Organic Chemistry · Structure Determination: Mass Spectrometry and Infrared Spectroscopy
Interpreting Infrared Spectra
On this page 9 sections
In 30 seconds
Interpreting an IR spectrum is systematic pattern matching, not memorization of every band. The spectrum has two working regions: the Functional-group region 4000–1400 cm⁻¹, where diagnostic stretches appear Full entry → (4000–1400 cm⁻¹), where the diagnostic stretches of C–H, O–H, N–H, C≡N, C=O, and C=C appear, and the Fingerprint region Below ~1400 cm⁻¹, dense unique band pattern Full entry → (below ~1400 cm⁻¹), a dense forest of bending and skeletal vibrations unique to each molecule — as individual as a human fingerprint.
Read a spectrum like a checklist: Is there a broad O–H band? A strong C=O band, and where exactly? Are the C–H stretches above or below 3000 cm⁻¹ (unsaturated versus saturated)? A sharp band near 2250 cm⁻¹ (nitrile or alkyne)? Crucially, absence is as informative as presence: no band between 1630 and 1800 cm⁻¹ almost certainly means no carbonyl group. This elimination logic makes IR interpretation fast and reliable.
Why this matters
Functional-group identification by IR is a daily task in synthetic, analytical, and industrial chemistry. A medicinal chemist confirms a ketone reduction by watching the C=O band vanish and an O–H band appear; a forensic analyst matches a paint chip to a car model through fingerprint-region patterns; a quality-control lab verifies aspirin batches by their ester and acid bands. On exams, IR problems give a molecular formula and a spectrum and ask you to choose among candidate structures — a skill built on the systematic checks in this topic.
The college version
Core Concepts
The functional-group region versus the fingerprint region
Everything above 1400 cm⁻¹ is dominated by stretching vibrations of bonds involving hydrogen or strong multiple bonds — the same few bonds repeat in all organic molecules, so this region is readable and assignable. Below 1400 cm⁻¹, bending and coupled skeletal motions produce dozens of overlapping bands; two different compounds almost never match here, so the region confirms identity rather than assigning groups.
Peak shape and intensity carry information
- Broad bands signal Hydrogen bonding O–H (and N–H) groups associating via H-bonds Full entry →: O–H stretches are wide humps (3200–3600 cm⁻¹); carboxylic acid O–H is even broader (2500–3300 cm⁻¹). N–H bands are narrower.
- Sharp, strong bands mean a large dipole-moment change: the C=O stretch is the classic example — strong and usually the most intense band.
- Weak bands can still be diagnostic: C≡C stretches (2100–2260 cm⁻¹) are characteristically weak, and the aldehyde C–H doublet (near 2720 and 2820 cm⁻¹) is easy to miss.
The carbonyl window: check it first
Because the C=O stretch is strong and sits in a relatively empty region (1630–1800 cm⁻¹), it is the natural anchor of interpretation. Its exact position narrows the group: ketones near 1715 cm⁻¹, aldehydes near 1725 cm⁻¹, esters near 1735 cm⁻¹, carboxylic acids near 1710 cm⁻¹ (with a broad O–H), amides near 1650 cm⁻¹ (resonance weakens the C=O), and acid chlorides near 1800 cm⁻¹.
A five-step interpretation protocol
- O–H / N–H (3200–3600): broad hump → alcohol or acid; sharper band (one or two peaks) → amine; nothing → neither.
- C–H (2850–3100): only below 3000 cm⁻¹ → sp³; above 3000 cm⁻¹ → sp²/aromatic; sharp 3300 cm⁻¹ → terminal alkyne C–H; weak 2720/2820 doublet → aldehyde C–H.
- Carbonyl (1630–1800): any strong band? Position and companions (O–H, C–O, N–H) identify the class.
- Triple bond (2100–2260): medium-sharp → nitrile (C≡N); weak → alkyne (C≡C).
- Confirmation (1000–1300 + fingerprint): strong C–O supports alcohols/ethers/esters; fingerprint match confirms identity.
Absence is diagnostic
The most powerful move in IR interpretation is ruling things out: no C=O band → no ketone, aldehyde, ester, acid, amide, or anhydride; no broad O–H → no alcohol or acid; no band above 3000 cm⁻¹ → no alkene, arene, or alkyne C–H. A molecule with no C=O, no O–H, and no multiple-bond stretches is consistent with an alkane — a conclusion reached entirely by elimination.
Common Confusions
| Do not confuse | With | Difference |
|---|---|---|
| Broad O–H band | Sharp N–H band | O–H is a wide hump (H-bonding); N–H is sharper; two N–H bands mean a primary amine |
| Alcohol O–H | Carboxylic acid O–H | Acid O–H is broader (~2500–3300 cm⁻¹) and pairs with C=O near 1710 cm⁻¹ |
| Absence of C=O | "No information" | Absence is strong evidence: no band in 1630–1800 cm⁻¹ rules out all carbonyl groups |
| Band above 3000 cm⁻¹ | Saturated C–H | Above 3000 = sp²/aromatic C–H; saturated C–H sits at 2850–2960 cm⁻¹ |
| Nitrile C≡N | Alkyne C≡C | C≡N is medium-sharp near 2240 cm⁻¹; C≡C is characteristically weak |
| The 2720/2820 cm⁻¹ doublet | C–H overtones | It is the aldehyde C–H — a reliable "aldehyde signature" |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Reading an IR spectrum is like a detective checking clues: "Is there a wide smudge for O–H? A strong mark near 1715 for C=O? A mark above 3000 for a double bond?" Missing clues count too — if there is no C=O mark, there is no carbonyl. The squiggly bottom part (the "fingerprint") is so unique that matching it is like matching real fingerprints.
