Organic Chemistry II
Organic Chemistry II CHEM 314
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Date Created: 09/28/15
Chapter 17 Carboxylic Acids and Their Derivatives Nucleophilic Addition Elimination at the Acyl Carbon Created by Professor William Tam amp Dr Philis Chang Ch 17 1 Abogt The Authors These PowerPoint Lecture Slides were created and prepared by Professor William Tam and his wife Dr Phillis Chang Professor William Tam received his BSc at the University of Hong Kong in 1990 and his PhD at the University of Toronto Canada in 1995 He was an NSERC postdoctoral fellow at the Imperial College UK and at Harvard University USA He joined the Department of Chemistry at the University of Guelph Ontario Canada in 1998 and is currently a Full Professor and Associate Chair in the department Professor Tam has received several awards in research and teaching and according to Essential Science Indicators he is currently ranked as the Top 1 most cited Chemists worldwide He has published four books and over 80 scientific papers in top international journals such as J Am Chem Soc Angew Chem Org Left and J Org Chem Dr Phillis Chang received her BSc at New York University USA in 1994 her MSc and PhD in 1997 and 2001 at the University of Guelph Canada She lives in Guelph with her husband William and their son Matthew Ch 172 1 Introduction Carboxylic Acid Derivatives i Rig Rick R OH carboxylic acid acid chloride acid anhydride 0 i RAOR39 R NR392 ester amide Ch 17 3 2 Nomenclature and Physical Properties st Nomenclature of Carboxylic Acids and Derivatives o Rules 0 Carboxylic acid as parent suffix ending with oic acid 0 Carboxylate as parent suffix ending with oate Ch 17 4 9999 Most anhydrides are named by dropping the word acid from the name of the carboxylic acid and then adding the word anhydride Acid chloride as parent suffix ending with oy chloride Ester as parent suffix ending with oate Amide as parent suffix ending with amide Nitrile as parent suffix ending with nitrile II Ch 17 5 3 Examples 0 JKOCH3 Methyl propanoate 0 km Ethanoic acid acetic acid 0 O O iok A NH39Z Ethanoic anhydride acetic anhydride Ethanamide Ch 17 6 3 Examples 0 N 0 C C39 09 N Sodium benzoate Benzoyl chloride H3C CEN Ethanenitrile Ch 17 7 2C Acidity of Carboxylic Acids pKa 45 00 Compare 0 pa of HZCO3 7 0 99 Of HF N 3 Ch 17 8 a When comparing acidity of organic compounds we compare the stability of their conjugate bases The more stable the conjugate base the stronger the acid CH3COOH CH3CH20H 79 475 16 Ch 17 9 0 n W o H26 A J H 0 O H CH3 09 3 A1 B1 CH3 CH3CH2 O H H26 CH3CH2 Oe H3O A2 82 Ch 17 10 a The conjugate base B1 is more stable the anion is more delocalized than B2 clue to resonance stabilization 5 0 09 s 0 ie x CH3 0 CH3 0 CH3 0 o Thus A1 is a stronger acid than A2 Ch 17 11 Acidity of Carboxylic Acids Phenols and Alcohols H H o o 0 pa 420 Mg 10 pa 17 Ch 17 12 Acidity of Carboxylic Acids Phenols and Alcohols 0 gm 40039 H26 H3o 9 Ch 17 13 Acidity of Carboxylic Acids Phenols and Alcohols oilH v09 g 115 l H26 7 94 Ch 17 14 a Acidity of Carboxylic Acids Phenols ancl Alcohols OjHA O9 H26 k H30 NO resonance stabilization Ch 17 15 Question 3 If you are given three unknown samples one is benzoic acid one is phenol and one is cyclohexyl alcohol how would you distinguish them by simple chemical tests 0 Recall acidity of Ch 17 16 O a e O AAH Na OH A 9 G H20 R O R O Na soluable in water 9 3 OH O Na