[Real world] observation: puddles and worms…
Inference: it must have rained
Observation [in infrared spectroscopy]: an IR band of this size and shape [at this location]
Inference: probably that functional group
All IR bands are equal but some are more equal than others.
[interpretation: all IR bands come from the vibrational excitation of bonds, but some bands are more distinctive than others and would therefore give us better information]
Moving down the [IR absorption] chart one at a time, matching bands as you go, is not a useful strategy.
Instead, let’s arrange [the bands] by their interpretive power, from great to poor:
[The 3400-3200 region] is the first region you should look at, and there are 4 different shapes of bands that you may see here:
- A smooth tongue means an alcohol [O–H bond]
- Vampire fangs means a primary amine [two N–H bonds]
- One knock-out fang means a 2 amine, or a terminal alkyne [one N–H or Csp–H bond]
- Hairy beard is a sign for a carboxylic acid [O–H bond].
...and if you don’t see any band here, you most likely don’t have any of these functional groups!
In the C=O region, it’s not the shape, but the exact location that reveals the identity of the functional group [~ means ‘around’, see ranges in the IR Absorption Table]:
- Acid chlorides: ~1800 cm–1
- Anhydrides: ~1810 AND 1760 cm–1
- Esters: ~1735 cm–1
- Aldehydes: ~1725 cm–1
- Ketones: ~1715 cm–1
- Acids: ~1710 cm–1
- Amides: ~1690 cm–1
Note that the exact position also depends on the surrounding environment:
- Strain, or conjugated at –O– [or –N–]: +30 cm–1
- Conjugated at C=O: -30 cm–1
At ~2200 cm–1, we find triple bonds: C≡C, and C≡N. Usually sharp, can be smallish.
Hint: Use 3300 cm–1 to distinguish terminal and internal alkynes!
Lastly, You should look at these features (but only after analysing the major bands in the previous paragraphs):
- C–H stretches: Unsaturated C-H stretch above 3000 cm–1, saturated C-H stretch above 3000 cm–1. Saturated and unsaturated C-H can be present at the same time!
- Aromatic C–H bend region: mono = 700, 690 cm–1; ortho = ~750 cm–1; meta = 880, 780, 690 cm–1; para = ~825 cm–1
- Alkene C–H bend region: mono = 990, 910 cm–1 (strong) ; gem (1,1-) disubstituted = ~900 cm–1 (strong); cis = ~700 cm–1 (weak); trans = ~970 cm–1 (strong)
- Aldehydes: a peak at 2700 cm–1, together with a C=O, is indicative of an aldehyde (C–H on the carbonyl carbon). Ketones have no C-H in them and can have no 2700 cm–1 C–H stretch peaks.
We looked at these peaks last because they can be ambiguous to interpret, or they have only a narrow niche of usefulness.
Notes:
- If you’re not sure what the difference between an acid and an alcohol is, you may need to review your functional groups…
- Be careful with molecules potentially having multiple similar functional groups! Two secondary amines would look like a primary amine!
