17.11 Spectroscopy of Alcohols and Phenols

Infrared Spectroscopy

Alcohols have a strong C–O stretching absorption near 1050 cm^–1 and a characteristic O–H stretching absorption at 3300 to 3600 cm^–1. The exact position of the O–H stretch depends on the extent of hydrogen-bonding in the molecule. Unassociated alcohols show a fairly sharp absorption near 3600 cm^–1, whereas hydrogen-bonded alcohols show a broader absorption in the 3300 to 3400 cm^–1 range. The hydrogen-bonded hydroxyl absorption appears at 3350 cm^–1 in the IR spectrum of cyclohexanol (Figure 17.12).

Figure 17.12 IR spectrum of cyclohexanol. Characteristic O–H and C–O stretching absorptions are indicated.

Phenols also show a characteristic broad IR absorption at 3500 cm^–1 due to the –OH group, as well as the usual 1500 and 1600 cm^–1 aromatic bands (Figure 17.13). In phenol itself, monosubstituted aromatic-ring peaks are visible at 690 and 760 cm^–1.

Figure 17.13 IR spectrum of phenol.

Nuclear Magnetic Resonance Spectroscopy

Carbon atoms bonded to electron-withdrawing –OH groups are deshielded and absorb at a lower field in the ¹³C NMR spectrum than do typical alkane carbons. Most alcohol carbon absorptions fall in the range 50 to 80 δ, as shown in the following drawing for cyclohexanol:

Alcohols also show characteristic absorptions in the ¹H NMR spectrum. Hydrogens on the oxygen-bearing carbon atom are deshielded by the electron-withdrawing effect of the nearby oxygen, and their absorptions occur in the range 3.4 to 4.5 δ. Spin–spin splitting, however, is not usually observed between the O–H proton of an alcohol and the neighboring protons on carbon. Most samples contain small amounts of acidic impurities, which catalyze an exchange of the O–H proton on a timescale so rapid that the effect of spin–spin splitting is removed. It’s often possible to take advantage of this rapid proton exchange to identify the position of the O–H absorption. If a small amount of deuterated water, D₂O, is added to an NMR sample tube, the O–H proton is rapidly exchanged for deuterium and the hydroxyl absorption disappears from the spectrum.

Typical spin–spin splitting is observed between protons on the oxygen-bearing carbon and other neighbors. For example, the signal of the two –CH₂O– protons in 1-propanol is split into a triplet by coupling with the neighboring –CH₂– protons (Figure 17.14).

Figure 17.14 ¹H NMR spectrum of 1-propanol. The protons on the oxygen-bearing carbon are split into a triplet at 3.58 δ.

Phenols, like all aromatic compounds, show ¹H NMR absorptions near 7 to 8 δ, the expected position for aromatic-ring protons (Section 15.7). In addition, phenol O–H protons absorb at 3 to 8 δ. In neither case are these absorptions uniquely diagnostic for phenols, since other kinds of protons absorb in the same range.

Mass Spectrometry

As noted in Section 12.3, alcohols undergo fragmentation in the mass spectrometer by two characteristic pathways, alpha cleavage and dehydration. In the alpha-cleavage pathway, a C–C bond nearest the hydroxyl group is broken, yielding a neutral radical plus a resonance-stabilized, oxygen-containing cation.

In the dehydration pathway, water is eliminated, yielding an alkene radical cation.

Both fragmentation modes are apparent in the mass spectrum of 1-butanol (Figure 17.15). The peak at m/z = 56 is due to loss of water from the molecular ion, and the peak at m/z = 31 is due to an alpha cleavage.

Figure 17.15 Mass spectrum of 1-butanol (M⁺ = 74). Dehydration gives a peak at m/z = 56, and fragmentation by alpha cleavage gives a peak at m/z = 31.