soluble in water OI 0 NaOH gt No Reaction immiscible W39th H20 Ch 17 17 O O H e o O Na9 NaHCO3 gt OH O NaHCO3 gt OH 0 NaHCO3 gt C02g H20 gas evolved No Reaction No Reaction Ch 17 18 o o o 0 CI gt CI gt CI gt H CI OH CI OH H OH H OH CI H H H pa 070 148 286 476 9 Stability of conjugate bases Cw Cw Cw w 0 gt O gt O gt 0 CI CI H H CI H H Ch 17 19 0 CI 0 Wf mi J OH CI 2Chlorobutanoic acid 3Chlorobutanoic acid mg23 mg4c 0 GM OH 4Chlorobutanoic acid pKa 43950 Ch 17 20 2D Dicarboxylic Acids PKa at 25 C Common Structure Name mp 0C pKI ng HOzC COZH Oxalic acid 189 dec 12 42 HOZCCHZCOZH Malonic acid 136 29 57 H02CCH24C02H Adipic acid 153 44 56 COzH G Phthalic acid 206208 dec 29 54 COzH Ch 17 21 2J Spectroscopic Properties of Acyl Compounds v IR Spectra o The CO stretching band occurs at different frequencies for acids esters and amides and its precise location is often helpful in structure determination o Conjugation and electrondonating groups bonded to the carbonyl shift the location of the CO absorption to lower frequencies Ch 17 22 6 IR Spectra o Electronwithdrawing groups bonded to the carbonyl shift the CO absorption to higher frequencies o The hydroxyl groups of carboxylic acids also give rise to a broad peak in the 25003100cm391 region arising from O H stretching vibrations The N H stretching vibrations of amides absorb between 3140 and 3500 cm391 Ch 17 23 Functional Group Acm ch unde Acm anhydnde Egemamane Cavboxyhc amd mdehyde Ketons Amwdy adam Cavbaxy alessh Figure 172 Approxwmcaybom w absovpn39on equencx39ea Fvequency vangeabaced on V J b n A w m r y m M19084 Ch 17 24 T0 60 50 40 Transmittance 91539 30 20 10 0 4000 1 3 v f I AL leumxx I hl Ifquot Iquot Ix n q iquot J II I 39139 quotI II j l II V L I OH 39639 X If I II 39I I39 I f I I l J l W l I I 39 1 fl I I 39 I I I 1 4 1 J1 a u l l f I 39 E39 I I I I I II II In I I 1 II II 39I 1 VA 039 1 39 39139 II t L11 II II 391 I Q H II II II 391 9 I outofplane bend J 0 39I I 0 H H 1quot 13 g 1 I c o H ll stretch Iquot stretch I Impleme hend l dimer L 7 stretch I l1 3ng stretch U 3000 3200 2800 2400 2000 1800 1000 1400 1200 1000 800 600 Wavenumber cm 1 Figure 173 The infrared spectrum of propanoic acid Ch 17 25 6 1H NMR Spectra o The acidic protons of carboxylic acids are highly deshielded and absorb far downfield in the 5 1012 region a The protons of the a carbon of carboxylic acids absorb in the 5 2025 region Ch 17 26 UGMB 5H ppm Figure 174 The BUG MHZ 1H NMR spectrum of methyl prapanoate Expansfans of the signals are Shawn in the offset plots Ch 17 27 6 13C NMR Spectra o The carbonyl carbon of carboxylic acids and their derivatives occurs downfield in the 5 160180 region see the following examples but not as far downfield as for aldehydes and ketones 5 180 220 o The nitrile carbon is not shifted so far downfield ancl absorbs in the 5 115120 region Ch 17 28 6 13C NMR chemical shifts for the carbonyl or nitrile carbon atom O O O 393 C H3C OH H3C 5 1772 5 1707 5 1703 0 C H3C CEN H3C NH2 8 1726 8 1174 Ch 17 29 3 Preparation of Carboxylic Acids 6 By oxidation cleavage of alkenes o Using KMnO4 1 KMnO4 OH heat WM 2 H3O OH K O W Ph 0 OH 0 Usmg ozonolySIs HO OH 103 O gt 2 H202 0 Ch 17 3O 6 By oxidation of aldehydes amp 1O alcohols o eg nH 1 A920 nOH gt 1 KMnO4 OH heat M MOH gt o 2 H3O O O H2CI O4 LH or OH gt OH Ch 17 31 6 By oxidation of alkyl benzene R 1 KMnO4 OH heat OH gt 2 H3O R 10 or 20 alkyl groups Ch 17 32 6 By oxidation of benzene