Organic Chemistry II
Organic Chemistry II CHM 2211
University of Central Florida
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This 37 page Class Notes was uploaded by Ellen Braun on Thursday October 22, 2015. The Class Notes belongs to CHM 2211 at University of Central Florida taught by Jingdong Ye in Fall. Since its upload, it has received 52 views. For similar materials see /class/227648/chm-2211-university-of-central-florida in Chemistry at University of Central Florida.
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Date Created: 10/22/15
15 Benzene and Aromaticity Based on McMurry s Organic Chemistry 7th edition Aromatic Compounds Aromatic was used to describe some fragrant compounds in early 19th century Although incorrectly later they are grouped by chemical behavior unsaturated compounds that undergo substitution rather than addition Current distinguished from aliphatic compounds by electronic configuration ii CH CH3 Biological compounds Benzene Benzaldehyde Toluene 151 Sources and Names of Aromatic Hydrocarbons From high temperature distillation of coal tar Heating petroleum at high temperature and pressure over a catalyst o o Benzene Toluene Xylene lndene bp 80 C bp 111 6 bp ortho 144 c39C bp 182 C meta 139 0 para 138 C Naphthalene Biphenyl Anthracene Phenanthrene mp 80 c39C mp 71 Cl mp 216 Cl mp 101 Cl 9 2007 Thomson Higher Education Naming Aromatic Compounds Many common names toluene methylbenzene aniline aminobenzene Monosubstituted benzenes systematic names similar to other hydrocarbons parent benzene alkyl substituenz S6 carbons CGH5Br bromobenzene CGH5NO2 nitrobenzene and CsH5CH2CHZCH3 is propylbenzene Br N02 CH2CH2CH3 Bromobenzene Nitrobenzene Propylbenzene G 2007 Thomson Hiuher Education The Phenyl Group When a benzene ring is a substituent alkyl substituent 27 carbons the term phenyl is used for CaHs You may also see Ph or I in place of CGH5 Benzyl refers to CaH5CH2 1CH3 CHCHZCHZCHZCHZCH3 042 3 02 3 4 5 6 7 O A phenyl group 2Phenylheptane A benzyl group Disubstituted Benzenes Relative positions on a benzene ring ortho 0 on adjacent carbons 12 meta m separated by one carbon 13 para p separated by two carbons 14 0 2 2 H 10 H30 3 1 CH3 3 1 CH 2 4 Cl CI orthoDichlorobenzene metaDimethylbenzene paraChlorobenzaldehyde 12 disubstituted metaxylene 14 disubstituted 13 disubstituted o 20m Thomson Higher Education x CH3 CH3 Ortho gt lt Ortho Brz Describes reaction patterns Meta 39V39e a FEB T Br Para Toluene p Bromotoluene 6 a 21m Thurman Higher Education Naming Benzenes With More Than Two Substituents Choose numbers to get lowest possible values List substituents alphabetically with hyphenated numbers Common names such as toluene can serve as root name as in TNT 3 Br 4 2 1 4Bromo1Zdimethylbenzene rEducation 2007 Thomson Hiqhe CH3 CH3 oH 1 CH3 2 3 5 H3C 4 25Dimethylphenol 246Trinitrotoluene TNT Table 151 1 Common Names of Some Aromatic Compounds Structure Name Structum e Name CH3 Toluene CHO Benzaldehycle hp 111 1 0 bp 178 C OH Phenol COZH Benzoic acid mp 43 C mp 122 C NH2 Aniline CH3 orthoXylene G bp 184 C 0 bp 144 C CH3 0 Acetophenone H Styrene II mp 21 C bp 145 C OC OC H ZDD39J39 Thomson Higher Education Give IUPAC names a H3 Cng 39l CHCHZ CHQCHCHg d Br CH2CH2CH3 2007 Thomson Higher Education b e co H F N02 Br N02 c Br H3C CH3 MHZ CI 152 Structure and Stability of Benzene Molecular Orbital Theory Benzene reacts slowly with Br2 to give bromobenzene where Br replaces H This is substitution rather than the rapid addition reaction common to compounds with CC suggesting that in benzene there is a higher barrier H Br Br Fe O e O catalyst H Benzene Bromobenzene Addition product substitution product NOTformed 20 Thomson Higher Education Heats of Hydrogenation as Indicators of Stability Benzene 0 r 150 kJmol difference 13Cyclohexadiene Q 356 kJmol expected Cvclohexene 0 quot 230 kJmo39 206 kJmol actual 1 18 kJmo Cyclohexane O i L ZDUT Thomson Higher Educahon Benzene s Unusual Structure C C bonds same length 139 pm between single 154 pm and double 134 pm bonds Electron density in all six CC bonds is identical Structure is planar hexagonal C C C bond angles 120 Each C is sp2 and has a p orbital perpendicular to the plane of the sixmembered ring LBbondsonavemge T i