Mass Spectrometry: Fragmentation of Ethers, Sulfides, Amines, Carbonyls, and Functional Gr, Lecture notes of Chemistry

Download Mass Spectrometry: Fragmentation of Ethers, Sulfides, Amines, Carbonyls, and Functional Gr and more Lecture notes Chemistry in PDF only on Docsity! Mass Spectrometry: Fragmentation Ethers & Sulfides ! ! ! ! ! Ethers • M+ usually stronger than corresponding alcohol; may be weak/absent • α-cleavage of an alkyl radical • Inductive cleavage • Rearrangement with loss of CHR=CHR’ Aryl Ethers • M+ strong • C-O cleavage β to aromatic ring with subsequent loss of CO • Cleavage adjacent to aryl ring also possible Sulfides • M+ usually stronger than corresponding ether • cleavage pattern similar to ethers Mass Spectrometry: Fragmentation Ethers fragmentation patterns α-cleavage inductive cleavage rearrangement R CH2 O R' O H H R' R + R CH2 O R' +R CH2 O R' O CH2 H H CHR H + H2C=CHR O H H H Mass Spectrometry: Fragmentation Ethers 2-ethyl-2-methyl-1,3-dioxolane O O MW = 116 101 M-15 87 M-29 M+ absent O O O O Mass Spectrometry: Fragmentation Aryl Ethers anisole 93 M-15 m/z = 78 MW = 108 OCH3 m/z = 65 O loss of C O M (108) H H Mass Spectrometry: Fragmentation Aryl Ethers fragmentation of aryl ethers O CH2 H m/z = 78 m/z = 77 O CH2 H H H - CH2O - H≡ O CH3 m/z = 93 m/z = 65 - CH3 O - CO Mass Spectrometry: Fragmentation Amines fragmentation patterns α-cleavage ring formation NH2 R NH2R + n n R N R + N R" H HR' R" R' n = 1, m/z = 72 n = 2, m/z = 86 loss of H radical R N R N R' R" H R' R" + H M-1 Mass Spectrometry: Fragmentation Amines ethylamine NH2 MW = 45 M (45) mz = 30 44 M-1 N H H H H base peak N H H H Mass Spectrometry: Fragmentation Amines diethylamine N H MW = 73 M (73) 72 M-1 58 M-15 m/z = 30 N H H H H α cleavage N H Mass Spectrometry: Fragmentation Cyclic Amines piperidine M (85) 84 M-1 MW = 85 N H 84 M-28 N H and N H Mass Spectrometry: Fragmentation Aromatic Amines aniline M (93) 92 M-1 NH2 MW = 93 NH HH -HCN M-1 m/z = 66 m/z = 65 - H m/z = 65 Mass Spectrometry: Fragmentation Carbonyl Compounds ! ! ! ! α-cleavage (two possibilities) β-cleavage McLafferty rearrangement Common Fragmentation Modes R G O C OR + G R G O C OGR + G O R R + G O G = H, R', OH, OR', NR'2 G OR H G OHR + Mass Spectrometry: Fragmentation Aldehydes 2-ethylbutanal M (100) mz = 29 m/z = 72 C OH H O MW = 100 H OH Mass Spectrometry: Fragmentation Aromatic Aldehydes benzaldehyde M (106) 105 M-1 mz = 77 H O MW = 106 Mass Spectrometry: Fragmentation Aliphatic Ketones 2-hexanone O MW = 100 M (100) mz = 58 85 M-15 43 M-57 C O OH CH3CO McLafferty α cleavage α cleavage Mass Spectrometry: Fragmentation Cyclic Ketones cyclohexanone O O H O H O + H O H m/z = 55 O + m/z = 42 - CO m/z = 70 Mass Spectrometry: Fragmentation Carboxylic Acids, Esters & Amides ! ! ! Carboxylic Acids • M+ weak in aliphatic acids; stronger in aromatic acids • Most important α-cleavage involves loss of OH radical (M-17) • α-cleavage with loss of alkyl radical less common; somewhat diagnostic (m/z = 45) • McLafferty rearrangement in appropriately substituted systems (m/z = 60 or higher) • Dehydration can occur in o-alkyl benzoic acids (M-18) Esters • M+ weak in most cases; aromatic esters give a stronger parent ion • Loss of alkoxy radical more important of the α-cleavage reactions • Loss of an alkyl radical by α-cleavage occurs mostly in methyl esters (m/z = 59) • McLafferty rearrangements are possible on both alkyl and alkoxy sides • Benzyloxy esters and o-alkyl benzoates fragment to lose ketene and alcohol, respectively Amides • M+ usually observed; Follow the Nitrogen Rule (odd # of N, odd MW) • α-cleavage affords a specific ion for primary amides (m/z = 44? • McLafferty rearrangement observed when γ-hydrogens are present Mass Spectrometry: Fragmentation Aliphpatic Carboxylic Acids butyric acid OH O MW = 88 M (88) mz = 45 71 M-17 weak M+ mz = 60 C O O OH H OHCO Mass Spectrometry: Fragmentation Esters butyl butyrate M (144) absent 101 M-43 89 O O MW = 144 McLafferty + 1 Pr O O H H 71 M-73 McLafferty Pr O O H 88 Mass Spectrometry: Fragmentation Esters fragmentation patterns McLafferty rearrangement McLafferty + 1 Pr