ring a eg 3 I 1 03 CH3COOH 0 2 H O gt 39 2 2 OH Ch 17 33 6 By hydrolysis of cyanohydrins and other nitriles elgl o 0 NC HO 2 HCN OH H OH k gt X gt X Ph CH3 Ph CH3 H20 Ph CH3 0 B HCN CN H E r gt gt N N H20 heat OH Ch 17 34 6 By carbonation of Grignard reagents o eg Br Mg gt EtZC MgBr Q 1C02 2H3O lt O OH QF Ch 17 35 4 Acyl Substitution Nucleophilic AdditionElimination at the Acyl Carbon 0 09 A NU ANN R Y R 4w e O A Y A R Nu Y leaving group eg OR NR2 CI oz This nucleophilic acyl substitution occurs through a nucleophilic additionelimination mechanism 0117 35 st This type of nucleophilic acyl substitution reaction is common for carboxylic acids and their derivatives 1 Rio Rick R OH carboxylic acid acid chloride acid anhydride 0 i RAOR39 R NR392 ester amide Ch 17 37 6 Unlike carboxylic acids and their derivatives aldehydes amp ketones usually do not undergo this type of nucleophilic acyl substitution due to the lack of an acyl leaving group A good 0 leaving 0 0 A group A A R Y R H R Rl a carboxylic acid 39 derivative Not a good leaving group cn 17 3s 6 Relative reactivity of carboxylic acid derivatives towards nucleophilic acyl substitution reactions c There are 2 steps in a nucleophilic acyl substitution 0 The addition of the nucleophile to the carbonyl group O The elimination of the leaving group in the tetrahedral intermediate Ch 17 39 o Usually the addition step the first step is the ratedetermining step rds As soon as the tetrahedral intermediate is formed elimination usually occurs spontaneously to regenerate the carbonyl group a Thus both steric and electronic factors that affect the rate of the addition of a nucleophile control the reactivity of the carboxylic acid derivative Ch 17 4O o Steric factor eg O reactivity of A gt Cl Cl o Electronic factor Q The strongly polarized acid derivatives react more readily than less polar ones Ch 17 41 o Thus reactivity of o oo o o AgtJKJKgtJK gtJk o R o R39 R 0R R NR2 R most least reactive reactive a An important consequence of this reactivity Q It is usually possible to convert a more reactive acid derivative to a less reactive one but not vice versa cm 17 42 5 Acyl Chlorides 5A Synthesis of Acyl Chlorides 00 Conversion of carboxylic acids to acid chlorides O A 1 R OH R CI 0 Common reagents O SOCI2 O COCI2 O PCI3 or PCI5 Ch 17 43 o Mechanism 0920 o e M AI gt A COZCOCI R o R CI CI 0 Ch 17 44 6 Nucleophilic acyl substitution reactions of acid chlorides o Conversion of acid chlorides to carboxylic acids Ch 17 45 AH 9 O C m R m o m H2 e M G gt O Ch 17 46 o Conversion of acid chlorides to other carboxylic derivatives 0 R39OH A ester pyridine R OR39 0 O R39 NH A 2 gt A amide R CI R NR392 O WAGS N a O O gt acid anhydride R O R39 Ch 17 47 6 Carboxylic Acid Anhydrides 6A Synthesis of Carboxylic Acid Anhydrides O O AJJI N OH R CI R O O O O A e G A A JL Na Ce R O Na R39 CI R O R39 O 0 OH 0 300 c O H20 OH 5 O UCCnC O Succinic acid anhydride 0 0 OH 0 gt230 C O H20 OH Phthalic Phthalic anhydride 0 acid 0 100 Ch 17 49 GB Reactions of Carboxylic Acid Anhydrides 00 Conversion of acid anhydrides to carboxylic acids Ch 17 50 o Mechanism Ch 17 51 6 Conversion of acid anhydrides to other carboxylic derivatives ROH O O ii R OR39 R OH UL A R O R Cgt R239NH R NR2 R O NR392H2 Ch 17 52 7 Esters 7A Synthesis of Esters Esterification H O R39OH gt A H20 R OR39 o RAOH Ch 17 53 Mechanism o 9 Ch 17 54 Esters from acyl chlorides eg 0 Cl EtOH 3939 Benzoyl chloride 