Chapter  19: Carboxylic Acids

Spectroscopic Analysis
 

• IR - The -O-H and C=O should be obvious

Absorbance (cm-1)	Interpretation
2500 - 3500 (very broad)	OH stretch
1700	C=O stretch
1200-1250	C-O stretch

• ¹H NMR - The -CO₂H proton is very deshielded

 

Resonance (ppm)	Interpretation
10 -12 (exchangeable)	-COOH proton
2 - 3	H-C-COOH

• ¹³C NMR

CO₂H carbon  160 - 185 ppm (deshielded due to O, but not as much as aldehydes and ketones, 190-215 ppm)
 
• UV-VIS

Simple carboxylic acids absorb at 210 nm, but this is too low to be particularly useful.
 
• Mass Spectrometry

Peak for the molecular ion, M⁺, is usually prominent.
Fragments due to loss of OH (M - 17)⁺ and then loss of CO (M - 45)⁺

© Dr. Ian Hunt, Department of Chemistry