Alcohol Spectroscopy
Alcohol Spectroscopy
IR Spectroscopy
The OH group of an alcohol appears as a large, broad peak around 3300 cm-1 in an IR spectrum. This is different than the OH of a carboxylic acid that appears at 3000 cm-1 and tapers off.
Mass Spectrometry
The molecular ion, M+, peak for an alcohol usually appears, but sometimes it is small or missing. A fragment peak that is often seen is M+-18. This is because a water molecule (which weighs 18) can easily be eliminated from the M+.
1H NMR
Hydroxyl protons are a little strange in the NMR. They can show up anywhere between 2-6 ppm. Because protons on OH 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 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 proton is to add D2O, heavy water, to the NMR sample. D2O interacts with the OH proton and the hydrogen atom and deuterium atoms switch to form O-D. The 1H NMR peak then disappears confirming OH. 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.
13C NMR
Of course, there is no carbon atom in a hydroxyl, OH group, so it is invisible in the 13C NMR. But, the carbon atom attached to the hydroxyl group usually appears between 50-100 ppm.