Difficulty: Intermediate | Prerequisites: General Chemistry, basic understanding of covalent bonding and functional groups.
Infrared spectroscopy is one of the primary tools organic chemists use to identify functional groups in a molecule. It works because different chemical bonds absorb infrared radiation at characteristic frequencies, producing an absorption spectrum that acts like a molecular fingerprint. In Organic Chemistry II, you will use IR data alongside NMR and mass spectrometry to determine unknown structures. If you are not yet comfortable naming common functional groups (alcohols, carbonyls, amines, carboxylic acids), review that material first.
IR spectroscopy identifies functional groups by measuring which frequencies of infrared light a compound absorbs. Each bond type (O-H, C=O, N-H, C-H, etc.) absorbs at a characteristic wavenumber range. You read a spectrum left to right, from about 4000 to 400 cm⁻¹, looking for diagnostic peaks.
Infrared (IR) spectroscopy
An analytical technique that passes infrared radiation through a sample and measures which frequencies are absorbed. In simple terms, you shine IR light through a compound and see which wavelengths it soaks up, telling you what bonds are present.
Wavenumber (cm⁻¹)
The unit used for the x-axis of an IR spectrum, measured in reciprocal centimetres. Higher wavenumber means higher energy (shorter wavelength). Think of it as a frequency scale: the further left on the spectrum, the higher the energy of the absorbed light.
Absorption band
A peak or trough on the IR spectrum where a particular bond absorbs energy. Each functional group produces characteristic bands at predictable wavenumber ranges.
Stretching vibration
A vibration in which the bond length increases and decreases along the bond axis. These generally appear at higher wavenumbers (left side of the spectrum) because they require more energy than bending.
Bending vibration
A vibration in which the bond angle changes rather than the bond length. These show up at lower wavenumbers (right side, the fingerprint region).
Fingerprint region (1500 – 500 cm⁻¹)
The lower-wavenumber portion of the spectrum containing a complex pattern of absorptions unique to each compound. Difficult to interpret peak by peak, but useful for confirming identity by direct comparison with a known reference spectrum.
Functional group region (4000 – 1500 cm⁻¹)
The higher-wavenumber portion where most diagnostic stretching absorptions appear: O-H, N-H, C-H, C≡C, C≡N, C=O, and C=C stretches all fall here.
Broad vs. narrow peaks
Broad peaks (wide, rounded) typically indicate hydrogen bonding, as in O-H and N-H stretches. Narrow (sharp) peaks suggest less or no hydrogen bonding.
O-H stretch (alcohols, phenols): ~3200 – 3600 cm⁻¹, broad and strong. The breadth comes from hydrogen bonding. A sharp O-H near 3600 cm⁻¹ suggests a free (non-hydrogen-bonded) hydroxyl.
O-H stretch (carboxylic acids): ~2500 – 3300 cm⁻¹, very broad, often overlapping the C-H region. This is one of the most recognisable patterns in IR: a wide, rounded absorption that can obscure neighbouring peaks.
N-H stretch (1° and 2° amines, amides): ~3300 – 3500 cm⁻¹, medium intensity. Primary amines and amides show two peaks (asymmetric and symmetric stretch). Secondary amines show one peak.
C-H stretch (alkane, alkene, aromatic, aldehyde):
sp³ C-H: ~2850 – 3000 cm⁻¹ (present in nearly all organic molecules)
sp² C-H (alkene/aromatic): ~3000 – 3100 cm⁻¹, medium
sp C-H (alkyne terminal): ~3300 cm⁻¹, narrow and strong
Aldehyde C-H: two peaks near 2720 and 2850 cm⁻¹ (the doublet around 2720 is a classic aldehyde tell)
≡C-H stretch (terminal alkyne): ~3300 cm⁻¹, narrow and strong.
C≡N stretch (nitrile): ~2200 – 2260 cm⁻¹, medium, sharp. This region is often empty, so a peak here is highly diagnostic.
C≡C stretch (alkyne): ~2100 – 2260 cm⁻¹, weak to medium. Symmetric internal alkynes may show no peak at all (no change in dipole moment).
C=O stretch (carbonyl): ~1630 – 1850 cm⁻¹, strong. This is typically the strongest and most obvious peak on a spectrum. The exact position narrows down the type of carbonyl:
Anhydride: two peaks near 1800 and 1750 cm⁻¹
Acyl chloride: ~1790 – 1815 cm⁻¹
Ester: ~1735 – 1750 cm⁻¹
Aldehyde: ~1720 – 1740 cm⁻¹
Ketone: ~1705 – 1720 cm⁻¹
Carboxylic acid: ~1700 – 1725 cm⁻¹
Amide: ~1630 – 1690 cm⁻¹ (lower because of resonance with the nitrogen lone pair)
C=C stretch (alkene): ~1620 – 1680 cm⁻¹, medium to weak.
C=C stretch (aromatic ring): ~1450 – 1600 cm⁻¹, medium, often two peaks.
C-O stretch (alcohols, ethers, esters): ~1000 – 1300 cm⁻¹, strong.
C-N stretch (amines): ~1000 – 1250 cm⁻¹, medium.
C-X stretch (haloalkanes):
C-Cl: ~600 – 850 cm⁻¹
C-Br: ~500 – 680 cm⁻¹
Alkene C-H bending: ~800 – 1000 cm⁻¹, strong and broad.