0 OEt e Cl B N Ethyl benzoate 80 H Ch 17 55 Esters from carboxylic acid anhydrides eg O O O OH U k O Acetic Benzoyl anhydride alcohol i0 0 Aw Benzoyl acetate Ch 17 56 7B BasePromoted Hydrolysis of Esters Saponification 00 Hydrolysis of esters under basic conditions saponification o OH o gt A R39OH H20 R 09 k R OR39 Ch 17 57 Mechanism o 9 R39OH Ch 17 58 Hydrolysis of esters under acidic conditions A gt A H20 Ch 17 59 Mechanism e H O H lt0 OH H V H A R Ch 17 60 7C Lactones zo Carboxylic acids whose molecules have a hydroxyl group on a y or 5 carbon undergo an intramolecular esterification to give cyclic esters known as y or 5lact0nes Ch 17 61 a 5hydroxyacid 6 T A H c H 0 H a 5Iactone Ch 17 62 Lactones are hydrolyzed by aqueous base just as other esters are O O H H O O 2 C6HSWUO HA slight excess C6H5 OH 0 C 0 HA exactly 1 equiv C H 6 SMOH OH Ch 17 63 8 Amides BB Amides from Acyl Chlorides R CI RJFQ quot H N Rquot NHR39Rquot FL 0 0 quot CI R39RquotNH2 inRquot lt i 9 R l 8C Amides from Carboxylic Anhydrides O O HRl A JL 2 I R o R Rquot o o H C RI RAN Rio FL I Rquot R39 Rquot can be H alkyl or aryl Ch 17 65 H20 NH2 0 2 NH3 gt warm 0 NH4 O Phthalamic O 0 Ammonium anhydride phthalamate 940 NHZ H30 0 OH NH4 Phthalamic acid 81 0 Ch 17 66 0 0 NH 0 2 150 160 c NH OH O H20 0 Phthalamic acid Phthalimide 100 Ch 17 67 8D Amides from Esters o o HR39 A II R I RIIIOH R OR39quot Rquot Rquot R39 andor Rquot may be H eg o 0 Me OMe MeNHz N gt I heat H MeOH Ch 17 68 8E Amides from Carboxylic Acids and Ammonium Carboxylates O O A NH3 2 A R OH R o NH4 heat 0 H20 L R NH2 Ch 17 69 DCC Promoted amide synthesis 0 O 1 DCC A I A R39 DCU R OH 2RNH2 R I H Ch 17 7O Mechanism 1 N O39k R N C6H11 R C E cQ C 39O H NP H o 5 N C6H11 C6Hll Dicyclohexyl carbodiimide R lN C6H11 C O C quot H Q C6H11 Ch 17 71 Mechanism Cont d O quot quot R N C6H11 proton C N C6H11 C39QC t fgt R 9 9 HQ lila rans er NHC6H11 C6H11 reactive intermediate 0 H2 NHC6H11 R l O INHC6H11 R C CC NHR39 iHC6H11 5 N c H ltgt 6 11 R39 NH2 RICO ltl N I an amide NN39Dicycohexyurea DCU Ch 17 72 8F Hydrolysis of Amides zo Acid hydrolysis of amides O H 3 A NH4 R NH2 H20 heat R OH Ch 17 73 Mechanism H R NHZ R NHZ RNH2 H O TH Ch 17 74 a Basic hydrolysis of amides 0 OH 0 A A 9 NH3 R NHZ H20 heat R 0 Ch 17 75 Mechanism 9 ill 90H 0 0 A A RNH A H 2 R NH 2 R o 2 HO 2 O NH3 A R 09 Ch 17 76 8G Nitriles from the Dehydration of Amides 0 P4010 or CH3CO20 JV h t gt R CEN H3PO4 R NH2 ea or CH3C02H H20 a nitrIIe ozo This is a useful synthetic method for preparing nitriles that are not available by nucleophilic substitution reactions between alkyl halides and cyanide ions Ch 17 77 CEN dehydration Ch 17 78 3 Example o Synthesis of MCEN NaCN MBr DMSO i 10 alkyl bromide gt 5N2 reaction with eCN works fine MCN Ch 17 79 CN But synthesns of X XBI39 No Reaction DMSO 30 alkyl bromide gt No SNZ reaction Ch 17 80 Solution 0 2 coz 3 H30 2 NH3 CN X P4010 dehydration Ch 17 81 8H Hydrolysis of Nitriles 0 base or acid HEN A H20 heat R OH ozo Catalyzed by both acid and base Ch 17 82 3 Examples H SO OH CN gt W H20 A o 82 OH CN gt 1 NaOH H20 A o 2 H30 68 Ch 17 83 otonated n39t 39Ie Mechanism pr A 39 r39 f e W I R CENH 4 R CNH Q H i39 V W H 1W amide tautomer H H H H HO la 0 w o 2 ltro H I H o H I IC C quot C RaINHZ V R NH R NH 6 t gtprotonated amide H e 1 several steps 0 a El amide h drol sis A NH4 R NHZJ y y R OH Ch 1784 mwth 0 O O 0 m 0 00 2 J NTil Ia Iolz O lt a IloI Go A 0 zoo 0 