Functional Group	Bond	Wavenumber Range (cm⁻¹)	Intensity / Shape
Alcohol, phenol	O–H	3200–3550	Strong, broad
Carboxylic acid	O–H	2500–3300	Strong, very broad (often overlaps with C–H stretches)
Primary amine	N–H	3300–3500	Medium; two bands (asymmetric + symmetric stretch)
Secondary amine	N–H	3300–3500	Medium; one band
Alkane	C–H (sp³)	2850–2960	Medium
Alkene, arene	C–H (sp²)	3000–3100	Medium
Alkyne	C–H (sp)	~3300	Sharp, medium–strong
Aldehyde	C–H (of C(=O)H)	2720 and 2820	Two weak bands, diagnostic alongside the C=O stretch
Alkyne	C≡C	2100–2260	Weak (can be absent if the alkyne is symmetric)
Ether	C–O	1050–1150	Strong
Alkene, arene	C=C	1450–1680	Medium; aromatic rings often show several bands in this range

Carbonyl (C=O) Stretch by Compound Class

The carbonyl stretch is one of the most diagnostic IR signals, and its exact position tracks the same reactivity/resonance trend developed in Chapters 12–13 and Appendix C: the more a heteroatom lone pair donates into the carbonyl (lowering its double-bond character), the lower the stretching frequency.

Compound Class	C=O Range (cm⁻¹)	Note
Acid chloride	1790–1815	Highest frequency — chlorine’s lone pairs overlap poorly with the carbonyl carbon (weak resonance donation), while its electronegativity withdraws electron density inductively; the net effect is high double-bond character and most electrophilic carbon (matches its position at the top of the reactivity order in Appendix C)
Anhydride	1750 and 1820	Two bands (asymmetric + symmetric stretch of the two carbonyls)
Ester	1735–1750	—
Aldehyde	1720–1740	Paired with the diagnostic 2720/2820 C–H doublet above
Ketone	1705–1725	—
Carboxylic acid	1710–1760	Broadened/shifted by hydrogen bonding (acids often exist as dimers)
Amide	1630–1695	Lowest frequency — nitrogen’s lone pair donates most effectively into the carbonyl, reducing its double-bond character (same resonance argument as amide’s low reactivity and low nitrogen basicity, Appendix A)

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