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Functional Groups in Organic Chemistry

Molecular Spectroscopy

Both the IR and 1H NMR spectroscopic analysis of alcohols have some interesting features making spectroscopic analysis of alcohols a very MCAT writer friendly topic. In the infrared spectrum of an alcohol, the O-H stretch is particularly noticeable, appearing as a broad peak in the 3200 to 3650 cm-1 region. The broad peak is due to the fact that variations in the intermolecular associations of the hydrogen bonded alcohol molecules lead to variations in the vibrational frequency of the O-H bond of their hydroxyl group. However, it is possible for the IR spectra of alcohols not to have this broad signal. In a very dilute solution of the sample, or in the gas phase, hydrogen bonding is prevented through lack of proximity. In these cases the broad O-H peak is replaced by a sharp signal.

Like IR spectrosophy, the 1H NMR spectroscopy of alcohols has some interesting features. In 1H NMR, the hydroxyl proton appears as a moderately shifted singlet. However, because the hydroxyl proton is acidic, this singlet will disappear if the alcohol is dissolved in D2O.

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