Chapter 24: Phenols

Spectroscopic Analysis

• IR - presence of -OH and aromatic CC and CH

Absorbance (cm-1)	Interpretation
3600 (strong, broad)	OH stretch
> 3000	aromatic CH stretch
1600	CC stretch (pairs)
1200-1250	C-O stretch

• ¹H NMR  -OH proton is more deshielded (higher ppm) in a phenol than in an alcohol.

Resonance (ppm)	Interpretation
4-12 (exchangeable)	-OH proton
7-8	aromatic protons, Ar-H

(exchangeable means that the peak "disappears" when shaken with D₂O)
 
• ¹³C NMR

C-O carbon in the aromatic ring is deshielded by 25 ppm vs the aromatic C-H carbons
Most shielded C-H carbons of ring are located ortho and para to the OH (recall resonance forms)

•   UV-VIS

-OH group affects benzene chromophore by shifting l_max to longer wavelengths.
The effect is very large in basic solution as a result of OH conversion to O^-.
 
• Mass Spectrometry

Peak for the molecular ion, M⁺, is usually prominent

© Dr. Ian Hunt, Department of Chemistry