IoII We N IO 60 Ilt oI oo 0 m 0 m Ila 2 El 2 4 o EmEmcumz A 81 Lacta ms 0 O 0 0i NH NH 3 y y B a BIactam a yIactam a 6Iactam H R C6H5CH2 Penicillin G I RnN 5 CH3 R C6HSCH Ampicillin 0 lN CH lllH o lt 3 2 C02H R C6H50CH2 Penicillin v Ch 17 86 9 Derivatives of Carbonic Acid 9A Alkyl Chloroformates and Carbamates Urethanes oz Alkyl chloroformate o o ROH A gt A HCI CI CI RO CI alkyl chloroformate Ch 17 87 OH O Jk RO CI 0 0km HCI Benzyl chloroformate Ch 17 88 Carbamates or urethanes O 0 JJ R39NHZ gt A OH RO CI RD N H R39 a carbamate or urethane Ch 17 89 rotected amine u Protection P A O 6 g O Deprotection gt CO 0 I H2 Pd R NHZ 2 Q WAD H HBr CHZCOZH r o A R A 0 CI OH N R NH2 gt ll Br 9 gt R NH3 C02 Ch 17 9O 10 Decarboxylation of Carboxylic Acids O decarboxylation A gt R H C02 R OH O O loo150 C O UU gt C02 R OH R A Bketo acid Ch 17 91 3 There are two reasons for this ease of decarboxylation H H O O O O CO m 2 x r k R O R R Bketo acid enol ketone O 0 C02 0 HA 0 Ke k R R R acylacetate ion be resonancestabilized anion R Ch 17 92 11 Chemical Tests for Acyl Compounds Recall acidity of Ch 17 93 O a e O AAH Na OH A 9 G H20 R O R O Na soluable in water 9 3 OH O Na soluble in water OI 0 NaOH gt No Reaction immiscible W39th H20 Ch 17 94 O O H e o O Na9 NaHCO3 gt OH O NaHCO3 gt OH 0 NaHCO3 gt C02g H20 gas evolved No Reaction No Reaction Ch 17 95 12 Polyesters and Polyamides StepGrowth Polymers Polyesters O O W mm H0 n OH m I iHZO V i f QWOWS L m a polyester Ch 17 96 Polyamides m I H i HC V H O O n m a polyamide Ch 17 97 3 Example Nylon 66 O WNH n IIOJKWOH n HZN 2 O Jheat I O H Nylon 66 n o Applications clothing fibers bearings Ch 17 98 3 Example Dacron Mylar O O n gt lt HoOH CH3O OCH3 2n CH3OH O n Dacron o Applications film recording tape Ch 17 99 13 Summary of the Reactions of Carboxylic Acids and Their Derivatives Reactions of carboxylic acids 8 O C e H 1 P x2 R O R COH 2 H20 Y NaOH or NaHCO3 or other bases 0 o RCHZOH 4 k gt E 1 LAH4 R OH R39OH H A R OR 2 H20 H SOCI o 2 o C C CI or PC5 RCC base Ch 17 100 3 Reactions of acyl chlorides C C JL k R NR R DH lt HZC R NH C RCJL CI R39OH base RCOOH base R O R R OR Ch 17 101 3 Reactions of acyl chlorides Cont d o R RAOH be nk All 394 AICI 2 H 0 3 O 3 RAG 1 LiAIHOtBu3 78 C 2 H3O OH 1 R39MgX o l 2 H 0 A RJFR 3 R H RI Ch 17 102 3 Reactions of acid anhydrides O O Ch 17 103 3 Reactions of esters o A o RAOH R H A 1 DIBAL 78 C R 0 2 H3O 1 LIAH4 4w A 2 H3O O O O A lt A RRll 1 R R ORI 1 R Ru 239 2 H O NH3 RquotOHHN O RiNHZ A R ORquot Ch 17 104 3 Reactions of nitriles O RANHZ A R OH 1 LiAH4 HI H20 A 2 H3O R CEN OH H20 A 1 LiAIHOtBu3 O or DIBAL 78 C 0 A 2 H30 A e R H Ch 17 105 3 Reactions of amides O A HNR392 R OH H20 H or 0H P4010 P205 or ACZO D o 1 LiAIH4 I I 2 H R H only A R 30 1 w R CEN R NR39Z Ch 17 106 8 END OF CHAPTER 17 8 Ch 17 107 Chapter 13 Conjugated Unsaturated Systems Created by Professor William Tam amp Dr Philis Chang Ch 13 1 Abogt The Authors These PowerPoint Lecture Slides were created and prepared by Professor William Tam and his wife Dr Phillis Chang Professor William Tam received his BSc at the University of Hong Kong in 1990 and his PhD at the University of Toronto Canada in 1995 He was an NSERC postdoctoral fellow at the Imperial College UK and at Harvard University USA He joined the Department of Chemistry at the University of Guelph Ontario Canada in 1998 and is currently a Full Professor and Associate Chair in the department Professor