Nitro compounds (NO₂): two peaks, asymmetric stretch ~1500 – 1570 cm⁻¹ and symmetric stretch ~1300 – 1370 cm⁻¹.
S = strong
M = medium
W = weak
B = broad
N = narrow
V = variable
"A peak at 3000 cm⁻¹ means an alkene." Not quite. sp³ C-H absorbs just below 3000 cm⁻¹ and sp² C-H just above it. The dividing line is right at 3000 cm⁻¹, so you need to check which side of 3000 the peak falls on.
"No carbonyl peak means no oxygen." Ethers and alcohols contain oxygen but lack a C=O, so they show no carbonyl absorption. The C-O stretch appears in the fingerprint region instead.
"A broad O-H peak always means an alcohol." Carboxylic acids also show a broad O-H, but theirs is even wider (extending down to ~2500 cm⁻¹) and typically overlaps C-H stretches. Always check for a C=O near 1710 cm⁻¹ to distinguish the two.
"The fingerprint region is useless." It is harder to assign individual peaks there, but it is essential for confirming compound identity by matching against a reference spectrum. It is also where C-O, C-N, and C-halogen stretches live.
⚠️ Be able to identify a carbonyl (C=O) peak and distinguish between ester, ketone, aldehyde, carboxylic acid, and amide based on peak position and the presence or absence of O-H or N-H stretches.
⚠️ Know the difference between sp³, sp², and sp C-H stretch positions (below, at, and above 3000 cm⁻¹).
⚠️ Recognise the broad O-H of a carboxylic acid versus the broad O-H of an alcohol.
⚠️ The triple-bond region (2100 – 2260 cm⁻¹) is commonly tested because it is usually empty, making nitrile and alkyne peaks easy to spot, or their absence easy to confirm.
⚠️ Exam questions often provide a spectrum and ask you to identify the functional groups present, or choose between candidate structures. Practice reading spectra left to right.
True or false: A broad absorption centred around 3300 cm⁻¹ could indicate either an alcohol O-H or an amine N-H.
True. Both appear in that region. Check for a carbonyl or other clues to distinguish them.
Fill in the blank: The C=O stretch of an amide appears at a ______ wavenumber than that of a ketone because of resonance with the nitrogen lone pair.
Lower.
True or false: A symmetric internal alkyne always shows a C≡C stretch in the IR spectrum.
False. If the alkyne is symmetric, there is no net change in dipole moment and the stretch is IR-inactive.
Fill in the blank: The two characteristic aldehyde C-H stretches appear near ______ and ______ cm⁻¹.
2720 and 2850.
True or false: The fingerprint region spans 4000 – 1500 cm⁻¹.
False. It spans 1500 – 500 cm⁻¹.
Q: An IR spectrum shows a strong, sharp absorption at 1735 cm⁻¹ and no broad O-H peak. Which carbonyl-containing functional group is most likely present?
A: An ester. Esters absorb near 1735 – 1750 cm⁻¹ and lack the broad O-H of a carboxylic acid.
Q: You observe a broad absorption from approximately 2500 to 3300 cm⁻¹ overlapping C-H peaks, plus a strong peak near 1710 cm⁻¹. What functional group does this pattern indicate?
A: A carboxylic acid. The very broad O-H stretch (extending to ~2500 cm⁻¹) combined with a C=O near 1710 cm⁻¹ is the classic carboxylic acid signature.
Q: How would you distinguish a terminal alkyne from an internal alkyne using IR spectroscopy?
A: A terminal alkyne shows a sharp, strong ≡C-H stretch near 3300 cm⁻¹ and a C≡C stretch near 2100 – 2150 cm⁻¹. An internal alkyne lacks the ≡C-H peak, and its C≡C stretch may be very weak or absent if the alkyne is symmetric.
Q: A compound shows two N-H stretches near 3350 and 3450 cm⁻¹ and a C=O stretch near 1650 cm⁻¹. What functional group is present?
A: A primary amide. The two N-H peaks indicate a primary amine or amide, and the carbonyl near 1650 cm⁻¹ (lower than a typical ketone or ester) confirms an amide.
Q: Why does the C=O stretch of an acyl chloride appear at a higher wavenumber (~1800 cm⁻¹) than that of an amide (~1650 cm⁻¹)?
A: Chlorine is electronegative but a poor electron donor by resonance, so it does not reduce the C=O double-bond character significantly. Nitrogen in amides is a good resonance donor, delocalising its lone pair into the carbonyl and reducing C=O bond order, which lowers the stretching frequency.
IR spectroscopy is typically used alongside ¹H NMR and ¹³C NMR to solve structure-determination problems. While NMR tells you about the carbon-hydrogen framework and connectivity, IR tells you which functional groups are present. Mass spectrometry adds the molecular weight. In exam problems, you will often receive all three data sets and need to piece them together.
This material also connects to reaction chemistry: if you run an oxidation of an alcohol to a ketone, IR lets you confirm the product by checking that the broad O-H disappears and a C=O appears near 1715 cm⁻¹.
IR spectroscopy, infrared spectroscopy, IR correlation table, functional group identification, wavenumber, cm⁻¹, carbonyl stretch, O-H stretch, N-H stretch, C-H stretch, fingerprint region, organic chemistry II spectroscopy, IR spectrum interpretation, vibrational spectroscopy