ORGANIC CHEMISTRY II
ORGANIC CHEMISTRY II CH 318N
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Date Created: 09/07/15
Lecture 10 Chemistry of Aromatics February 19 2008 Chemimy 318N W First Midterm Exam When Wednesday 220 0 When 79PM Where WEL 1308 0 What Covers material through last Thursday Review 219 600 PM Painter 302 Practice Exams will be posted on the web site Please bring pencils an eraser and a calculator only and D0 agood job Chemimy 318N I Some Nomenclature A A A f f B B OIThO Meta Para or 12 or 13 orl4 Please read about naming in Chapter 213 Chemistry 318N D s Some Nomenclature Please read about naming in Chapter 213 benzene phenol toluene xylene aniline styrene etc Chemimy 318N I IR spectrum of toluene lin39mmIvrx 4 5 7 X ll ll 12 UNIS 2 2 an g 20 CH 39 c u m mulling nrunmn main 0 mrmxmlim mnmmliu 4 ll MINI 320quot ZSIIIII MINI ZINIII mill lblll I400 12th Itlltll MINI lvOll 4 0 an ruuullm 1cm animation Chmdmyszxzv Y Benzene ringssubstitution patterns 51 m R 750700 p 250 R R R 350730 700 320 R R Umelmble wnh N02 COZH subs From Crewesy Rodrigu ez and ch 8 Outrofrplane bending cambmattam te smallbutm anormally elem Ieglon 0fR Reliable even wnh nmo m cmboxyl s s tunon Chmlinty 3131 Y If the region between 16672000 cm391 w is free of interference 00 stretching frequency a weak grouping of peaks is observed for aromatic systems Analysis of this region can lead to a determination of the substitution pattern on the aromatic ring 5 r i 8 20m 1667th I Mono E V Di EEG o CCquot G l f quott 0 ilrulrhiugi G aromatic N p f S liiilll Itiilii hilli l4liii l2 G quot 39 CemmiymN 742cm 39 Ring Current in Benzene 439 2 3961 a x 9463 53 x l iii 6 l v J Secondary magnetic eld generated by circulating TE electrons deshields aromatic protons Chemistry 318N I NMR Spectrum of Toluene m Chemistry 318N I Coupling Constants Aromatics OrthoH s9Hz Meta H s 3 Hz 0CH3 ParaHslt1Hz H N02 M H H IuIIJLIIIIIIIIIIIII 85 84 ppm N02 IIIIIJIIIIIIIIIIIIIL IIIIIJILIIIIILIIIIIL 88 87 ppm 73 72 ppm CH34SI I 1H 1H THE It I I l I I I I l lllIlIIllllIllllIlllllllllllllllIIIIIIIIIIIIIIIIIIIIIllIllllllllllllllIIIIIlllIIIIIIIIIIIIIII 9 8 7 6 5 4 3 2 1 0 PPm5 NMR Spectrum of 1iodo4methoxybenzene I 14ltgt700H3 139 CHCI3 impurity e I It I W Fquot J I I I I l I I I I I I J I RD 70 50 50 10 In E U D Chemistry 318N 39 T NMR Spectrum of 1bromo4ethoxybenzene 350 397 l I BrQOCH2CH3 3 7 4 l l l l l x m u w w l a 9 Chemistry 318N I THE pDISUBSTITUTED PATTERN CHANGES AS THE TWO GROUPS BECOME MORE AND MORE SIMILAR All peaks move closer Outer peaks get smaller and nally disappear Inner peaks get taller and nally merge lllli xa v XX XX Chemistry 318N all H equivalent Chemistry 318N I Phenols o The functional group of a phenol is an OH group bonded to a benzene ring O H o H CH3 Phe no 3Me thylphe no m Cresol Chemistry 318N if Acidity of Phenols o Phenols are much more acidic than aliphatic alcohols that also contain the OH group Phenol pK a 995 OOH HZOamp O39 H30 Ethanol pKa 159 CH3CH20H HZOCH3CH2039 H30 o delocalization of the negative charge by resonance stabilizes the phenoxide ion relative to the alkoxide ion Chemimy 318N l Acidity of Phenols QOH OH39 gt 0039 H20 pKa 157 pKa 995 Strong Strong Weak Weak acid b as e b as e acid Strong acids have weak conjugate bases Stabilization of anions leads to weakly conjugate bases Chemistry 318N D s Lets move electrons together PLEASE FOLLOW ME STEP BY STEP Draw this gt Chemimy 318N I Acidity of Phenols o Alkyl and halogen substituents effect acidities by inductive effects 7 alkyl groups are electronreleasing by induction 7 halogens are electronwithdrawing by induction OH OH OH OH OH