thehydroge in e newith anothe sp2ns the r hybridize carbon give a prope d s nylic or allylic syste . m Allylic position Obse rvations: a. b. H 87 kcal m -1: We ol ak! H 101 kcal m -1 ol L S1 N S 1 re activity of allylic carbon likethat of RsecX, N e n though it is prim ve ary! B H B 50 c. H pKa ~ 40: Acidic! + Clearly: Allylic - are stabilized. Why? Re sonance ! CH 2 H 2 C CH 2 H 2 C CH 2 H 2 C S notation: Dotte line hort d s MO Picture of 2-Propenyl (Allyl) 3 pOs 3 MOs of E H+ HNode Now le look again at e ne t's the Re : Bonds m by ove call ade rlap orbitals. Le look again at H2. t's .+ H. H H H Bonding Antibonding H H +H H+ + H+ I n phase H+ + HOut of phase S of thewavefunction, ign Not charge ! H2 +H * E * CH2 CH2 What happe to this picturewhe we ns n inte with anothe p orbital? ract r Interactions of a singly occupied p-orbital with each of the molecular orbitals 1) Inte raction with the bonding orbital cause e rgy splitting: thep s ne orbital le l m s up and the ve ove bonding le l m s down. ve ove Inte raction with the* antibonding orbital cause thee rgy of thep s ne orbital to m down to where it ove was originally and that of the* antibonding orbital to m up. The ove two e cts on thep orbital cancel ffe e othe out; thetwo orbitals are ach r pushe apart; im d portantly, the bonding orbital goes down. * * 0 up Nonbonding MO 2) p E unchange d down Ethe ne Allyl p Orbital Allylic radical is stabilize by 13.5 kcal m -1. d ol Re sulting picture : # of e de nds on pe +,,- * 0 ? p E H 2C H C CH2 Location of +,,is at te ini rm Reactivity of Allylic Position A. Radical Haloge nation C C H H HC Me ch. I nitiation: 1. Br2 h or + Br2 Low conc. C C C Br H H H Faste than addition! r + HBr . 2 Br . + Br . C C C H H H C C C H H H + HBr . Propagation: 2. C C H H HC C C C H H H . Br2 . C C C Br + H H H C C C H H H . Br Te ination: rm . + . CH CH Br 3. C H Br2 BrC C C H H H . + Br . Br . CH CH 2 C H C C H H HC C C C H H H Anything that traps radicals, including the"dirt" on thewalls of theflask, contribute to te ination. s rm Trace s Low conc. Allylic Prope ge rate a sym e ne ne s m trical allylic radical and only one product. For unsym e m trical syste s: m m ixture . Ratios de nd s pe on %radical characte on e carbon and TS le r ach s ading to products. Br 2 Br 2 Br + B. S 1: TheAllylic C ation is S tabilize d N 2 + C C C C H H H H + C C C C H H H H slow fast C C C HC H H HC l -H lO C H + C C C H OH H H HC C C H C H HC H OH +H Two products C S 2: Fast! Theallylic TSis stabilize and theallylic carbon . N d is re lative e ctrophilic. ly le sp = e -withdrawing 2 C l .. .. . . - C C C HC H H HC l 100 tim s faste than e r + NaI C C C C H H H I - Cl C C C HI H H HC + C l - D. Allylic Organom tallics e Li + CH 3 CH2 Li H 2C C CH 3 RX CH 2R H 2C C CH 3 CH 3 Alte rnativepre paration via allylic Grignard re nts: age Br + Mg M gBr Weshall e ncounte ne r utral analogs of allylic anions: X X : : X = OR, S NR2 R, NMR shie d! lde 1 H NMR S ctrumof e nylm thyl e r pe the e the C 3 H C B A Conjugated Double Bonds What about C C C C C C C C ? Nom nclature e : C is/trans E/Z re w vie S tability: He of hydroge ats nation (kcal m -1) ol H C 3(C 2)3C C 2 H H H H + H2 + 2 H2 1,5-He xadie ne -30.3 -60.5 But: 1,3-Butadie ne + 2 H2 -57.1 Re sonancee rgy of butadie ~ 3.5 ne ne Structure Fast S re hort lativeto an alkane C Csinglebond (1.54 ). But is this a good com parison? This is hot stuff! Equilibrium Structures for Butadiene and Ethylene: Compelling Evidence for -Electron Delocalization in Butadiene Norman C. Craig* Obe C ge rlin olle Peter Groner Unive rsity of MissourisKansas C ity Donald C. McKean Unive rsity of Edinburgh J. Phys. C m A 2006,110, 7461-7469 he . I t is shown for thefirst tim that -e ctron de e le localization has thestructural conse nce of incre