11-11 - 11/12/2009 Compound A has the formula C5H11Br. The...

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Unformatted text preview: 11/12/2009 Compound A has the formula C5H11Br. The peaks for the 13C NMR are shown below. Give structures for compounds A-D. 13C Which one of these compounds (A, B, C or D) is 2-bromo-2-methylbutane. 13C peaks peaks A=5 A O A=5 A O B H Br C B=5 C=4 D=5 O Et B H Br C B=5 C=4 D=5 O Et O H 3C C CH3 + H3C O C H 1. O3 2.Zn/HOAc D H 3C O C CH3 + H3C O C H 1. O3 2.Zn/HOAc D Compound A has the formula C5H11Br. The peaks for the 13C NMR are shown below. Give structures for compounds A-D. 13C H Br O Br NMR spectroscopy (MRS) measures differences in nuclear spin transition energies. (molecular structure) peaks N H H H A=5 B=5 C=4 D=5 N O A B H Br C Br H O NH NH H O H O O H H H O O OH OH H E E H HO H HO O OO O O H O Et O H 3C C CH3 + H3C O C H 1. O3 2.Zn/HOAc D energy E = 300,000,000 Hz S S 4 Is the frequency required to flip the magnetic moment of H2 (A) greater or (B) less than H1? A. greater N Does H1 experience a (A) stronger or (B) weaker magnetic field than H2? A. stronger N B. less N B. weaker N E H1 H2 O E H E H1 H2 O E H energy E = 300,000,000 Hz S E = 300,000,900 Hz S 5 energy E = 300,000,000 Hz S E = 300,000,900 Hz S 6 1 11/12/2009 an NMR spectrum at 300 MHz = 900 Hz 3.0 ppm Why do different hydrogen nuclei in an organic molecule experience a different magnetic field? 0 10 9 8 7 6 5 4 3 2 1 0 10 9 8 7 6 ppm 5 4 3 2 1 ppm E E energy E = 300 MHz = 300,000,000 Hz E = 300,000,900 Hz E  E = 900 Hz 7 E E energy local magnetic fields 8 Why do different hydrogen nuclei in an organic molecule experience a different magnetic field? H H H O H O NH H O H O O H H H HO O OO O O H H H O O OH Why do different hydrogen nuclei in an organic molecule experience a different magnetic field? 9 10 What is the magnetic field at a hydrogen nucleus? induced magnetic field What is the magnetic field at a hydrogen nucleus? induced magnetic field H H E E What is the actual field experienced by the hydrogen in a molecule applied magnetic field required for H resonance actual field at shielded nucleus is less than the applied field. 11 applied magnetic field required for H resonance 12 2 11/12/2009 The The magnetic field experienced by a given proton depends depends upon the fields induced by the electrons. If the electron density at the hydrogen atom is decreased would the strength of the actual magnetic field (A) increase or (B) decrease? A. Increase B. Decrease C H O C H actual actual magnetic magnetic field applied applied magnetic magnetic field13 actual actual magnetic magnetic field applied magnetic field14 The graph below displays the chemical shifts of the circled hydrogens. Which hydrogen atom would you predict to be at 4 ppm? E E Would the circled hydrogen be expected to occur in region A or B of the following spectrum? A 1 0 B 4 3 10 9 8 7 H H 6 ppm 5 4 3 H A. H O H O H B. O O O OH OH deshielding of hydrogen nuclei 2 H O NH NH H H deshielding deshielding of hydrogen nuclei requires higher energy to induce a nuclear spin transition. 10 9 8 7 H H 6 ppm 5 shielding shielding of hydrogen nuclei 2 1 0 H O H O NH NH H O H O O H H H O O OH OH H O H H HO H HO O OO O O H H H HO H HO O OO O O H 15 16 chemical chemical shift ? H H H O H O NH NH H O H O O H H HO H HO O OO O O H H H O O OH OH chemical chemical shift H H C H H C H H H H H applied applied field 17 actual field at H actual field at H 18 3 11/12/2009 chemical chemical shift Assign the indicated hydrogen atoms to the nmr spectrum in the following molecule. aromatic aromatic H H H H O H H H CH CH3 methyl methylene methylene A. A. aromatic = methylene = methyl = B. aromatic = methylene = methyl = C. aromatic = methylene = methyl = D. aromatic = methylene = methyl = H H C E E 10 9 8 7 6 ppm 5 4 3 2 1 0 19 20 20 The area under the peaks in a H nmr spectrum are proportional to the different types of hydrogens. aromatic aromatic H H H H O H H H CH CH3 methyl methylene methylene Which of the following hydrogens would you expect to be furthest to the right in the 1H nmr spectrum. A CH methyl - H methylene H 3 B Electronegativities: O = 3.5 C = 2.5 H = 2.1 O CH3 methyl - peak peak area = 5 2 3 C deshielding of hydrogen nuclei 21 22 Assign the indicated hydrogen atoms to the nmr spectrum in the following molecule. A. methyl - = A. methylene = methyl - = B. methyl - = methylene = methyl - = C. methyl - = methylene = methyl - = D. methyl - = methylene = methyl - = H CH3 methyl O methylene H CH3 methyl - 23 24 4 ...
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