CH3 i CI CI CH3 Phenol pKa 995 m C hloro p C hloro phenol phenol pKa 885 pKa 918 Chemistry 318N D s m C resol p Cresol pKa 1001 pKa 1017 Acidities of Phenols Nitro groups increase the acidity of phenols by both an electronwithdrawing inductive effect and a resonance effect OH OH OH N02 N02 Phenol m Nitrophenol p Nitrophenol pKa 995 pKa 828 pKa 715 Chemimy 318N I Acidities of Phenols 7 Part of the acidstrengthening effect of N02 is due to its electronwithdrawing inductive effect 7 In addition NO2 substituents in the ortho and para positions help to delocalize the negative charge by Resonance 208 delocalization of negative charge onto oxygen Chemistry 318N D s Acidity of Phenols 7 Most phenols do not react with weak bases such as NaHCO3 they do not dissolve in aqueous NaHCO3 Q OH NaHCOa lt2 lt 039 Na H2C03 Sodium 11 13195 bicarbonate Sodium phe noxide C3302 2 p a 39 Weaker base Stronger base p a Weaker acid Stronger acid Chemimy 318N I Flow chart for separation of cyclohexanol and phenol OH OH Q E Dksolve in dierhyl elher Mix will 01M NnDH lilher layer containing 39 Aqueous layer cnnuaiuuing cydnhrxanol sudium pllennxid Cy luluMum Pllvnul Clumixtty 3 MN I Finally Chemistry AB gtC Clumixtty 3 MN I Synthesis AlkyIAryl Ethers o Alkylaryl ethers can be prepared by the Williamson ether synthesis 7 but only using phenoxide salts and alkyl halides 7 aryl halides are unreactive to SNZ reactions QX RO39Na no reaction Chemistry 318N D s AI kyIAryl Ethers c1 00 6 gt CV Sn2 reactions are accelerated by polar aprotic solvents phase transfer catalysis crown ethers Remember methyl gt 10 gt20 and 30 is ano go Review chapter 8 Chemimy 318A T Reactions at Benzyl Carbons Chemistry 318N D s Benzylic Reactions 0 Benzylic radicals and cations are easily formed because of the resonance stabilization of these intermediates 7 the benzyl radical is a hybrid of five contributing structures Chemimy 318N I Benzylic Bromination o Bromination is by a radical mechanism Br OV NBS CCJ4 A Regioselective reaction Slow Bro O N O O N O l 0 Br NBS Nbromosuccinimide Chemistry 318N D s Flash Card Tricks 0 front Br 0 Back HNBS ch Chemimy 318N I Benzylic Oxidation o Benzene is unalTected by strong oxidizing agents such as H2CrO4 and KMnO4 7 halogen and nitro substituents are also unaffected by these reagents 7 alkyl groups with at least one hydrogen on the benzylic carbon are oxidized to a carboxyl group CI CI 02 N K2 CrO Z 02 N H2 304 2 Chlor0 4 nitr0 2 Chlor0 4 nitr0 toluene benzoic acid Chemistry 318N if Benzylic Oxidation if there is more than one alkyl group on the benzene ring each is oxidized to a C02H group ll CH c CE 3 KzCrzO7 CE OH H HSO CHfCHg Z 4 cOH o o l KZCrZO7 OH H st04 ch CH3 Chemimy 318N l Electrophilic Aromatic Substitution o Electrophilic aromatic substitution a reaction in which a hydrogen atom of an aromatic ring is replaced by an electrophile 0 We study 7 several common types of electrophiles 7 how they are generated and 7 the mechanism by which they replace hydrogen is the same for all Cltmutry39 51m Y Electrophilic Aromatic Substiution PE diagram for the reaction If benzene with bromine Energy Unfavored step addition of BIquot Favored slep elimination of H y QBr HBr ChemiwryIIXN Y Electrophilic Aromatic Substitution H GAL H E Please be sure that you can do this and that it makes sense to you 1 a at E Chemistry 318N D s The Energetics H H HQB H H H Y Chemistry 318N Nitration N02 st04 gt HNo3 H u I H o Noz Ho 303 H q 5 H O N02 H804quot Nitric acid H H I I H fo Noz F B H o em 18N Y Chlorination o Chlorination requires requires a Lewis acid catalyst such as AlCl3 or FeCl3 Step 1 formation of a chloronium