OChem IR Spectroscopy

Alcohol (R–OH)

• O–H stretch: 3200–3600 cm⁻¹
• Broad, rounded peak
• C–O stretch: 1000–1300 cm⁻¹

Identification:
• Broad O–H peak
• No carbonyl (~1700 cm⁻¹)

Carboxylic Acid (R–COOH)

• O–H stretch: 2500–3300 cm⁻¹
• Extremely broad smear
• C=O stretch: ~1710 cm⁻¹
• Strong, sharp peak

Identification:
• Must have:
→ Huge broad O–H
→ Strong carbonyl peak
• One of the easiest groups to recognize

Primary Amine (RNH₂)

• N–H stretch: 3300–3500 cm⁻¹
• Two peaks:
→ Symmetric N–H stretch
→ Asymmetric N–H stretch
• N–H bend: ~1600 cm⁻¹

Identification:
• Two N–H peaks
• No carbonyl

Secondary Amine (R₂NH)

• N–H stretch: ~3300 cm⁻¹
• One peak

Identification:
• One N–H stretch
• No carbonyl

Tertiary Amine (R₃N)

• No N–H stretch
• No characteristic N–H peak

Identification:
• No N–H
• No carbonyl
• Difficult to identify by IR alone

Primary Amide (RCONH₂)

• C=O stretch: 1650–1690 cm⁻¹
• Strong peak
• N–H stretch: 3100–3350 cm⁻¹
• Two N–H peaks
• N–H bend: ~1600 cm⁻¹

Identification:
• Carbonyl + two N–H peaks
• C=O lower than ketones due to resonance

Secondary Amide (RCONHR)

• C=O stretch: 1650–1690 cm⁻¹
• Strong peak
• N–H stretch: ~3300 cm⁻¹
• One peak

Identification:
• Carbonyl + one N–H peak

Tertiary Amide (RCONR₂)

• C=O stretch: 1650–1690 cm⁻¹
• Strong peak
• No N–H stretch

Identification:
• Amide carbonyl present
• No N–H peaks

Alkane (sp³ C–H)

• C–H stretch: 2850–2960 cm⁻¹

Appearance:
• Several medium peaks

Identification:
• Peaks just below 3000 cm⁻¹
• Present in most organic molecules

Alkene (C=C)

• =C–H stretch: ~3050 cm⁻¹
• Above 3000 cm⁻¹
• C=C stretch: 1600–1650 cm⁻¹

Identification:
• sp² C–H above 3000
• C=C peak around 1650

Aromatic Ring (Benzene)

• Aromatic C–H stretch: ~3050 cm⁻¹
• Aromatic C=C stretches:
→ ~1600 cm⁻¹
→ ~1500 cm⁻¹

Identification:
• Classic aromatic pattern:
→ 3050
→ 1600
→ 1500

Terminal Alkyne (RC≡CH)

• ≡C–H stretch: ~3300 cm⁻¹
• Sharp, narrow peak
• C≡C stretch: 2100–2200 cm⁻¹

Identification:
• Sharp 3300 peak
• Not broad like O–H

Internal Alkyne (RC≡CR)

• C≡C stretch: 2100–2200 cm⁻¹

Appearance:
• Weak or sometimes absent

Identification:
• No ≡C–H stretch
• No terminal hydrogen

Nitrile (C≡N)

• C≡N stretch: ~2250 cm⁻¹

Appearance:
• Sharp
• Strong
• Isolated peak

Identification:
• One of the easiest peaks to recognize

Ketone (RCOR)

• C=O stretch: ~1715 cm⁻¹

Appearance:
• Very strong
• Sharp peak

Identification:
• Large carbonyl peak
• No O–H
• No aldehyde C–H peaks

Aldehyde (RCHO)

• C=O stretch: ~1725 cm⁻¹
• Strong peak
• Aldehyde C–H stretch:
→ 2720–2820 cm⁻¹
→ 2820–2900 cm⁻¹

Identification:
• Carbonyl + aldehyde C–H doublet near 2800 cm⁻¹

Ester (RCOOR)

• C=O stretch: ~1735 cm⁻¹
• Strong, sharp peak
• C–O stretch: 1050–1300 cm⁻¹

Identification:
• Highest common carbonyl frequency
• Oxygen strengthens C=O bond

Ether (ROR)

• C–O stretch: 1050–1150 cm⁻¹

Appearance:
• Strong peak

Identification:
• No O–H
• No C=O
• Identify by C–O stretch