17.11 Spectroscopy of Alcohols and Phenols

Spectroscopic Analysis of Alcohols and Phenols

Alcohol and phenol IR spectra

The IR spectrum is often the fastest way to confirm an alcohol or phenol is present. The O-H stretch is broad and hard to miss, and the C-O stretch region can help you distinguish between primary, secondary, and tertiary alcohols.

Alcohols show two key absorptions:

• O-H stretch: Broad, strong band at 3200–3600 $\text{cm}^{-1}$. The broadness comes from hydrogen bonding between molecules. A free (non-hydrogen-bonded) O-H would appear as a sharp peak near 3600 $\text{cm}^{-1}$, but in practice you almost always see the broad, hydrogen-bonded version.
• C-O stretch: Strong band at 1000–1260 $\text{cm}^{-1}$. The exact position helps classify the alcohol:
  • Primary alcohols: ~1050 $\text{cm}^{-1}$
  • Secondary alcohols: ~1100 $\text{cm}^{-1}$
  • Tertiary alcohols: ~1150 $\text{cm}^{-1}$

Phenols share the broad O-H stretch (3200–3600 $\text{cm}^{-1}$) but differ in two ways:

• C-O stretch appears at higher wavenumbers, around 1180–1260 $\text{cm}^{-1}$, because the C-O bond has partial double-bond character from resonance with the aromatic ring.
• Aromatic C=C stretches show up as medium-intensity bands near 1450–1600 $\text{cm}^{-1}$, confirming the aromatic ring.

$^1$H NMR analysis of alcohols

NMR gives you the most detailed structural picture. Three features matter: chemical shifts, splitting patterns, and integration.

Chemical shifts:

• The hydroxyl proton (–OH) typically appears as a broad singlet between 1–5 ppm. Its exact position varies with concentration, solvent, and temperature. You can confirm it's an –OH by adding $\text{D}_2\text{O}$ to the sample: the –OH signal disappears because the proton exchanges with deuterium.
• Protons on the carbon bearing the –OH (the $\alpha$-carbon) are deshielded by the electronegative oxygen and shift downfield, typically appearing around 3.4–4.0 ppm. Compare this to a simple alkane $\text{CH}_2$, which would appear near 1.2 ppm.

Splitting patterns:

• Protons on the $\alpha$-carbon couple with neighboring protons following the $n + 1$ rule. For example, in 1-propanol ($\text{CH}_3\text{CH}_2\text{CH}_2\text{OH}$), the $\text{CH}_2$ next to the –OH is split into a triplet by the adjacent $\text{CH}_2$.
• The –OH proton itself usually does not show coupling in routine spectra because rapid proton exchange averages out the splitting. This is why it appears as a broad singlet.
• Protons farther from the –OH group split normally, just as they would in an alkane.

Integration:

• The area under each signal is proportional to the number of protons producing that signal. This lets you determine ratios (e.g., a 3:2:2:1 pattern for 1-propanol).
• The –OH proton's integration can be unreliable because exchange with trace water in the solvent may inflate or reduce its apparent area.

Mass spectrometry of alcohols

Alcohols rarely show a strong molecular ion peak ($\text{M}^+$) because they fragment easily. Two fragmentation pathways dominate:

Alpha cleavage:

1. The C–C bond next to the –OH group breaks, producing a resonance-stabilized oxocarbenium ion ($\text{R}_2\text{C}=\text{OH}^+$) and a neutral radical.
2. More substituted cations are more stable, so the fragment that gives the more stable cation will be more abundant. Tertiary > secondary > primary.
3. Common fragments from alpha cleavage:
   • $[\text{M} - 15]^+$: loss of a methyl group (e.g., 2-propanol loses $\text{CH}_3 \cdot$)
   • $[\text{M} - 29]^+$: loss of an ethyl group
   • $[\text{M} - 1]^+$: loss of a hydrogen atom from the $\alpha$-carbon

Dehydration (loss of water):

1. The molecular ion loses $\text{H}_2\text{O}$ (mass 18) to form an alkene radical cation.
2. This pathway is especially favored for secondary and tertiary alcohols, where the resulting carbocation is more stable.
3. Key fragments:
   • $[\text{M} - 18]^+$: loss of water (the most diagnostic dehydration peak)
   • $[\text{M} - 33]^+$: loss of water plus a methyl radical (seen in some branched alcohols)

Additional spectroscopic techniques

• UV-Vis spectroscopy is most useful for phenols. Simple alcohols absorb only in the far UV (below 200 nm), but phenols absorb near 270 nm due to the aromatic $\pi \to \pi^*$ transition. Substituents on the ring shift this absorption, which can help identify specific phenols.
• $^{13}$C NMR complements proton NMR. The carbon bearing the –OH group in alcohols typically resonates at 50–90 ppm, well downfield from a typical alkane carbon (~10–50 ppm). In phenols, the C–OH carbon appears around 150–160 ppm due to the aromatic environment.