HC C H H C CH I ll lt gt II I HCC H H C CH Drawing Benzene and Its Derivatives The two benzene resonance forms can be represented by a single structure with a circle in the center to indicate the equivalence of the carbon carbon bonds This doesn t indicate the number of 7 electrons in the ring but reminds us of the delocalized structure We shall use one of the resonance structures to represent benzene for ease In keeping track of bonding changes In reactions Alternative representations of benzene The quotcirclequot representation must be lt gt l used carefully Since it doesn t Indicate the number of 17 electrons in the ring 2007 Thomson Hiaher Education Molecular Orbital Description of Benzene The 6 porbitals combine to give Three bonding orbitals with 6 7 electrons Three antibonding with no electrons Orbitals with the same energy are degenerate Antibonding 1116 I l 414quot upsquot I L l Ll 3 lt l 5 Six benzene molecular orbitals 153 Aromaticity and the Hiickel 4n2 Rule Unusually stable heat of hydrogenation 150 kJmol less negative than a cyclic triene Planar hexagon bond angles are 120 carbon carbon bond lengths 139 pm Undergoes substitution rather than electrophilic addition Resonance hybrid with structure between two line bond structures Aromaticity and the 4n 2 Rule Huckel s rule based on calculations a planar cyclic molecule of conjugation has aromatic stability if it has 4n 2 72 electrons n is 0 123 4 For n1 4n2 6 benzene is stable and the electrons are delocalized Benzene Three double bonds six 7r electrons Compounds With 4n 7 Electrons Are Not Aromatic May be Antiaromatic Planar cyclic molecules with 4 n 7relectrons are much less stable than expected antiaromatic They will distort out of plane and behave like ordinary alkenes Cyclobutadiene is so unstable that it dimerizes by a selfDiels Alder reaction at low temperature Cyclooctatetraene has four double bonds reacting with Br2 KMnO4 and HCI as if it were four alkenes UT I n Diels Alder Not conjugated Cyclooctatetraene Four double bonds eight 77 electrons 154 Aromatic Ions The 4n 2 rule applies to ions as well as neutral species Both the cyclopentadienyl anion and the cycloheptatrienyl cation are aromatic The key feature of both is that they contain 6 7c electrons in a ring of continuous p orbitals i I H c H H Clt H C C C C Hc cH CC H H Cyclopentadienyl anion Cycloheptatrienyl cation Six 7quot electrons aromatic ions 7Thummn war ummn Aromaticity of the Cyclopentadienyl Anion HH Cyclopentadiene 13Cyclopentadiene contains gconjugated double bonds H H H joined by a CH2 that blocks H H 39idelocalization quot Removal of H at the CH2 produces a Cyclic 6eectron Cyclopentadienyl Cyclopentadienyl Cyclopentadienyl Wh is Stable cation four 7139 electrons radical ve 11 electrons anion six qr electrons Removal of H39 or H generate honaromatlc 4 and 5 electron systems Relatively acidic pKa 16 H H because the anion is stable Aromatic cyclopentadienyl anion with six 1 electrons nnnnnnnn EEEEEEE W Cycloheptatriene Cycloheptatriene has 3 conjugated double bonds joined by a CH2 Removal of leaves the cation the cation has 6 electrons and is aromatic H H Cycloheptatriene l H Hquot H H H 3 Cycloheptatrienyl Cycloheptatrienyl Cycloheptatrienyl cation six 1r electrons radical seven 1139 electrons anion eight 1r electrons 2007 Thomson Higher Education H H H 4 Br H H k HlBr H H Cyclohepta Cyclnhepta triene trienylium bromide Cycloheptatrienyl cation six 1 aloctrons 9 2M Thomson Hiqher Education 21 155 Aromatic Heterocycles Pyridine and Pyrrole Heterocyclic compounds contain elements other than carbon in a ring such as NSOP Aromatic compounds can have elements other than carbon in the ring There are many heterocyclic aromatic compounds and many are very common Cyclic compounds that contain only carbon are called carbocycles Pyridine and Pyrimidine D Six membered heterocycle nitrogen atom 0 6 7c electrons D The nitrogen lone pair electrons are not part of the aromatic system perpendicular orbital 4 H 3 H H 2 N 1 Lone pair in Pyridine Six 7r electrons 3 92 orbital Lone pair 5102 4 3 Lone pair in 5 Nquot sp2 orbital I H H e 2 Protonatlon N 1 Lone pair in 2 Pyrimidine Six 1139 electrons 5P orb39ta Lone pair spZ 23 e 2007 Thomson Higher EdUca an Pyrrole