Organic Chemistry II
Organic Chemistry II CHEM 548
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Chemistry 548 Name Homework for Monday Propose a synthetic route complete with all reagents and intermediate structures d acetylene 0 Chapter 13 Spectroscopic Methods Spectroscopy answers the questions What is it Is it pure What s in this mixture Spectroscopy provides molecular fingerprints Common types of spectroscopy Nuclear Magnetic Resonance Spectroscopy NMR uses radiowaves to flip nuclear spins for samples in a magnetic field Provides detailed information about hydrogens and carbons in the structure Generally considered the most informative of all spectra Infrared spectroscopy absorption of infrared light vs wavelength identifies functional groups and provides a molecular fingerprint IR light causes molecular vibrations UltravioletNisible Spectroscopy measures the absorption of UVVIS light vs wavelength Provides information on functional groups and level of conjugation UVVIS light promotes electrons from bonding to antibonding molecular orbitals Mass spectrometry samples are ionized broken into fragments and the mass of each fragment is measured Provides a molecular mass from which the formula can be determined Fragments give information about the molecular structure The Electromagnetic Spectrum energy in different flavors High energy low waverengm mm p 102 lo 02 10 I to to 10 I I I I x l I I I I I I Gamma Ultra 2 i ray era Male lnlvamd Microwave I Radiolvequanoy I s I I l I I I I t L t l l t 1 102 10 5 m39 In i m 10 105 1 10 lt FrequenCy s4 AOX O 45quot A Quantum Leap E hv hCk Energy absorption is quantized frequency and energy gap must match Nuclear Magnetic Resonance NMR Spectroscopy Enormoust powerful method allows rapid assignment of complex molecular structures NMR provides information about molecular structure through a map of the 0H framework Nuclei with spin such as 1H 2H 13C 14N 19F 31P can be seen by NMR Nuclei without spin 12C 16O 328 etc are invisible to NMR MRI medical imaging evolved from NMR GrammarAert one spectrum two spectra Magnetic resonance imaging MRI The nucleus measured is usually H in water Nuclei with spin act like tiny magnets and become aligned in the presence ofa magnetic field There are now two spin states In No mm magnetic eld 1m Apply mama Ilmgnc c new 1 5 mmmm myan r Iv M No energy Llrllultncc t f m nuclear m 1 H H lncrezmn lrenglh nr enema us I39 cld The energy gap between spin states corresponds to radiofrequency RF energy For 1H a field strength of 116 Tesla 116000 Gauss corresponds to 500 MHz energy gap is 5 x 10395 kcalmol Modern spectrometers range from 60 MHz to 920 MHz A Typical Superconducting NMR Spectrometer Dmlvz samp z m Khmermed curamlm 6091 no glacs m NMR ma lnsen NMR mm mm vnmcal cava nova a Ihe magnex In El magnum comam a 3 Bo pmhe Ina ads as a 439 llansmmer m rad unmusncy ms mum and receiver cl Q 5 The magnahc m4 Waxed Mm me new onemauan 1 me nudes lelums values m we mid changes a gensvzka exeanw Imuulse mm s mansmlued mm was probe a mm n ma census 5 39iree nducliwn uecay39 puvss lalax sequence 5 5 men In a computer m In comm A mamsmaucal weaken semen or prmLi n AVarian 500 MHz NMR spectrometer at UNH One basic feature of NMR spectra is the chemical shift the location of a particular resonance in the spectrum m J C85 36 35 34 3 I80 I7u mu CI BrCaCHf CHCI T1145 90 50 70 20 m 00 L0 50 JD Cncmmi shift a pplui Deshielded or down eld Shielded or u p eldquot What determines chemical shift The nucleus is surrounded by electrons Electron motion causes local magnetic fields which can shield or deshield a nucleus from the field around it thus changing its observed resonance frequency In general proximate electron withdrawing groups deshield nearby nuclei and push resonances downfield Substitution is important In general tertiary ltsecondaryltprimary 7 bonds from alkenes and benzene rings also have a deshielding effect 1 u 3 3 J 33 XL 7U 64 CH BrCmCH39g CHCI3 TITLE 9 J 240 7 0 3 0 2 H M n u 50 40 Chemiclesluih5p1um Tetramethylsilane TMS is used as an internal reference in both carbon and hydrogen spectra This is assigned a chemical shift of zero k 36 35 34 33 Ixn I7n LMi C Brc CH3 H CH0 T1145 90 Ni 70 n u 