Infra-red spectroscopy and identifying compounds
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The teaching part (2 min) is free to watch. A free account opens the whole lecture: 1 worked example, 2 practice questions, 4 real past-paper questions with the answers explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
What this lecture covers
- Bonds that vibrate like springs
- Reading an infra-red spectrum
- The absorption table
Read the transcript of the teaching part
Last time
Before we start: last time, in Reaction routes and functional-group tests, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
Bonds that vibrate like springs
Every bond in a molecule stretches and bends like a spring. Shine infra-red radiation through a sample and each type of bond absorbs its own particular frequency. The absorptions appear as dips in a spectrum, and from them you can read which bonds are present, and so which functional groups the molecule has.
Reading an infra-red spectrum
The horizontal axis is wavenumber in reciprocal centimetres, running from high on the left to low on the right. The vertical axis is transmittance, so absorptions point downwards. A given bond absorbs in a characteristic range. You are given a table, and you match each dip to a bond. A bond absorbs infra-red only if its vibration changes the dipole moment. Polar bonds absorb strongly.
The absorption table
C–O in alcohols and esters: 1040 to 1300. C double bond C in alkenes and aromatics: 1500 to 1680. C double bond O: amide 1640 to 1690, carboxylic acid 1670 to 1740, ester 1710 to 1750. C triple bond N in nitriles: 2200 to 2250. C–H in alkanes: 2850 to 2950. N–H in amines and amides: 3300 to 3500. O–H in alcohols: 3200 to 3600, broad. O–H in carboxylic acids: 2500 to 3300, very broad.
Chapter 22: Analytical techniques
- Mass spectrometry 7 min
- Infra-red spectroscopy and identifying compounds 8 min
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