and Imidazole D A fivemembered heterocycle nitrogen m 6 7c electrons m Nitrogen lone pairs Lone pair in p orbital 3 l2 N1 H l H i H Delocalized Pyrrole Six 1r electrons 8 pair Lone pair 3 in p orbital I h 4 N Lone pair Protonatlon Lone pair In SP2 5 quot2 sp2 orbital N1 H H H Imidazole Six 7r electrons Delocalized lone pair p a 2007 Tnulmon Highef Educaliim 24 156 Why 4n 2 w4 p596 lt 12 H 3 Degenerate MO pairs AH 11 Six p atomic orbitals Benzene Single lowest MO 9 2007 Thomson Hugher Educa on Cyclapentadiene lH H H H H r Cyclopentadienyl Cyclopentadienyl Cyclopentadienyl cation four 17 electrons radical five 77 electrons anion six qr electrons 2001 Thmnson mth Educmion lt Five p atomic orbitals Z 12 ms H39 H 1L m H Five cyclopentadienyl Cyclopentadienvl Cyclopentadienyl Cyclopentadienyl molecular orbitals cation radical anion four 1139 electrons ve 1139 electrons six 7139 electrons Q 2007 Thomson Higher Education 26 Draw the resonance structures of anthracene total of 4 including the one shown Anthracene Arrows 157 Polycyclic Aromatic Compounds Aromatic compounds can have rings that share a set of carbon atoms fused rings Compounds from fused benzene or aromatic heterocycle rings are themselves aromatic Naphthalene Anthracene Benzolapyrene Co ronene 2007 Thomson Hiuher Education Naphthalene Orbitals 1 Three resonance forms and delocalized electrons Ha Naphthalene 9 2W7 Thamson Hiqher Education ugh Edumion Naphthalene Q 2007 Thomson H ex 29 Br Br2 Fe gt Heat HBr Naphthalene LBromonaphthalene 75 200 Thomson Higher Education 5 4 6 3 7 N 2 8 1 Quinoline 2007 Thomson Higher Education 5 4 4 3 6 3 5 2 7 N2 6 N1 8 1 7 H Isoquinoline Indole Nitrogen lone pairs 6 7N 5 1 8lt N 42 9N 4 N H 3 Pu ne 158 Spectroscopy of Aromatic Compounds IR Aromatic compounds C H 3030 cm 1 weak 1660 2000 cm 1 weak 1450 1600 cm 1 medium 2007 Thomson vaher Educanon O O CO 0 60 4o Monosubstituted ring C H Transmittance 20 0 llIllIllll 1 4000 3500 3000 2500 2000 1500 Wavenumbercm1 2D07 Thomson Higher Education UV Peak near 205 nm and a less intense peak in 255275 nm range 1H NMR Aromatic H s strongly deshielded by ring and absorb between 8 65 and 8 80 Peak pattern is characteristic of positions of substituents Ring Currents Aromatic ring oriented perpendicular to a strong magnetic field delocalized 7c electrons producing a small local magnetic field Opposes applied field in middle of ring but reinforces applied field outside of ring Circulating 77 electrons ring current f Proton deshielded by I l induced field H 5 Il Induced magnetic field because of ring current 39 x A Wrong direction Applied magnetic field 5 2007 Thomson Higher Education 34 181Annulene Inside H 30 5 Outside H 93 5 Q 2007 Thumson Higher Education 35 130 NMR of Aromatic Compounds Carbons in aromatic ring absorb at 8 110 to 140 Shift is distinct from alkane carbons but in same range as alkene carbons 213 i X1281 39 Ci CH3 1284 1250 1256 1293 1254 1275 1285 1284 Benzene Toluene Chlorobenzene Naphthalene 2007 Thomson Hiuher Education Appendix httpwwwacdlabscomiupacnomenclature79r7983htma614 612 When there is no generally recognized trivial name for the hydrocarbon then 1 the radical name denoting the aliphatic chain is prefixed to the name of the cyclic hydrocarbon or 2 the radical name for the cyclic hydrOCarbon is prefixed to the name of the aliphatic compound Choice between these methods is made according to the more appropriate of the following principles a the maximum number of substitutions into a single unit of structure b treatment of a smaller of structure as a substituent into a larger Numbering of double and triple bonds in chains or nonaromatic rings is assigned according to the principles of Rule numbering and citation of substituents are effected as described in Rule 613 In accordance with the principle a of Part 2 ofthis rule hydrocarbons containing several chains attached to one cyclic nucleus are generally named as derivatives of the cyclic compound and compounds containing several side chains andor cyclic radicals attached to one chain are named as derivatives ofthe acyclic compound u i 39i 1 H3 n H quotha 3939 quot quot 39T39 quotf d39 a 2 ElEI irnethyl 39l phEan39rl 39l herterm
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