5x 4 u 30 10 iLi 00 Chemical shift 5 ppm Higher energy necessary for the spin flipquot Lower Energy The chemical shift is given in parts per million 5 or ppm ofthe resonance frequency This makes chemical shifts independenti the magnetic field strength What determines chemical shift The nucleus is surrounded by electrons Electron motion causes local magnetic fields which can shield or deshield a nucleus from the field around it thus changing its observed resonance frequency In general proximate electron withdrawing groups deshield nearby nuclei and push resonances downfield to the left Substitution is important In general tertiary ltsecondaryltprimary 7 bonds from alkenes and benzene rings usually have a deshielding effect 90 m 70 1 U 50 J 0 Chemical um 5 mm In bromoethane hydrogens on the carbon with bromine attached are strongly deshielded Bromine is more electronegative than carbon 5 1 9 H CEC CHZCHzCH2CH3 1 Hexync H H C C LU Tb III C C a 1 4 IL 4 BO 17 Proximate 7 bonds can 3 have a dramatic effect 1 Tum on chemical shifts due to llxnnulcnc nnllulcne m In aromatic antiaromatic 1H NMR Chemical Shifts Wm H mm um Mu acorn H w 271 o H Alu uvl muum Rcill m ml Ari 705 urc illmum m m Ar w39 u humm 395 um lhm mmmm ummm a m w i c J V 7 u mquot w mm mm chon 39 m o v n m 7quot u m Chrllun al lnfl 5 mm Chemical shifts in benzyl alcohol are typical for aryl benzylic and hydroxyl hydrogens Note that peak areas integration give information about the Ratio of different hydrogens NECCHEOCHE 70 60 50 40 Chemical mm 5 ppm 30 20 I 0 00 Electronic integration of the area under resonances gives the ratio of different kinds of hydrogen Here the integral is 26 This indicates a ratio of 1 3 for the two types of hydrogen 01ml 11 LC C C O CIIJ I MS CH Intensity 6 5 4 3 2 1 0 ppm mam EmoksCnieTnumsnn Learning Chemical shift L8 Spectral Databases Several large databases are available free on the WWW SDBSWeb National Institute of Advanced Industrial Science and Technoloav Jaban 33 ppm SpinSpin Coupling The Icing on the Cake Proximate nuclei couple through electron spin this results in complex but informative NMR patterns The magnitude of coupling is usually 0 20 Hz Coupling is observed for nuclei that are different and usually on adjacent atoms CarbonHydrogen coupling is usually removed n 1 Rule A nucleus with n neighbors will be split into n1 signals 9 With no adjacent hydrogens each type of hydrogen appears as a singlet N E CCH10CH3 CH3 u CH Km 9 m 70 r l x I Chalan mm 5 ppm CI CI I T H Clicl l HiCliCl CH CH1 1 Spin of mclhinc prulon rmm39u no Mehy signal appear at Imvcr new highcr frequency Spin ul39mmhine prawn shields methyl promquot 39 Mcmyl signal mm m higlmr eld lower frequency 2 p v 4 4 v v 4 v 4 v v 4 4 H t v v v y y Tln 39VI 0 uhiualmm 1 ul um 11 m ul39dlu mm mum pmmm m CH 1 CL my pm 11m wglll cmnhlnullnm mmc m g 1mm cm lmlun m m mm mum 4 m M39 mmmu Ilcll mu The typical pattern for an ethyl group is a downfield quartet and an upfield triplet Ii il iilixpruummiltxihu C Timmxpmmm giml lm mu quotmm C mm m quotwhim Cl CHX wgnui mm mm unmm mm 4 Immlm g CH3 ulcec icn3 C1 LS in II J 43 42 4 M 1 CH mu x u 7 u no it 40 30 20 0 n 0 Chemical mm 5 ppm IOquot One line singlet Two lines doublet Three lines triplet Four lines quartet Five linesquot quintet Six lines sextet Seven lines septet Chemical shi dif 39k ucc Ax I much 14 ch lhan coupling I consiuni AM I l I I 7 I For a two spin system ine intensities depend on the difference in chemical shifts A7 Same Iclxiemimi shift no 5p m g E 31 OCHa CI CI 74 72 70 ax m TMS i M L in u 30 10 LU DJ 1 u 39Ilcmicu mm 45 ppm More complex patterns sometimes resonances have overlapping signals and don t follow the n1 rule The pattern can be predicted by a tree diagram Co ogN r H L 39s s i x a I r x xx mm 69 58 6 If h Intensity In flexible structures an average coupling constant J of ca 7 Hz is commonly observed In more rigid compounds J can vary from 0 to 20 Hz and the n 1 rule fails n a II I I mic UIIIII Inalucnyuc u23 6 5 4 3 2 1 0 ppm mum BronksCoieThnmsnn Learning Chemical shift 16 Transcinnamaldehyde gives the odor to cinnamc A tree diagram is used to understand coupling Larger coupling are usually shown first mm Brooksmum Learning 6 3973 