Practice Problems on the NMR of Amino Acids, Exercises of Chemistry

Download Practice Problems on the NMR of Amino Acids and more Exercises Chemistry in PDF only on Docsity! Practice problems on the NMR of amino acids Test your ability to correlate NMR spectra with structure by trying the following problems. Use the correlation tables, the relative sizes of the signal, electronegativity and the number of signals and logic to connect the structures with the spectra. These are the signals of protons (1H NMR). We think of the position (in ppm) of the NMR signals in the same way we viewed the frequency of IR signals. The 1H NMR signals are characteristic of certain types of protons in molecules NMR units are ppm (parts per million). We discuss where this ppm unit comes from in class. Leucine and isoleucine are isomers. Two 13C NMR spectra A and B are shown below. One is leucine the other is isoleucine. Which is which? 020406080100120140160180 PPM 020406080100120140160180 PPM A B CH2 CH3 CH3O HO NH2 H isoleucine H NH2 CH2 CH3 CH3 O HO H H leucine The 13C NMR spectrum of valine and leucine are shown. Which is valine and which is leucine? 020406080100120140160180 PPM NH2 CH3 CH3O OH H H valine NH2 CH2 CH3 CH3 O HO H H leucine 020406080100120140160180 PPM signal signal 020406080100120140160180 PPM 020406080100120140160180 PPM Two signals Very close together NH2 CH2O HO H NH2 CH2' OHO HO H tyrosine phenylalanine The 13C NMR spectrum of phenylalanine and tyrosine (both possess aromatic rings) are shown. Which is phenylalanine and which is tyrosine? The 13C NMR spectrum of valine and leucine are shown. Which is valine and which is leucine? 020406080100120140160180 PPM NH2 CH3 CH3O OH H H valine NH2 CH2 CH3 CH3 O HO H H leucine 020406080100120140160180 PPM signal signal An essential difference between valine and leucine is that leucine has one more carbon atom. In the bottom spectrum the CH2 carbon atom of leucine is just to the left of the CH3 carbon atom. 020406080100120140160180 PPM NH2 CH2' OHO HO H tyrosine 020406080100120140160180 PPM Two signals Very close together NH2 CH2O HO H phenylalanine The 13C NMR spectrum of phenylalanine and tyrosine (both possess aromatic rings) are shown. Which is phenylalanine and which is tyrosine? The most significant difference between the two structures is the OH on the benzene ring of the tyrosine. This substituent will cause the carbons of the benzene ring of tyrosine to be much more different chemically than those of phenylalanine. This difference will also be reflected in the differences in the separation of the carbon signal in the NMR