ion CI I gl c m le C lt2 chloronium Cl ion CI C l amp Cl eC394 CI Chemistry 318N I 20 Sulfonation Q H 03 mgtgt 803H Benzenesulfonic acid H O SO Sulfonation can be reversed by Heating in H20 Chemistry 318N I The FriedelCrafts Reaction Circa I 8 77 Charles Friedel James Craft Making CC bonds is a deal Chemistry 318N I 21 FriedelCrafz s Alkylaz ion CH CH3CH2C1 2 CH3 1gt A1013 ICI R lAII CI Cl 39 R CEI Al Cl IAICM l Anionpair containing a carbocation Chemistry 318N if A word about the FriedelCrafts Alkyation J I Carbocation rearran ements are common au Ionquot g CH3 I AICI 3 Q CH3 CHCH2 Cl gt G CCH33 Hc Benzene Isobutyl chloride tert Butylbenzene Chemimy 318N I Role ofACl3 Acts as a Lewis acid to promote ionization of the alkyl halideB CH33c c l AICI3 CH33C 5 AICI3 CH33C cl AICI3 Chemimy318NW FriedelCrafts Alkyation 2 They are tough to stop Product is more reactive than the starting material CH3CH2C1 em A1013 Chemimy 318N I 23 FriedelCrafts Alkyation Gautiohilllljl alkylation fails on benzene n39ngs bearing one or more strongly electronwithdran groups o o o 0 H CH CR COH COR CNH2 SO3H cN No2 NR3 CF3 CCI3 Chemimy318NNirf F riedel C rafts Acylatl39on o o Om REX gt QJER HX An acylbenzene I ICI 39IE IC n RC lzmAlcl 2 R If II a mequot l Cl Anionpalr Cl conmining an acylium ion Chemimy 318N I FriedelCrafts Acyation 0 An acylium ion is a resonance hybrid of two major contributing structures RCO lt gt RCEO w 7 FC acylations are free of ONE major limitation of FC alkylations acylium ions do Lot rearrange 7 They still do not work on deactivated Rings 7 They stop after one substitution Chemistry 318N D s amp a E H Y Electrophiic aromatic substitutions include Nitration Sulfonation Halo genation FriedelCrafts Alkylation FriedelCrafts Acylation Chemimy 318N I 25 Hydrogenolysis O CH3COC1 H2 gt gt A1C13 Pdc There are two nice tricks hidden here Please be sure to remember this reaction Chemistry 318N D s Di and Polysubstitution Existing groups on a benzene ring in uence further substitution in both orientation and rate Orientation 7 certain substituents direct preferentially to ortho amp para positions others direct preferentially to meta positions 7 substituents are classified as either orthopara directing or meta directing Chemimy 318N I 26 Di and Polysubstitution o Iate certain substituents cause the rate of a second substitution to be greater than that for benzene itself others cause the rate to be lower substituents are classi ed as activating toward further substitution or deactivating Chemistry 318N D s Di and Polysubstitution o OCH3 is orthopara directing and activating OCH3 OCH3 0CH3 1 B G 3 D HBr CH 3 CO 2 H Anisole Br o Bromo p Bromo anisole anisole 4 96 Chemimy 318N I Di and Polysubstitution 0 N02 is meta directing and deactivating NO 2 HNO 3 D NO 2 NO 2 NO 2 Nitro Q 1 No 2 benzene NO 2 NO 2 mDinitro 0Dinitl 0 p Dinitro benzene benzene benzene J 93 Less than 7 combined Chemistry 318N D s Methyl Group CH3 Toluene undergoes nitration 1000 times faster than benzene A methyl group is an activating substituent Chemimy 318N I 28 Trifluoromethyl Group CF3 Tri uoromethylbenzene undergoes nitration 40000 times more slowly than benzene The tri uoromethyl group is a deactivating substituent Chemimy 318N E Relative rates of Nitration 1000 10 0033 6XlO398 w Chemimy 318N I 29 Effects of substitution on further electrophilic aromatic substitution Real Slow Strungly 41111 N391ut QH R931 Fast attivattng o 0 0 ii i ll V I Modmteiy NHCR NHCAr 9R 9cn activating a t a Weakly R lt gt 9 activating Ev 2 Weakly Cl 43 I O deactivating In Moderately I ll ll l g deactivating CH CR cou cou my 5 Strongly 2n deactivating