Worked example
Example 1: Identifying a ketone — butanone (C₄H₈O)
A compound of formula C₄H₈O shows: strong band at 1715 cm⁻¹; C–H stretches at 2960 and 2875 cm⁻¹ (sp³ only); no broad band at 3200–3600 cm⁻¹; nothing near 2720 cm⁻¹.
Reasoning: The 1715 cm⁻¹ band lands in the ketone range of the carbonyl window. No broad O–H rules out alcohol and carboxylic acid; no aldehyde C–H doublet rules out an aldehyde; no strong C–O companion pattern rules out an ester. The structure is consistent with butanone (methyl ethyl ketone), CH3C(=O)CH2CH3.
Example 2: Distinguishing an alcohol from an aldehyde — 1-hexanol vs hexanal
Both are C₆H₁₄O isomers, so mass spectrometry would not separate them, but IR does instantly:
- 1-hexanol: broad, strong band near 3330 cm⁻¹ (O–H, hydrogen bonded); strong C–O band near 1060 cm⁻¹; no carbonyl band.
- Hexanal: no broad O–H; strong C=O at ~1725 cm⁻¹; weak 2720/2820 doublet (aldehyde C–H); C–H stretches straddling 3000 cm⁻¹.
The deciding evidence is the broad O–H versus the C=O + aldehyde C–H doublet: check both the presence of O–H and the position of any carbonyl band.
Example 3: Quantitative IR with the Beer–Lambert law
A carbonyl band reads 5% T through a cell of path length b = 0.10 cm. Find the absorbance, then the concentration if the molar absorptivity is ε= 200 L·mol-1·cm-1.
Step 1 — absorbance from transmittance:
A = 2 - log10(%T) = 2 - log10(5) = 2 - 0.70 = 1.30
Step 2 — Beer–Lambert, solved for c:
A = εb c ⇒ c = Aεb
Substitute with units:
c = 1.30(200 L·mol-1·cm-1)(0.10 cm) = 0.065 mol/L
Dimensional check: A is unitless; L·mol-1·cm-1 × cm = L·mol-1, so 1/(L·mol-1) = mol/L. ✓ The linear A-vs-c relationship is why quantitative work uses absorbance, never raw percent transmittance.
Key takeaways
- Two regions: functional-group region (4000–1400 cm⁻¹, readable) and fingerprint region (<1400 cm⁻¹, identity check).
- Check the carbonyl window (1630–1800 cm⁻¹) first — the C=O stretch is strong, and its position identifies the carbonyl class.
- Broad band at 3200–3600 cm⁻¹ → hydrogen-bonded O–H (alcohol or acid); acid O–H extends down to ~2500 cm⁻¹.
- C–H above 3000 cm⁻¹ → sp²/aromatic; only below 3000 → sp³; sharp 3300 → terminal alkyne; weak 2720/2820 doublet → aldehyde.
- Nitrile C≡N ≈ 2210–2260 cm⁻¹ (medium, sharp); alkyne C≡C ≈ 2100–2260 cm⁻¹ (weak).
- Absence is diagnostic: no C=O band means no carbonyl-containing group.
- Quantitative link: A = 2 - log10(%T) and A = εb c (Beer–Lambert).
Check yourself
5 review questions from the chapter. Try each one, then open the answer.
What are the two working regions of an IR spectrum, and what is each used for?
Show answer
Functional-group region (4000–1400 cm⁻¹) for identifying groups; fingerprint region (<1400 cm⁻¹) for confirming identity.
A spectrum shows a strong band at 1715 cm⁻¹ and a broad hump from 2500–3300 cm⁻¹, with nothing else diagnostic above 1400 cm⁻¹. What functional group is present?
Show answer
A carboxylic acid: the C=O near 1710 cm⁻¹ plus the exceptionally broad O–H (2500–3300 cm⁻¹) is the acid signature.
How would you tell a terminal alkyne C–H from an aromatic C–H?
Show answer
Terminal alkyne C–H is a sharp band near 3300 cm⁻¹ (often with a weak C≡C band ~2100–2140 cm⁻¹); aromatic C–H is weaker, near 3030–3100 cm⁻¹, with ring bands at 1450–1600 cm⁻¹.
A band reads 10% T. What is its absorbance?
Show answer
A = 2 - log10(10) = 2 - 1 = 1.00.
Why is absence of a carbonyl band decisive, even in a noisy spectrum?
Show answer
The C=O stretch is strong and sits in an otherwise quiet region (1630–1800 cm⁻¹); if it is absent, no carbonyl-containing group can be present — absence rules out an entire family at once.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Functional-group region
- 4000–1400 cm⁻¹, where diagnostic stretches appear
- Fingerprint region
- Below ~1400 cm⁻¹, dense unique band pattern
- Transmittance (%T)
- Percent of IR light passing through the sample
- Absorbance (A)
- 2 - log10(%T); scales linearly with concentration
- Hydrogen bonding
- O–H (and N–H) groups associating via H-bonds
- Carbonyl window
- 1630–1800 cm⁻¹, where C=O stretches appear
- Diagnostic band
- A band whose position identifies a functional group
- Elimination reasoning
- Using absent bands to rule out functional groups
- transmittance, T
- Fraction of incident light that passes through the sample (I/I0)
Sources & references
This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.
Educational content only. It is not medical, legal or professional advice. Found an error? Tell us.