Spectroscopic Analysis of Alcohols and Thiols

The S—H stretching frequency of thiols gives rise to a weak band in the range 2550-2700 cm  

NMR The most helpful signals in the NMR spectrum of alcohols result from the O—H proton and the proton in the H—C—O unit of primary and secondary alcohols. 

NMR The electronegative oxygen of an alcohol decreases the shielding of the carbon to which it is attached. The chemical shift for the carbon of the C—OH is 60-75 ppm for most alcohols. Carbon of a C—S group is more shielded than carbon of C—O. 

UV-VIS Unless the molecule has other chromophores, alcohols are transparent above about 200 nm Xj ax for methanol, for example, is 177 nm. 

Mass Spectrometry The molecular ion peak is usually quite small in the mass spectrum of an alcohol. A peak corresponding to loss of water is often evident. Alcohols also fragment readily by a pathway in which the molecular ion loses an alkyl group from the hydroxyl-bearing carbon to form a stable cation. Thus, the mass spectra of most primary alcohols exhibit a prominent peak at m/z 31. 

Interpreting the mass spectra of sulfur compounds is aided by the observation of an M-l-2 peak because of the presence of the mass-34 isotope of sulfiir. The major cleavage pathway of thiols is analogous to that of alcohols. 

---