No Nut 1F CC3 Chanixtty 318N Tr Di and Polysubstitution Some observations 0 Alkyl groups phenyl groups m all groups in which the atom bonded to the ring has an unshared pair of electrons are orthopara directing All other groups are meta directing o All orthopara directing groups except the halogens are activating toward lrther substitution The halogens are weakly deactivating Chemimy318N Y Effect on Regioselectivity Orthopara directors direct an incoming electrophile to positions ortho andor para to themselves Meta directors direct an incoming electrophile to positions meta to themselves All meta directors are deactivating All orthopara directors are activating except halogen Chemistry 318N D s Theory of Directing Effects So what s going on here The rate of EAS is limited by the slowest step in the mechanism duh For EAS the ratelimiting step is attack of E on the aromatic ring to form a resonancestabilized cation intermediate The more stable this cation intermediate the faster the rate limiting step and the faster the overall reaction Chemimy 318N I 31 Adding a Second Substiuent Inela Illzlck OCHJ OCHJ OCH OCH3 0cm 901 CPR r QMQH gt s ow N0 N0 Noz h N03 para alluck ocHJ 139H3 CH3 OCH CH3 OCH ow bb slow 25 H 3 NO H No H N02 H NO The mos impo colllrihuling struclure N Methoxy is is therefore an op director Chemimy 318N Y Adding a Second Substiuent nlelu attack 0 Mol N02 N01 Noz l01 l gt H4 b H4 gt H gt sow 1 No2 No2 No2 5 NO puru ulluck No2 No2 N01 NoZ N02 o 0 shm quot39 I39asl HM HNoZ H N02 N02 h most uvored contributing S ucmre N1tro 1s therefore a meta director Chemimy318N V 32 Di and Polysubstitution CH3 COZH HN03 D K20r207 H2504 H2804 CH3 N02 NO2 p Nitrobenzoic co H co H K20quot207 0 H2804 D H2 504 N02 m Nitrobenzoic Chemimy318N 5 l ortho Nitration of Toluene CH3 No2 H H Chemimy 318N I 33 ortho Nitration of Toluene CH3 CH3 N02 N02 H H H H H H H H Chemimy318N D s ortho Nitration of Toluene CH CH 3N02 3N02 3N02 H H H H H H H H H H H H H this resonance form is a teltialy carbocation Chemimy 318N I 34 H H ortho Nitration of Toluene CH3 CH3 N02 N02 H H H H H H H H CH3 No2 H H H H H the ratedetermining intennediate in the 01tho nitration of toluene has teItiag carbocation character Chemistry 318N D s meta Nitration of Toluene CH3 H H NO H 2 Chemimy 318N I 35 meta Nitration of Toluene CH3 CH3 H H H H H H NO NO H 2 H 2 Chemimy318N 5 meta Nitration of Toluene CH3 CH3 CH3 H H H H H H H H H H NO NO NO H 2 H 2 H 2 all the resonance forms of the ratedetermining intermediate in the meta nitration of toluene have their positive charge on a secondary carbon Chemimy 318N I 36 Nitration of Toluene Interpretation 0 The ratedetermining intermediates for ortho and para nitration each have a resonance form that is a tertiary carbocation All of the resonance forms for the ratedetennining intermediate in meta nitration are secondary carbocations Tertiary carbocations being more stable are formed faster than secondary ones Therefore the intermediates for attack at the ortho and para positions are formed faster than the intermediate for attack at the meta position This explains why the major products are 0 and pnitrotoluene Chemistry 318N WT Nitration of Toluene Partial Rate Factors 0 The experimentally determined reaction rate can be combined with the orthometapara distribution to give partial rate factors for substitution at the various ring positions 0 Expressed as a numerical value a partial rate factor tells you by how much the rate of substitution at a particular position is faster or slower than at a single position of benzene Chemimy 318N I 37
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