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Each bond absorbs infrared light in its own region: a strong, sharp C=O near 1700–1750 cm−1, a broad O–H above 3200, a very broad acid O–H down to 2500, triple bonds near 2200 and C–O at 1000–1300; present and absent peaks together identify the functional groups.
Paper packet. Every task here also exists on screen, where it is checked automatically; answers written on paper are not assessed by Nydus. When you are back at a device, enter your answers there.
You will match bonds to absorption regions, identify the group behind a described peak, place a ketone's C=O on the scale, check which peaks a compound's bonds explain, and tell two isomers apart by IR.
You can name a molecule's functional groups from its structure. Infrared spectroscopy runs the other way: it reads the groups from the molecule, by which bonds absorb which light.
An infrared spectrum plots how much light a sample absorbs against wavenumber, in cm−1. A peak or band is an absorption. A broad band is spread over hundreds of cm−1, usually from hydrogen bonding; a sharp peak is narrow.
A bond vibrates like two masses on a spring: the stiffer the spring and the lighter the masses, the higher the frequency, and the higher the wavenumber it absorbs.
| Bond | Region, cm−1 | Look |
|---|---|---|
| O–H, alcohol | 3200–3550 | strong, broad |
| O–H, carboxylic acid | 2500–3300 | very broad |
| N–H | 3300–3500 | medium |
| C–H | 2850–3100 | medium |
| C≡N, C≡C | 2100–2260 | medium |
| C=O | 1680–1750 | strong, sharp |
| C=C | 1620–1680 | medium |
| C–O | 1000–1300 | strong |
Two peaks settle most questions. A strong, sharp peak near 1700–1750 means a C=O. A broad band above 3200 means an O–H; a band so broad it reaches down to 2500, together with a C=O, means a carboxylic acid.
Another way: steps
To read a spectrum:
A missing peak is as informative as a present one. Propanone and propan-1-ol are both C3; propanone shows a strong C=O at 1715 cm−1 and no O–H, propan-1-ol a broad O–H and no C=O. A single spectrum tells them apart. The region below 1500 cm−1, the fingerprint region, is crowded with bending vibrations and is used mainly to match a spectrum against a known one rather than to read groups off. The next lesson looks at what infrared cannot tell you.
An IR peak alone supplies a complete molecular structure. It shows bonds, not the whole molecule.
Every peak above 3000 cm−1 is an O–H. C–H and N–H are there too; O–H is the broad one.
A C=O is an O–H. C=O is sharp near 1700 cm−1.
A missing peak tells you nothing. It rules a group out.
Broad at 3400 cm−1: an alcohol O–H.
High and broad.
Strong at 1050 cm−1: a C–O.
Low region.
Nothing near 1700 cm−1: no C=O. So the compound is an alcohol.
Absence matters too.
What bond absorbs at 2250 cm−1?
A C≡N (or C≡C).
Is there a C=O or an O–H?
Neither: the compound is a nitrile such as ethanenitrile.
Match each bond to the region where it absorbs.
| 3200–3550 cm−1, broad | 2210–2260 cm−1 | 1680–1750 cm−1, strong | 1000–1300 cm−1 | |
|---|---|---|---|---|
| an alcohol O–H | ||||
| C≡N | ||||
| C=O | ||||
| C–O |
An infrared spectrum shows a medium, sharp peak at 2250 cm−1. Which functional group does it point to?
On a wavenumber scale from 1000 to 4000 cm−1, mark where the C=O of a simple ketone such as propanone absorbs.
1000 |——————————| 4000
Mark the position with a cross, then write the value:
A spectrum is claimed to be of ethanol, CH3CH2OH. Mark every peak that ethanol's bonds would explain.
This task has no paper form; do it on a device.
A bottle is labelled either propan-1-ol or propanone. Its infrared spectrum has a strong, sharp peak at 1715 cm−1 and no broad band above 3200 cm−1. Which is it?
Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.
Would the infrared spectrum of ethanoic acid, CH3COOH, show absorption in each region?
| absorption expected | |
|---|---|
| 2500–3300 cm−1, very broad (O–H) | |
| 1680–1750 cm−1 (C=O) | |
| 2210–2260 cm−1 (C≡N) | |
| 1000–1300 cm−1 (C–O) |
You can read functional groups off an infrared spectrum. Tell someone how IR tells propanone from propan-1-ol. Next: what an infrared spectrum cannot tell you.
8. Your turn: a sharp peak at 2250 cm−1 and nothing near 1700 or above 3200, step 3