Tam has received several awards in research and teaching and according to Essential Science Indicators he is currently ranked as the Top 1 most cited Chemists worldwide He has published four books and over 80 scientific papers in top international journals such as J Am Chem Soc Angew Chem Org Left and J Org Chem Dr Phillis Chang received her BSc at New York University USA in 1994 her MSc and PhD in 1997 and 2001 at the University of Guelph Canada She lives in Guelph with her husband William and their son Matthew Ch 13 2 1 Introduction 3 A conjugated system involves at least one atom with a p orbital adjacent to at least one 71 bond 0 ELg 7gt 59 50 455d3 5 3 conjugated aHyHc aHyHc aHyHc diene radical cation anion O en0 enyne Ch 13 3 2 Allylic Substitution and the Allyl Radical vinylic X carbons X2 spz gt 1 low temp k CCI4 X f xz gt allylic high temp A H X carbon and low conc x 593 of X2 Ch 13 4 2A Allylic Chlorination High Temperature 400 C CI HCl A CIZ gas phase W Ch 135 Mechanism a Chain initiation AA Cl CI gt 2039 o Chain propagation Ah NH 039 gt gt H CI 9 F gt allylic radical 0113 6 Mechanism o Chain propagation A CI I gt W0 CI 0 Chain termination h Ar 039 gt W0 Ch 13 7 DHO 369 kJmoI391 H gt A H DHO 465 kJmoI391 Ch 13 8 rvx Eact HA XvWHX Relative stability of radicals allyllc gt 3 gt 2 gt 1 gt VlnyC Ch 13 9 OH MH zU TE 2 mg m in an n 55 TE 3 me u in 1 Emma TE 2 LE 3 an n in m E H 92D 1 Emma am Emma an I a Kmxurxf 1 F165 3 mmm H aim d Ema Ea Ifmmnx ZB Allylic Bromination with NBromo succinimide Low Concentration of Brz NBS is a solid and nearly insoluble in CCI4 a Low concentration of Br lr H O N O v hv or ROOR NBS heat CC4 l39 Br O N O v Ch 13 11 3 Examples Br NBS gt ROORCCM heat NBS z gt z ROORCCM Br heat Ch 13 12 3 The Stability of the Allyl Radical 3A Molecular Orbital Description of the Allyl Radical 1 All carbons are Sp hybridized sp3 Hleridized Hydrogen abstraction Transition state Allyl radical delocalized Ch 13 13 gt Thvee lsomked parmms Wm an e ecunn in each Norm mmms w 39 1 CH 1 Ammundmg Hzc K 77 mm 1 Natl Num i CH J Nonbunding r20 x M m 1 Hana CH Eundm l l JL g 33920 2 my mum Schemmm ma ecu ar mums Stimulated mo ecular mm W V Ch 13 14 3B Resonance Description of the Allyl Radical Ch 13 15 4 The Allyl Cation a Relative order of Carbocation stability 669 gt k gt AGE 3 allylic 3 allylic gt V gt gt 63 639 2 1 vinylic Ch 13 16 5 Resonance Theory Revisited 5A Rules for Writing Resonance Structures 9 O O O Resonance structures exist only on paper Although they have no real existence of their own resonance structures are useful because they allow us to describe molecules radicals and ions for which a single Lewis structure is inadequate We connect these structures by double headed arrows lt gt and we say that the hybrid of all of them represents the real molecule radical or ion Ch 13 17 a In writing resonance structures we are only allowed to move electrons M N6 J V resonance structures H J V not resonance structures Ch 13 18 a All of the structures must be proper Lewis structures Jng Jaci 10 electrons W not a proper Lewis structure Ch 13 19 a All resonance structures must have the same number of unpaired electrons Ch 13 20 a All atoms that are part of the delocalized nelectron system must lie in a plane or be nearly planar delocalization of nelectrons delocalization of nelectrons Ch 13 21 a The energy of the actual molecule is lower than the energy that might