O O H Pr O O H Pr O O H H Pr O O H H + m/z = 89 O O H Pr O O H + m/z = 88 O OH O OH +Bu Bu m/z = 116 (not observed) Mass Spectrometry: Fragmentation Esters benzyl acetate M (150) 108 M-42 91 M-59 O O MW = 150 OH 43 M-108 α cleavage tropylium ion O Mass Spectrometry: Fragmentation Amides N-ethylpropionamide M (101) 72 M-29 57 M-44 N H O MW = 101 mz = 29 86 M-15 mz = 30 N CH2 H H CH3CH2 N H O Mass Spectrometry: Fragmentation Aryl Amides N-methylbenzamimde M (135) 105 M-29 mz = 77 N CH3 H O MW = 135 M-1 Mass Spectrometry: Fragmentation Nitriles Nitriles • M+ may be weak/absent; strong M+ in aromatic nitriles; follow nitrogen rule • Fragment readily to give M-1 • Loss of HC≡N fequently obsterved (M-27); aromatic nitriles also show loss of •CN (M-26) • McLafferty rearrangement in nitriles of appropriate length (m/z = 41) Mass Spectrometry: Fragmentation Nitro Compounds & Halides Nitro Compounds • M+ almost never observed unless aromatic; follow nitrogen rule • Principle degradation is loss of NO+ (m/z = 30) and loss of NO2 + (m/z = 46) • Aromatic nitro compounds show additional fragmentation patterns Halides • M+ often weak; stronger in aromatic halides • chloro and bromo compounds show strong M+2 peaks Cl – M : M+2 3 : 1 Br – M : M+2 1 : 1 • principle fragmentation is loss of halogen • Loss of HX also common • α-cleavage sometimes observed Mass Spectrometry: Fragmentation Nitro Compounds fragmentation patterns R N O O R + N O O m/z = 46 R N O O R O N O R O N O m/z = 30 + NO2 O + NO C O+ m/z = 65m/z = 93 NO2 + NO2 HHC4H3 + m/z = 77 m/z = 51 Mass Spectrometry: Fragmentation Nitro Compounds 1-nitropropane 43 M-46 MW = 89 NO2 M (89) absent mz = 30 mz = 46 N O N O O CH3CH2CH2 Mass Spectrometry: Fragmentation Alkyl Halides 1-chloropropane M (78) mz = 49, 51 80 M+2 Cl MW = 78 42 M-HCl 43 M-Cl CH3CH2CH2 C Cl H H α cleavage Mass Spectrometry: Fragmentation Alkyl Halides 2-chloropropane m/z = 63, 65 43 M-Cl Cl MW = 78 M (78) 80 M+2 C Cl H CH3 Mass Spectrometry: Fragmentation Alkyl Halides 2-chloroheptane M (134) M+2 (136) m/z = 105, 107 56 M-78 Cl MW = 134 98 M - HCl Cl H Cl H Cl rearrngement Mass Spectrometry: Fragmentation Alkyl Halides bromobenzene M (156) 158 M+2 mz = 77 Br MW = 157 Mass Spectrometry: Fragmentation What Can the MS Tell You? ! Evaluation of UnknownCompounds by Mass Spectr 1. Get an overview of the spectrum. Is it simple? Complex? Are there groups of peaks? 2. Identify and evaluate the molecular ion. - Is M+ strong or weak? - Are their significant peaks due to isotopes (e.g. M+1, M+2, etc.)? - Is the molecular ion an odd number (Apply the Nitrogen Rule)? - Is there an M-1 Peak? - If a molecular formular is not provided, check tables or on-line calculators to determine possible formulas 3. Evaluate the major fragments - What mass is lost from M+ to give these peaks? - What ions could give these peaks? - If available, use IR data to identify functionality, and consider known fragmentation patterns of these groups. - Consider the loss of small neutral molecules (e.g. H2O, HOR, H2C=CH2, HC≡CH, HX, CO2, etc.) - Consider possible diagnostic peaks (e.g. m/z = 29, 30, 31, 39, 41, 44, 91, 45, 59, etc.) 4. Use fragmentation information to piece together possible structure Mass Spectrometry: Fragmentation Commonly Lost Fragments Pavia Appendix 11 Fragment lost Peak obtained Fragment lost Peak obtained “CH, Mf-15 “OCH, Mt. 31 “OH Mi-17 “cl Mt .35 “CN MP6 CH.C=o Mt- 43 H,C—= CH, Pag *OCH,CH, Mt. 45 *CH,CH, M?-29 Cp M? . oy in/z values lon miz= 43 cH—é=0 mi2=91 ‘O : O CH, + . n/z= M*-1 + Mass Spectrometry Reporting Mass Spec Data Low Resolution Mass Spec OCH3 O MW = 102 MS (EI, 75 eV): m/z 102 (M+, 1%), 87 (16), 74 (64), 71 (50), 59 (22), 43 (100) .... ionization technique/method mass peak assignment height of peak relative to base peak Mass Spectrometry Reporting Mass Spec Data ! ! ! ! ! High Resolution Mass Spec O CO2Et OH Mass Spectrometry Reporting Mass Spec Data High Resolution Mass Spec HRMS (ESI): calcd for C12H18O4Na ([M+Na]+) 249.1097; found 249.1094. ionization method exact mass calculated mass found molecular ion observed chemical formula of (quasi) molecular ion