IR Spectroscopy Functional Groups

How should an IR spectroscopy graph be read

First look at peaks at and above 3000 to determine hybridization

Then look at diagnostic region (1500 cm^-1 and above) and analyze peaks to determine the groups present

Look at fingerprint region (below 1500 cm^-1) if needed

How can the regions of a IR graph be used to determine the bonds

• Bonds to H (C-H, O-H, N-H) found at and above 2700 cm^-1

• Triple bonds (CN, CC) found between 2100 and 2300 cm^-1

• Double bonds (CC, CN, CO) found between 1600 and 1850 cm^-1

• Single bonds (CC, CN, CO) found below 1600 cm^-1 (finger print region)

What factors should be considered when viewing an IR spectroscopy graph

Wavenumber

Intensity

Shape

Alkane C-H (sp³)

Found at wavenumber 2850-3000

Sharp

Alkene C-H (sp²)

Found at 3000-3100 cm^-1

Sharp

Alkyne C-H (sp)

Found at ~3300 cm^-1

Sharp

CO double bond, ketone

Found at 1650-1850 cm^-1

Sharp, tall

O-H, alcohol

Found at 3200-3600 cm^-1

COOH, carboxylic acid

Two peaks:

• OH peak around 3000 to 3600 (broad)

• CO double bond around 1700 to 1900 (a little broad)

NH₂, amine

Found at 3355-3500 cm^-1

A little broad, short

NH, secondary amine

Found at 3355-3500 cm^-1

A little broad, short

C-O bond

Found at 1000-1350 cm^-1

Sharp

Triple bond CC

Found at 2100-2200 cm^-1

Sharp

aromatic ring bonds

Found at 1450-2000 cm^-1 and 1650-2000 cm^-1

Multiple short peaks

What do the peaks of symmetric compounds look like

Peaks disappear or become very short

C-H, aldehyde

Found between 2750 and 2850 cm^-1

Two weak signals

• CO double bond between 1700 to 1900 cm^-1

Triple bonded CN, nitrile

Found at 2200-2300 cm^-1

sp² C-O bond

Found at 1250-1350 cm^-1

sp³ C-O bond

Found at 1000-1100

sp³ C-N bond

Found at 1000-1200

sp³ C-Cl bond

Found at 600-800

sp³ C-Br bond

Found at 500-600