be estimated for any contributing structure 3 Equivalent resonance structures make equal contributions to the hybrid and a system described by them has a large resonance stabilization Ch 13 22 a The more stable a structure is when taken by itself the greater is its contribution to the hybrid ema 3O ayic cation 20 ayic cation greater contribution Ch 13 23 SB Estimating the Relative Stability of Resonance Structures o The more covalent bonds a structure has the more stable it is 7H gt9 more stable less stable 9 W3 gt O more stable less stable Ch13 24 3 Structures in which all of the atoms have a complete valence shell of electrons ie the noble gas structure are especially stable and make large contributions to the hybrid 1 3 this carbon has this carbon has 6 electrons 8 electrons Ch 13 25 3 Charge separation decreases stability ll J Me eAgMe more stable less stable Ch 13 26 6 Alkadienes and Polyunsaturated Hydrocarbons a Alkadienes Dienes 2 4 1M 3 2 3 13Butadiene 14 2 4 6 6 5 1 W 13Cyclohexadiene 3 5 2E4E 24Hexadiene Ch 13 27 a Alkatrienes Trienes Z 4 6 8 W 1 3 5 7 2E4E6E Octa246triene Ch 13 28 a Alkadiynes Diynesquot 123456 24Hexadiynes a Alkenynes Enynes 6 5 4 1 3 5 6 7 8 W 3 2 2 4 HexlenSyne 2E Oct2en6yne Ch 13 29 Cumulenes enantiomers A F gtH I H CCC CCC b I 4 H H 39 H H Allene a 12diene Ch 13 3O a Conjugated dienes NW 3 Isolated double bonds WW Ch 13 31 7 13Butadiene Electron Delocalization 7A Bond Lengths of 13Butadiene 141M M4 1 3f 1343 3 3 2 3 Sp Sp if A V 154A 139503 13946 lg h1332 7B Conformations of 13Butadiene trans gt single L single bond bond R 239 cis scis strans 9 less stable Ch 13 33 7C Molecular Orbitals of 1 3Butadiene wwwwwww av nnnn Vs mummy nnnnn 5 F F F F I O nnnnnnnnnnnnnnnnnnnnnnn 5 Ch 13 34 8 The Stability of Conjugated Dienes a Conjugated alkadienes are thermodynamically more stable than isomeric isolated alkadienes AH kJmol391 2 W 2 H2 gt 2 N 2X 127254 N2H2 gt 239 Difference 15 Ch 13 35 2W 2H2 Difference I 15 kJ mutzrrl 1 254 kJ med 1 AHQ 239 kJ ml1 N Ch 13 36 9 Ultraviolet Visible Spectroscopy The absorption of UV Vis radiation is caused by transfer of energy from the radiation beam to electrons that can be excited to higher energy orbitals Ch 13 37 9A The Electromagnetic Spectrum m 1019 Hz 1015 Hz 10 3Hz Cosmic U UV NIR and Xrays Vacuum Near Visible Near Ing ed M39 gm39jve y rays ultraviolet ultraviolet infrared 01 nm 200 nm 400 nm 700 nm 2pm 50 pm Ch 13 38 9B UV Vis Spectrophotometers UV light source Diffraction grating g I I Mirror Slit Slit Finer mi e 77 Visible light source Mirror I Reference beam V In Reference Reference Lens de ecior Half mirror cuvette Mirror WIS Mirror Sample beam 1 Lens samp 9 Sample detector cuvette Ch 13 39 Absorbance Wavelength nm Ch 13 40 a Beer s law A chxz A absorbance 8 molar absorptivity or g A c concentration C X2 2 path length o eg 25Dimethyl24hexadiene kmaXmethanol 2425 nm a 13100 Ch 13 41 9C Absorption Maxima for Nonconjugated and Conjugated Dienes 71 4 w Antibonding I W gt molecular T 3 LUMO orbitals excrtatlon excitation J l I l g L B d39 P on Ing IL 2 HOMO gt molecular HOMO orbitals L A 1342 Acetone gtXlt 0 I i TI 0 Smax Ground state 7c Excited state 0 Vk f gt 75 Lmax nm Smax 71 gt 713 kmaX nm Smax Ch 13 43 9D Analytical Uses of UV Vis Spectroscopy ozo UV Vis spectroscopy can be used in the structure elucidation of organic molecules to indicate whether conjugation is present in a given sample o A more