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Infrared shows which bonds a compound has but not where they are, how long the chain is, or the molecular mass; it rules candidates out, and matching group peaks shows consistency, not identity.
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 sort questions IR can and cannot answer, decide which pairs IR can tell apart, name what IR cannot give, count the structures a spectrum leaves, and judge what a peak match proves.
You can read which functional groups a spectrum shows. The other half of using evidence well is saying what it leaves open.
A candidate structure is one consistent with the evidence so far. Consistent with means not ruled out; it is weaker than identified as. The fingerprint region, below 1500 cm−1, is unique to each compound but hard to interpret by eye.
Infrared sees bonds. So it cannot tell apart compounds that have the same bonds in different places:
What it does well is rule things out. For C3H8O the isomers are propan-1-ol, propan-2-ol and methoxyethane. A broad O–H band removes the ether and leaves two alcohols; no O–H band leaves only the ether.
Another way: steps
To use IR honestly:
A spectrum that shows an O–H, a C=O and a benzene ring is consistent with aspirin and with dozens of other compounds. To identify a substance, a chemist matches its whole spectrum, fingerprint region included, against a reference, and usually adds other evidence: a mass spectrum for the molecular mass, NMR for which atoms are next to which. The next lessons add the mass spectrum and then combine the evidence.
Every functional group has one exact IR frequency. Groups absorb over a range.
Matching peaks identifies a compound. It shows consistency.
IR tells where a group is on the chain. It shows the group, not its position.
IR gives the molecular mass. It gives none.
Isomers of C4H10O: four alcohols and three ethers.
List the candidates.
No O–H rules out all four alcohols.
Strike out what is contradicted.
Three ethers remain, which IR alone cannot separate.
Say what is left.
Which isomers have no O–H?
Only methoxyethane.
How many candidates remain?
One: the compound is methoxyethane.
A chemist runs an infrared spectrum on an unknown liquid. Mark every question the spectrum alone can answer.
This task has no paper form; do it on a device.
Match each pair of compounds to whether infrared group peaks can tell them apart.
| yes: one has a bond the other lacks | no: same functional group, same group peaks | |
|---|---|---|
| propanone and propan-2-ol | ||
| butan-1-ol and butan-2-ol | ||
| hexane and hex-1-ene |
Which of these can an infrared spectrum not tell you about a pure compound?
A compound is C3H8O. Its IR spectrum shows a broad O–H band and no C=O. How many structures are still possible?
Answer:
An analyst reports that a seized powder 'is aspirin' because its IR spectrum shows an O–H band, a C=O peak and aromatic peaks, all of which aspirin has. How strong is the claim?
Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.
A compound is C3H8O. Its IR spectrum shows no O–H band and no C=O. How many structures are still possible?
The number of structures still possible is a.
You can say what an infrared spectrum leaves open. Tell someone why IR cannot tell butan-1-ol from butan-2-ol. Next: the mass spectrum, which gives the molecular mass IR cannot.
8. Your turn: C3H8O with no O–H band, step 3