que s asing thele ngth of theform doublebond by 0.007 and al de asing thele cre ngth of theform singlebond by 0.016 . al Orbitals Antibonding Bonding NMR ( ppm, J Hz) J = 10 = 6.27 (effect of 2 doublebond) J = 10 H H H H = 5.06 J = 17 = 5.16 J ~1-2 H H H H 137.2 116.6 C onjugation stabilize the odynam s rm ically, but it also incre s re ase activity, for e plein xam e ctrophilic additions (re w C le vie hapte 12). r + HC l Fast CH 3 CH 3 I nte e rm diatecation is also stabilize d 1,2-Addition (kine tic) Te inal alke le stable rm ne ss than inte rnal C l - - 1,4-Addition C l CH C l + cis (the odynam rm ic) C l CH HC lAddn Kinetic vs Thermodynamic Control Extended Conjugation + HBr Quitere active CH 3 CH 3 Br The o and rm kine tic Kine tic Thre products e CH 3 The o rm Them doublebonds, them se ore ore nsitive(re active is thepolye , ye it ) ne t (products) is also m stable ore . Cyclohexatriene is Special Benzene C yclic array of six e ctrons has spe stability, calle le cial d arom aticity (C hapte 15). r Be neis re nze lative ine to H2-cat, e ctrophile oxidants, in ly rt le s, com parison with he xatrie . ne Extended Conjugation in Natural and Unnatural Products Orangecolor of carrots Biological de gradation Vision Organic Conductors He ge MacDiarm S e r, id, hirakawa, Nobe Prize2000 l Light e itting diode (LEDs) m s Conjugated Systems Undergo Special Transformations: Pericyclic Reactions Theconjugate syste can re as a unit, involving both d m act e . For e ple nds xam , 1. C ycloadditions: TheDiels-Alder reaction, a [4+2] cycloaddition HC HC CH 2 CH 2 CH 2 + CH 2 20% HC HC H2 C CH 2 CH 2 Otto Die ls 1876-1954 Kurt Alde r 1902-1958 4-4C Die ne 2-2C Die nophile C H2 Nobe Prize1950 l C ycloadduct The Diels-Alder Reaction is Chemoselective Die ls-Alde re r actions work be whe wepair an st n e -rich (push) die with an e ne -poor (pull) die nophile , or an e -poor die with an e ne -rich die nophile De nds on substitue pe nts: e -Donating: Alkyl, alkoxy, alkylthio 3 3 3 2 Hype rconjugation C , H Re sonance COCH 3 H O, COCH 3 S OCH 3 HC H Eve though O is e gative(inductivee ct), re n -ne ffe sonancewins out. O e -Withdrawing: C , C , C N, NO F R F I nductive : C F F O CR H2 C C H O CR O CR H2 C C H O CR H2 C C H O CR 90% Re sonance : Exam : ple + Doe not com tewith die s pe nophile re : lative e ly -rich. Mechanism: Concerted + Orbital de scription: sp2 sp2 sp3 Consequences S re cific: Re ntion of Die te ospe te nophile S re m te oche istry (ne C w --Cbonds) O COCH 3 O CC3 OH 80% + CH 3 C3 H C is O COCH 3 C is O COCH 3 90% + H 3C CH 3 Trans Trans Re ntion of Die S re m te ne te oche istry OCH 3 NC CN OCH 3 + OCH 3 Trans,trans (sam for cis,cis-die ) e ne NC CN CN CN CN CN OCH 3 OCH 3 OCH 3 NC CN + OCH 3 C is,trans NC CN CN CN CN CN H 3 CO Whe both partne areste oche ically de d: "Endo rule n rs re m fine " de rm s the approach. te ine ir Endo/Exo Addition S ubstitue point away nts fromdie ne S ubstitue point nts toward die ne Usually faster, even though product less stable: Kinetic control DA Exam : ple C3 H C3 H HC N 3 C Endo N C H H o HC 3 C N C N http://csi.che ie m .tu-darm /ak/im e m l/ Ge rally: ne o i i A o i A A + A Walba Dylan o i Re asons for e rulecom x. ndo ple Lipshutz S govia e Quiz C yclope ntadie has a lim d she life eat roomte pe ne ite lf tim m rature(8% is conve d into its dim r in 4 h and 50%in 24 h), be rte e causeit re acts with itse lf. 2 ? or Answer: Alkynes as Dienophiles Ge rate1,4-cyclohe ne xadie s ne CO 2 CH 3 + CO2 CH 3 CO2 CH 3 75% C re an act again CO 2 CH 3 C 2C 3 O H C 2C 3 O H 2. Ele ctrocyclic Re actions: Intramolecular ring closureand ope nings TheC yclobute 1,3-Butadie Equilibrium ne ne , Ea = 32.9 kcal m -1 ol Exothe ic (ring rm strain re ase le d) h Light drive C be the odynam . n: an at rm ics