widespread use of UV Vis spectroscopy however has to do with determining the concentration of an unknown sample ozo Quantitative analysis using UV Vis spectroscopy is routinely used in biochemical studies to measure the rates of enzymatic reactions Ch1344 10 Electrophilic Attack on Conjugated Dienes 14 Addition 2 4 H CI 1N 25 C l Cl N HCI 7800 2200 12Addition 14Addition Ch 13 45 Mechanism H JV i lt9 C H gt H a b b b Ch 13 46 10A Kinetic Control versus Thermodynamic Control of a Chen calReac on Br o amp JV NE N 80 20 Br gt JVNBW 40 C 20 80 Ch 13 47 L 40 C HBr Br 12Addition 14Addition product product Ch 13 48 1 2 Addition 14 Addition Reaclion coordinate Ch 13 49 11 The Diels Alder Reaction A 14Cycloaddition Reaction of Dienes K 47c27c H gt diene dienophile adduct Ch 13 50 O benzene o gt 100 C 0 13Butadiene Maleic diene anhydride dienophile 0 go 0 Add uct 10096 Ch 13 51 11A Factors Favoring the DielsAlder Reaction Type A EDG EDG EWG EWG T Type B EWG EWG I W U EDG EDG a Type A and Type B are normal DielsAlder reactions Ch 13 52 EWG EWG EDG EDG V O I gt Type D EDG EDG I W EWG EWG o Type C and Type D are Inverse Demand DielsAlder reactions Type C Ch 13 53 a Relative rate O 30 C Diene I O gt DA cycloadduct O OMe Diene gt gt 4 h tlZ 20 min 70 min Ch 1354 a Relative rate 20 C Dienophile gt DA cycloadduct NC CN CN CN Dienophile I gt E gt f NC CN CN t12 0002 sec 20 min 28 h Ch 13 55 Steric effects Dienophile gt gt COO Et COOEt COOEt Relative rate 1 014 0007 Ch 13 56 113 Stereochemistry of the DielsAlder Reaction 1 The Diels Alder reaction is stereospecific The reaction is a syn addition and the configuration of the dienophile is retained in the producot H E H OMe J0Me gt H OMe I0Me H 1 O O Dimethyl maleate Dimethyl cyclohex4 ene a cis dienophile cis 12dicarboxylate Ch 13 57 O Dimethyl fumarate Dimethyl cycloheX a trans dienophile 4enetran5 12 dicarboxylate Ch 13 58 The diene of necessity reacts in the SCS rather than in the strans conformation s as Config uration s trans Config uration O O W H Highly strained Ch 13 59 diene locked in sCl39s conformation COOMe h f i No Reaction diene locked in strans conformation Ch 13 60 oz Cyclic dienes in which the double bonds are held in the scs conformation are usually highly reactive in the Diels Alder reaction oz Relative rate O 30 C Diene o gt DAcycloadduct O Diene lgt gt gt tlz 11 sec 130 sec 4 h Ch 13 61 3 The Diels Alder reaction occurs primarily in an endo rather than an em fashion when the reaction is kinetically controlled H H longest bridgegt R lt R is 8X0 H H H H R R is endo 1 Ch 13 62 AlderEndo Rule o If a dienophile contains activating groups with 7 bonds they will prefer an ENDO orientation in the transition state Ch 13 63 100 endo 0 Ch 13 64 Stereospecific reaction i X X W gt X X K IX X X quot X Ch 13 65 a Stereospecific reaction ii Y Y gt Y Y Y Y Y V Ch 13 66 3 Examples A Me Me NC CN CN I DA CN a CN NC CN CN B Me Me Me NC CN D A CgN Me I 4 0 C N NC CN CN Ch 13 67 Diene A reacts 103 times faster than diene B even though diene B has two electrondonating methyl groups Me Me Me Me gtltgtlt H SCS s trans Ch 13 68 3 Examples C o I O O D o 3 Examples E o DA I O gt No Reaction O 0 Rate of Diene C gt Diene D 27 times but Diene D gtgt Diene E o In Diene C tBu group 9 electron donating group 9 increase rate 0 In Diene E 2 tBu group 9 steric effect cannot adopt sCis conformation Ch 13 7O
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