5 Many spectra are complex because of overlapping resonance mnrn 13C NMR Spectroscopy 13C has a random natural abundance of ca 11 and can be seen by NMR Low sensitivity requires time averaging of spectra Hundreds or thousands of spectra usually are measured and the signal is averaged Random electronic noise averages to zero while the signal continues to go up A pulsed Fourier transform method is used to measure spectra rapidly The sample is irradiated with a short radiofrequency pulse rather than sweeping through frequencies Carbon resonances usually appear as sharp lines in the spectrum in the range of O to 200 ppm compare to O to 10 for hydrogen CH coupling is removed and 130130 coupling is nearly absent Hybridization is a major factor in chemical shift sp2 lt sp lt sp3 Substitution is important quaternarylt tertiary lt secondary lt primary Chemical shift is also dependent on electron density Electron withdrawing groups push resonances downfield NMR Spectra for 1chloropentane CIC CthIhCIIECH1 Im 017 x U 10 3 u 10 u 00 o 0 50 40 Chmniml min 46 ppm zoo 180 150 140 420 100 as so av 20 o Intensity 13C NMR Chemical Shifts 2 60 140 12 60 40 20 0 ppm 20 200 1 80 mum monksunis i homsnn Learning 0 100 80 Chemical shift 5i Carbon displays a broad range of chemical shifts 50 m arbltrarg unlLs 5 a 210 190 170 150 130 110 90 70 50 30 10 PP quot Symmetry is readily determined from 13C spectra This substance shows only six unique carbons because it has a plane of symmetry Quaternary with no attached hydrogen atoms carbons usually have greatly reduced intensity CHlt OH 200 um mn I441 4U 1H 0 Assign carbon resonances DEPT Distortionless Enhancement by Polarization Tran sfer CH H H CH 7 H N ormal H CCHZCHICHZCH H H CH1 Cll CHZCIIg CH3 mi C l 100 lel loll Hi ill 00 Si 6 Jll le ll Chum luh l5 mum W CH CH H H D E PT H Q CCHJClthHICHx 7 CH 2m l l 1m Hll zu inn S m in 20 n 1 Chemical emu 613pm li More complex RF pulse sequences such as DEPT135 are used to assign the number of attached hydrogens CH3 and CH go UP CH2 goes down C is nulled out 7 2 3 4 Possible Relationships Between Nuclei Homotopic or identical Enantiotopic replacement gives enantiomers Diastereotopic replacement gives diastereomers Chemically unrelated In 1H NMR homotopic and enantiotopic Hydrogens show identical NMR signals Unrelated and diastereotopic hydrogens show different NMR signals Infrared spectroscopy absorption of infrared light vs wavelen identifies functional groups and prOVI es a molecular fingerprint IR light causes molecular vibrations Wavelenglh mm 104 10quot mg m 06 me mm 012 l l I l l I l l I l l i Gamma 1 Ullr a 1 i my i may Mole g mlrmd Microwave Radio lrequency l i S I l 4 l I l i i l l l l in 1013 miquot 10 inquot mm 103 105 10 lt Frequency S W 400 500 600 750 nm some 5mm Views 75m Infrared spectroscopy absorption of infrared light vs wavelen identifies functional groups and provides a molecular fingerprint IR light causes molecularvibrations stretching and bending CC CN lllhl 0 L N mm 0 xiw a bowl mum and licmliilg mung l OllNlmulCll coaxch humid iliplcrbnml Ammicrm menmug Mung Iii JEWJRSUCMquot zanmzummi l85 lvllum WSOOCnIquot Funcrinmu Groups Fingerprint mgim l 39e l 5mm 2mm 0 mm sin wilemum ltlil 1 441m Infrared spectra are a unique molecular fingerprint u Hexane CH3CH2CH3CH3CH2CH3 100 80 7 725 14601380 cmquot 60 C H and C C bending u 40 7 20 2958 2859 cmquot C H stretching i 7 r i i 1 3500 3000 2500 2000 1500 1000 500 Wavenumbers m l Infrared spectra indicate which functional groups are present humx our 1 m m m bmu punkulllhuJH lvm39i ulnulwhm ul ludmganrlvmulrd on gum in unnu nhninn Ilydmgun Imnmng ik m m u Hquot 39 39 mu 1 k ravine 1n gmup mun w r uu V cpumnmum l mlwmngc v Mquot My Man 4200cquot39Haniihlrclvhi w V 7 andmnl cmugnmlmil w quotquot N nun Wu Nummw m l tnle H thilth39 u39 nplcbumllthrvrplllnv may Munlllmhlc m um IR svucllmu m n lulnlc m 2 u my quot hurp mm at medium mm v m mm rm Var m mm Imum uh wquot a Inplu mm mr quotMummc l leuxpzzmmu 39yv u um um um mm mm m a mm n m Infrared spectra indicate which functional groups are present O H Hexanoic acid CH3CH34COH 100 7 80 O H P Q 60 40 7 0 C06 0 0 1711 cmquot I I I I I I I 3500 3000 2500 2000 1500 1000 500 Wavenumbers cm l