Amines and Nitrile Spectroscopy
IR Spectroscopy
NH, NH2
NH bond stretches all appear around 3300.
For benzylamine, there are sp3 CH peaks below 3000 for the CH2 and sp2 CH peaks above 3000 for the aromatic ring. The aromatic overtones below 2000 confirm this. At 3300, we see a two-hump peak. This means there are two N-H bonds present (an NH2).
IR spectrum of benzylamine
For N-methylaniline, we see the sp2 and sp3 CH peaks surrounding 3000. We also see the aromatic overtones indicating aromatic ring. We do see a one-hump peak at 3300. We know this is an N-H stretch or an sp CH stretch. It is fairly sharp. An OH stretch is usually broader and NH2 has two humps. If it was an sp CH stretch, we would also see the C≡C peak below 2200. Therefore, we know that we have an amine with one N-H bond.
IR spectrum of N-methylaniline
Nitriles
In acetonitrile, the C≡N nitrile is the sharp peak just above 2200. C≡C IR peaks also show up near C≡N. We can tell this is not C≡C because C≡N shows up just above 2200 while C≡C is usually just below 2200. Additionally, we usually only see C≡C if it is at the end of a molecule (terminal alkyne). In that case, we also see the sp CH stretch at 3300, which is absent here.
IR spectrum of acetonitrile
Amides
Amide carbonyl shifts are lower than normal carbonyl shifts around 1700. Amide carbonyl shifts are typically in the 1660s range. For acetamide, we see that the carbonyl peak is quite low in the 1660s indicating an amide. We look for secondary peaks at 3300 and find two N-H bonds. We also see sp3 CH peaks below 3000. It is fairly small since there are only three C-H bonds.
IR spectrum of acetamide
Mass Spectrometry
Nitrogen atoms do not have M+2 isotope peaks to help us identify them. But, they do give a clue when they are present. All of the M+ peaks you have seen so far have something in common; they all have an even number m/z. This is remarkable. But, nitrogen compounds are different. If a molecule has an odd number of nitrogen atoms in it, it has an odd molecular mass. So, a typical nitrogen-containing compound with one nitrogen atom will be odd, but if there are two nitrogen atoms present, it is even. Below is the mass spectrum for methylamine, CH3NH2.
Mass Spectrum of methylamine
1H NMR Spectroscopy
Protons on oxygen or nitrogen atoms are a little strange. They can show up anywhere between 2-6 ppm. Because protons on OH and NH are involved in hydrogen bonding, their chemical shift can change depending on the temperature and concentration of the solution. They are not always broad peaks. Sometimes they stay on the molecule enough to couple.
Usually, these peaks are broad, hump-like peaks. This is because the hydrogen atom disassociates from the OH or NH bond. It falls off and returns to the molecule, so there usually is not time for it to couple with other protons.
One laboratory trick to help determine if a 1H NMR peak is from an OH or NH proton is to add D2O, heavy water, to the NMR sample. D2O interacts with the OH or NH proton and the hydrogen atom and deuterium atoms switch to form O-D or N-D. The 1H NMR peak then disappears confirming OH or NH. Protons on carbon atoms will not trade with deuterium of D2O. A new peak will grow in the spectrum, though. HOD is formed in the reaction.
The protons on carbon atoms next door to a nitrile usually show up at 2-3 ppm, like when they are next door to a carbonyl. Protons on carbon atoms next door to the nitrogen atom of an amine usually show up at 3-4 ppm.
13C NMR Spectroscopy
Of course, nitrogen atoms do not show up in 13C NMR spectra. But, carbon atoms next to nitrogen atoms show up between 50-100 ppm in a 13C NMR spectrum. These carbon atoms are “next to something special”. Nitrile carbon atoms typically show up 115-125 ppm.
2. Fill in the blanks to complete the following statements about amine, nitrile, and amide spectroscopy.
a) If the M+ peak in a mass spectrum is ________, it tells us we have an odd number of nitrogen atoms in the compound.
b) If a compound is a primary amine and has an –NH2 group, the IR spectrum should show a broad, double humped peak (rabbit ears) around _______ cm-1.
c) If a compound is a nitrile, it should show a strong, sharp C≡N peak in the IR spectrum around _______ cm-1.
d) If a compound is a secondary amine and has an –NH group, the IR spectrum should show a broad peak with one hump on it (a shark’s tooth) around _______ cm-1.
e) The C=O carbonyl peak for an amide shows up at a __________ wavenumber than “regular” ketone or aldehyde C=O peaks around 1710 cm-1. The carbonyl peak for an amide is closer to _________ cm-1.
f) In the 13C NMR spectrum, the carbon atom for the C=O of an amide appears in the ______________ ppm region. In the 13C NMR spectrum, the carbon atom in the C≡N of a nitrile appears in the ____________ ppm region.
Answers
2.
a) If the M+ peak in a mass spectrum is odd, it tells us we have an odd number of nitrogen atoms in the compound.
b) If a compound is a primary amine and has an –NH2 group, the IR spectrum should show a broad, double humped peak (rabbit ears) around 3200-3300 cm-1.
c) If a compound is a nitrile, it should show a strong, sharp C≡N peak in the IR spectrum around 2200-2300 cm-1.
d) If a compound is a secondary amine and has an –NH group, the IR spectrum should show a broad peak with one hump on it (a shark’s tooth) around 3200-3300 cm-1.
e) The C=O carbonyl peak for an amide shows up at a lower wavenumber than “regular” ketone or aldehyde C=O peaks around 1710 cm-1. The carbonyl peak for an amide is closer to 1660 cm-1.
f) In the 13C NMR spectrum, the carbon atom for the C=O of an amide appears in the 150-200 ppm region. In the 13C NMR spectrum, the carbon atom in the C≡N of a nitrile appears in the 100-150 ppm region.