318-6 - CH 318 N LECTURE 6 Textbook Assignment: Chapter 13...

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Unformatted text preview: CH 318 N LECTURE 6 Textbook Assignment: Chapter 13 (finish)-Begin Mass Spec (Chap 14) Homework (for credit): POW 3 posted Today’s Topics: 13CNMR; Mass Spec intro Notice & Announcements: Organic Lecture Series Stereochemistry & Topicity Topicity is the stereochemical relationship of substituents relative to the structure to which they are attached. • Depending on the symmetry of a molecule, otherwise equivalent hydrogens may be – homotopic – enantiotopic – diastereotopic • The simplest way to visualize topicity is to substitute an atom or group by an isotope; is the resulting compound – the same as its mirror image – different from its mirror image – are diastereomers possible 2 1 Organic Lecture Series • Homotopic atoms or groups H C H Cl Cl Substitute one H by D H Substitution does not produce a stereocenter; Cl therefore hydrogens D are homotopic. Achiral Cl C Dichloromethane (achiral) – homotopic atoms or groups have identical chemical shifts under all conditions – i.e. the protons have the same stereochemical relationship to the molecule 3 Organic Lecture Series • Enantiotopic groups H C H Cl F Substitute one H by D Substitution produces a stereocenter; Cl therefore, hydrogens are C F enantiotopic. Both hydrogens are prochiral; D one is pro-R-chiral, the Chiral other is pro-S-chiral. H Chlorofluoromethane (achiral) – enantiotopic atoms or groups have identical chemical shifts in achiral environments – they have different chemical shifts in chiral environments E.G. chiral deuterated solvent 4 2 Organic Lecture Series • Diastereotopic groups – H atoms on C-3 of 2-butanol are diastereotopic – substitution by deuterium creates a chiral center – because there is already a chiral center in the molecule, diastereomers are now possible H OH Substitute one H on CH 2 by D H OH H H 2-Butanol (chiral) H D Chiral – diastereotopic hydrogens have different chemical shifts under all conditions 5 Organic Lecture Series 13C-NMR Spectroscopy • Each nonequivalent 13C gives a different signal – a 13C signal is split by the 1H bonded to it according to the (n + 1) rule – coupling constants of 100-250 Hz are common, which means that there is often significant overlap between signals, and splitting patterns can be very difficult to determine, so: • The most common mode of operation of a 13C-NMR spectrometer is a hydrogendecoupled mode, which results in singlets. 6 3 Organic Lecture Series • In a hydrogen-decoupled mode, a sample is irradiated with two different radio frequencies – one to excite all 13C nuclei – a second broad spectrum of frequencies to cause all hydrogens in the molecule to undergo rapid transitions between their nuclear spin states. • On the time scale of a 13C-NMR spectrum, each hydrogen is in an average or effectively constant nuclear spin state, with the result that 1H-13C spin-spin interactions are not observed; they are decoupled. 7 Organic Lecture Series Chemical Shift - 13C-NMR Much wider range of ppm; Much broader range for carbon types 8 4 Organic Lecture Series – hydrogen-decoupled 13C-NMR spectrum of 1-bromobutane 9 Organic Lecture Series – hydrogen-decoupled 13C-NMR spectrum of Ethyl Acetate O O 10 5 Organic Lecture Series – hydrogen-decoupled 13C-NMR spectrum of Ethyl Benzene 2 peaks 11 Organic Lecture Series Chemical Shift - 13C-NMR Typ e of Carbon RCH3 RCH2 R R3 CH RCH2 I RCH2 Br RCH2 Cl R3 COH R3 COR RC CR R2 C=CR2 Chemical S hift (δ) 10-40 15-55 20-60 0-40 25-65 35-80 40-80 40-80 65-85 100-150 Type of Carb on CR O RCOR O RCNR2 O RCCOH O O RCH, RCR Chemical Sh ift (δ) 110-160 160 - 180 165 - 180 165 - 185 180 - 215 12 6 Organic Lecture Series Chemical Shift - 13C-NMR Similar deshielding trends as seen in 1HNMR 13 Organic Lecture Series Mass Spectrometry (MS) • An analytical technique for measuring the mass-to-charge ratio (m/z) of ions in the gas phase – mass spectrometry is our most valuable analytical tool for determining accurate molecular masses – also can