3600–3650 Sharp, strong Free O–H (phenol/alcohol) Not H-bonded 3500–3450 Sharp, medium 2° amine N–H (–NH) Single absorption 3400–3350 Sharp, medium 1° amine N–H (–NH₂) Usually two peaks 3300–3100 Sharp, medium Aromatic =C–H Aromatic feature; alkene weak 3000–2850 Med–strong Alkyl C–H (sp³) Normal alkane CH₃, CH₂ 2850 & 2750 Weak, doublet Aldehyde –CHO Typical aldehyde feature 2260–2240 Sharp, strong Nitrile C≡N Characteristic band 2110–2100 Sharp, weak–m Terminal alkyne C≡C–H Internal not always clear 1745–1735 Sharp, strong Ester C=O R–COOR′ 1725–1715 Sharp, strong Ketone C=O (non-conjugated) R–CO–R′ 1705–1695 Sharp, strong α,β-Unsat ketone C=O Conjugation lowers freq 1690–1680 Sharp, strong Carboxylic acid C=O R–COOH + broad O–H 1660–1640 Med–strong α,β-Unsat C=C Conjugated double bond 1600–1580 Med–strong,mul Aromatic C=C stretch Aromatic fingerprint 1500–1485 Med–strong Aromatic C–C stretch Aromatic feature 900–700 Sharp, strong Aromatic C–H oop bend Substitution pattern 1370–1365 Weak–med Methyl –CH₃ bend Symmetric bending 1220–1210 Medium, sharp Ester C–O–C stretch R–CO–O–R′ 1100–1080 Med–strong Ether C–O–C stretch R–O–R
Mono 750, 690 Two sharp peaks; 750 > 690 Ortho 735–770 One broad (or two close) peaks Meta 780, 730, 700 Three peaks, medium strength
Determination:
Notes:
IR Absorption Table
Wavenumber (cm⁻¹) Absorption Group / Structure Notes
3600–3650 Sharp, strong Free O–H (phenol/alcohol) Not H-bonded
3500–3450 Sharp, medium 2° amine N–H (–NH) Single absorption
3400–3350 Sharp, medium 1° amine N–H (–NH₂) Usually two peaks
3300–3100 Sharp, medium Aromatic =C–H Aromatic feature; alkene weak
3000–2850 Med–strong Alkyl C–H (sp³) Normal alkane CH₃, CH₂
2850 & 2750 Weak, doublet Aldehyde –CHO Typical aldehyde feature
2260–2240 Sharp, strong Nitrile C≡N Characteristic band
2110–2100 Sharp, weak–m Terminal alkyne C≡C–H Internal not always clear
1745–1735 Sharp, strong Ester C=O R–COOR′
1725–1715 Sharp, strong Ketone C=O (non-conjugated) R–CO–R′
1705–1695 Sharp, strong α,β-Unsat ketone C=O Conjugation lowers freq
1690–1680 Sharp, strong Carboxylic acid C=O R–COOH + broad O–H
1660–1640 Med–strong α,β-Unsat C=C Conjugated double bond
1600–1580 Med–strong,mul Aromatic C=C stretch Aromatic fingerprint
1500–1485 Med–strong Aromatic C–C stretch Aromatic feature
900–700 Sharp, strong Aromatic C–H oop bend Substitution pattern
1370–1365 Weak–med Methyl –CH₃ bend Symmetric bending
1220–1210 Medium, sharp Ester C–O–C stretch R–CO–O–R′
1100–1080 Med–strong Ether C–O–C stretch R–O–R
1040–1020 Medium Alcohol C–O stretch Stronger w/ O–H stretch
Aromatic C–H Out-of-plane Bending & Substitution
Substitution Peaks (cm⁻¹) Description
Mono 750, 690 Two sharp peaks; 750 > 690
Ortho 735–770 One broad (or two close) peaks
Meta 780, 730, 700 Three peaks, medium strength
Para 825–845, 800 Two peaks; one strong, one weak
Determination:
Notes:
- Substituent type may shift by a few cm⁻¹, pattern unchanged
- Multiple substituents → overlapping peaks, check aromatic C=C region too
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