Analysis of Infrared Spectra

Step 1: Visually divide the spectrum into 4 regions

• 4000-3000 cm-1

• 3000-2000 cm-1

• 2000-1400 cm-1

• 1400-400 cm-1

 

Step 2: 4000-3000 cm-1 region

• Note down whether there is a large, broad peak

  • If present, this is an O-H stretch in an alcohol

• Note down whether there are 1 or 2 medium, broad peaks

  • If present, there is likely a 1º or 2º amine or amide

• Note down if there is a sharp medium peak

  • If present, it is likely the C-H stretch of a terminal alkyne

  • Look for a weak, sharp peak at ~2250 cm-1 which will correspond to the C≡C stretch

• Note down any medium peaks just above 3000 cm-1

  • If present, there are likely Csp2-H alkene or aromatic stretches

 

Step 3: 3000-2000 cm-1 region

• Note down any medium peaks just below 3000 cm-1

  • If present, there are likely Csp3-H alkane stretches (-CH2- or -CH3)

• Note down a broad peak which would extend from ~3300 cm-1 to ~2500 cm-1

  • If present, this belongs to the O-H stretches of a COOH carboxylic acid dimer

• Look for a sharp, medium peak ~2500-2250

  • If present, it’s either C≡C or C≡N

 

Step 4: 2000-1400 cm-1 region

• Note down any strong, sharp peaks between 1750-1680 cm-1

  • If present, this like a C=O carbonyl stretch

• Note down any sharp, weak peaks between 1660-1600 cm-1

  • If present, this is likely an alkene C=C stretch

• Note down any sharp, weak double peaks between 1600-1500 cm-1

  • If present, this is likely an arene (aromatic ring) C=C stretch

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Step 5: 1400-400 cm-1 (OPTIONAL)

• Note down any strong, sharp peaks between 1300-1150

  • If present, they are likely C-O ester stretches