give information about structure – proteins can now be sequenced by MS 14 7 Organic Lecture Series 15 Organic Lecture Series 16 8 Organic Lecture Series Mass Spectrometry (MS) Electron Ionization MS 17 Organic Lecture Series A Mass Spectrometer A mass spectrometer is designed to do three things 1. convert neutral atoms or molecules into a beam of positive (or negative) ions 2. separate the ions on the basis of their mass-to-charge (m/z) ratio 3. measure the relative abundance of each ion 18 9 Organic Lecture Series A Mass Spectrometer • Electron Ionization MS – in the ionization chamber, the sample is bombarded with a beam of high-energy electrons – collisions between these electrons and the sample result in loss of electrons from sample molecules and formation of positive ions H H C H + eH + H HC H + 2 eH Molecular ion (a radical cation) 19 Organic Lecture Series • Molecular ion (M): a radical cation formed by removal of a single electron from a parent molecule in a mass spectrometer • It does not matter which electron is lost; radical cation character is delocalized throughout the molecule; therefore, write the molecular formula of the parent molecule in brackets with – a plus sign to show that it is a cation – a dot to show that it has an odd number of electrons 20 10 Organic Lecture Series – at times, however, it is useful to depict the radical cation at a certain position in order to better understand its reactions CH3 CH2 OCH(CH3 ) 2 . . CH 3 CH 2 OCH(CH3 ) 2 21 Organic Lecture Series Mass Spectrum • Mass spectrum: a plot of the relative abundance of ions versus their mass-tocharge ratio • Base peak: the most abundant peak – assigned an arbitrary intensity of 100 • The relative abundance of all other ions relative is reported as a % of abundance of the base peak 22 11 Organic Lecture Series MS of dopamine – a partial MS of dopamine showing all peaks with intensity equal to or greater than 0.5% of base peak M-29 23 Organic Lecture Series Resolution Resolution: a measure of how well a mass spectrometer separates ions of different mass – low resolution: refers to instruments capable of separating only ions that differ in nominal mass; that is ions that differ by at least 1 or more atomic mass units – high resolution: refers to instruments capable of separating ions that differ in mass by as little as 0.0001 (10-4) atomic mass unit 24 12 For Example: Organic Lecture Series – C3H6O and C3H8O have nominal masses of 58 and 60, and can be distinguished by lowresolution MS – C3H8O and C2H4O2 have nominal masses of 60 – distinguish between them by high-resolution MS M o lecular N o m inal Form ula M ass C3 H8 O 60 C2 H4 O2 60 P recise M as s 60.05754 60.02112 25 MS is especially Useful for: Organic Lecture Series Isotopes Atomic Mas s Relative Element w eigh t Isotope (amu ) A bun dance hydrogen 1.0079 1H 1.00783 100 2 H 2.01410 0.016 12 carbon 12.011 13 C 12.0000 100 C 13.0034 1.11 14 N 14.0031 100 nitrogen 14.007 15 N 15.0001 0.38 oxygen 15.999 1 6O 15.9949 100 18 O 17.9992 0.20 32 su lfu r 32.066 S 31.9721 100 34 S 33.9679 4.40 ch lorine 35.453 3 5Cl 34.9689 100 37 Cl 36.9659 32.5 79 Br bromine 79.904 78.9183 100 81 Br 80.9163 98.0 26 –virtually all elements common to organic compounds are mixtures of isotopes 13 Organic Lecture Series Isotopes – carbon, for example, in nature is: 98.90% 12C and 1.10% 13C – there are 1.11 atoms of carbon-13 in nature for every 100 atoms of carbon-12 – So, MS is not as helpful as 13CNMR – However, MS is very valuable for confirming halogens 27 Organic Lecture Series M+2 and M+1 Peaks • The most common elements giving rise to significant M + 2 peaks are chlorine and bromine – chlorine in nature is 75.77% 35Cl and 24.23% 37Cl – a ratio of M to M + 2 of approximately 3:1 indicates the presence of a single chlorine in a compound 28 14 M+2 and M+1 Peaks Organic Lecture Series – bromine in nature is: 50.7% 79Br and 49.3% 81Br – a ratio of M to M + 2 of approximately 1:1 indicates the presence of a single bromine in a compound 29 15 ...
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