Chapter 14 - ASUC Lecture Notes for Chem 3B...

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Unformatted text preview: ASUC Lecture Notes for Chem 3B notes.berkeley.edu Transcribed verbatim notes available after each lecture includes figures, diagrams, slides, and past exams Peter Vollhardt University of California at Berkeley I, too, star TigreDe Bue Aire 1952 lta, nos s, 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 F 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 of thep atom orbital with the ic bonding orbital cause thep orbital le l s ve to m up and the bonding le l to m ove ve ove down. Inte raction of thep atom orbital with the ic * antibonding orbital cause thep orbital s le l to m down to where it was ve ove originally and that of the* antibonding orbital to m up. ove Thetwo e cts on thep orbital cancel ffe e othe out; thetwo orbitals are ach r pushe apart: "e rgy splitting"; d ne im portantly, the bonding orbital goes down. Ethe ne * * 0 up Nonbonding MO 2) p E unchange d 3) down 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 chanism : 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, always pre nt in NBS se Low conc. S toichiom try: Br2 doe not show up, but is theactual brom e s inating spe s! cie AllylicF11 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 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 TSde localize d 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: allylic Grignard re nts age Br + Mg M gBr Weshall e ncounte ne r utral analogs of allylic anions: CH2 X isoe ctronic to le X : : X = OR, S NR2 R, NMR shie d! lde 1 H NMR S ctrumof m thoxye ne pe e the 3H C 3 H e -rich : : : : 13CNMR: 141.2 109.6 1H 1H A 1H C B 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 4H H H H H = 5.06 2H 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 Markovnikov addition with a twist: Fast + HC l CH 3 CH3 Re ason: I nte e rm diatecation is also stabilize d 1,2-Addition ("kine tic") C l - - 1,4-Addition C l CH ("the odynam rm ic") C l Te inal alke le stable rm ne ss than inte rnal CH Le stable ss + cis C l Morestable HC lAddnF11 Kinetic vs Thermodynamic Control Extended Conjugation + HBr Quitere active e n , ve though stabilize by d conjugation Product of nucle ophilic attack: CH 3 CH 3 The o and rm kine tic Kine tic Thre products e Br CH 3 The o rm C ation also stabilize by conjugation d The odynam stability doe not always rm ic s e qual lack of re activity 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. S eExam s of theTre in Re om ple nd activity of Die nophile and Die s s ne H3 C < < F3 C < Dienophile H3 C H3 C H3 C Diene Increasing reactivity NC < NC NC < < < H 3CO Mechanism: Concerted + Orbital de scription: sp2 sp2 sp3 Die ls-Alde re r action re quire acce s ssing thele stables-cis ss conform ation CH3 CH3 H3C CH 3 H s-cis s-trans s-cis s-trans Whe s-cis formis hinde d or n re im possible there , action slows or doe not s occur. Whe die is constraine s-cis, the n ne d transform ation is acce rate le d. Consequences of Concertedness 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 DAF11 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 stadt.de /ak/im e m l/ Ge rally: ne o i i A o i A A + A WalbaC tF Dylan o i Re asons for e rulecom x. ndo ple LipshutzC tF S govia e 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 CH 3 H CH 3 H C onrotatory (clockwise ) CH 3 H H3C H C ounte rclockwiseconrotation in principlepossiblebut ste rically prohibite : d CH3 H CH 3 H 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 in ringope d ne partne r 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 !) E (kcal m -1) = 28,600/ (nm ol ) 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 trie d ne max = 217 nm Unconjugate trie d ne Azule ne ...
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This note was uploaded on 05/09/2010 for the course BIO biology taught by Professor Meighan during the Spring '09 term at Berkeley.

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