Wave ngth de nde (can go e r le pe nt ithe way). Im e ml The1,3-C yclohe xadie 1,3,5ne He xatrie Equilibrium ne Light drive C be the odynam . n: an at rm ics Wave ngth de nde (can go e r le pe nt ithe way). , Ea = 29.9 kcal m -1 ol Endothe ic rm (C Cbe r than C C tte , and no ring strain pre nt) se h Im e ml Electrocyclic Reactions are Stereospecific CH 3 Only! cis3,4-Dim thylcyclobute e ne CH 3 CH 3 cis,trans2,4-He xadie ne Only! Trans CH 3 Trans,trans Movement of Substituents C onrotatory: sam dire e ction CH 3 CH 3 Both e r ithe clockwiseor counte rclockwise S e : am product. The rotatein the y H C onrotatory H CH 3 H CH 3 H Im e ml H CH 3 H C onrotatory CH 3 H H3C H CH 3 (counte rclockwise ) C lockwiseconrotation in principlepossiblebut ste rically prohibite : d H CH 3 H CH 3 H H CH 3 CH 3 Fascinatingly, h goe disrotatory (rotation s in oppositedire ctions) CH 3 CH 3 h dis CH 3 CH 3 CH 3 CH 3 CH 3 h dis CH 3 Eve m startling: Thehe n ore xatrie /cyclohe ne xadie inte ne rconve rsion is also ste ospe re cific, but follows theoppositerule of se of rotation, s nse com d to thebutadie /cyclobute syste : pare ne ne m = dis Im e ml h = con Robe B. Woodward rt 1917-1979 Roald Hoffm ann b. 1937; NP 1981 Orbital S m try: An inkling of why this goe ym e s...... C ontrols photo-che ical m closure dis : Disrotatory h-provide eto s C 3 the al ontrols rm closure con : C onrotatory Theorbital signs at thete ini alte rm rnate with # of doublebonds. C pare om he xatrie with butadie . ne ne C ontrols photo-che ical m closure con : C ontrols the al rm closure dis : Electronic Spectroscopy (Ultraviolet-Visible or UV) White(sun) light is com d of the pose visiblespe ctrum Re e be mm r spe ctroscopy (C hapte 10): r Excite state d E Ground state E = h = hc/ UV-Vis spe ctroscopy re quire m highe e rgy than NMR (kcals vs s uch r ne calorie doe not ne d e rnal "condition" (m t). Ele s), s e xte agne ctronic e xcitation frombonding to antibonding le ls, particularly e for syste s, be ve asy m cause occupie ounoccupie E re d d lative sm ly all. For e ple look at a sim bond, as in e ne xam , ple the : No bond le ft! I .e Light cause ., s cis-trans isom rization, e radical re actions, .... UV Spectrum of Ethene Quote as max d Broad, be causeof rotational and vibrational state Ele s. ctronic spe ctroscopy is fast, no "ave raging" A 171 nm Wave ngth (give in nm units of 10-9 m not in fre ncy = c/ , as we le n , ; que did in NMR, whe ~ 100 m to 1m re m !) UV spe ctroscopy be 200 nmre low quire vacuum be s , causeair absorbs. Norm (in atm ally osphe ) onescans 220-400 (UV), 400-800 nm(visible re ). This allows lowe e rgy transitions to bere r ne corde e 1,3-butadie : d, .g. ne E Re lative ly low e rgy ne max Pe he ak ights arere porte as : d Extinction coe fficie , which is nt absorbancenorm d by conce alize ntration: S houlde sh r, max = 222.5 nm ( = 10,800) = A/ c Visible Absorption: Color Ne wton Light e rs theprismfromthetop right, and is nte re fracte by theglass. Theviole is be m than d t nt ore theye llow and re so thecolors se d, parate . Absorption in thevisible 450 nmorange re - d 550 nmviole t 650 nmblue gre n - e C of substance olor I n e nde syste s m transitions are possible giving rise xte d m any , to m com x and not re ore ple adily inte tablespe rpre ctra, but HOMO-LUMO gap ge sm r: Longe wave ngth ts alle st le absorption is indicativeof thee nt of conjugation, e xte .g., Gre r conjugation: ate S alle HOMO/LUMO gap m r CH 3 max = 271 nm C onjugate d max = 217 nm Unconjugate d Azule ne ...
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This note was uploaded on 09/10/2008 for the course CHEM 3BL taught by Professor Chunmei during the Fall '08 term at University of California, Berkeley.

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