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UB CHE 202 ir values
IR Stretching Frequencies
| Question | Answer |
|---|---|
| C-C | 1200 cm-1 |
| isolated C=C | 1640-1680 cm-1 |
| conjugated C=C | 1620-1640 cm-1 |
| aromatic C=C | approx 1600 cm-1 |
| C=-C | just below 2200 cm-1, usually does not appear for internal, only terminal appears |
| C=-N | just above 2200 cm-1 |
| in general C=C and variations | 1660cm-1, conjugation lowers frequency, aromatic is around 1600cm-1 |
| distinguishing between C=-C and C=-N | nirtogen form is just above 2200cm-1 while carbon only is just below 2200cm-1. There is a greater polarity and stronger bond so nitrogen form is higher |
| sp3 C-H | 2800-3000 cm-1 (just below 3000cm-1) |
| sp2 C-H | 3000-3100 cm-1(just above 3000 cm-1) |
| sp C-H | 3300 cm-1 sharp, usually strong |
| O-H, alcohol | 3300 cm-1 broad |
| O-H, acid | 3000 cm-1, broad - stronger H bonding makes it lower than an alcohol, probably shoulder/spikes around 2500-2700cm-1 |
| C-O | 1060 cm-1 usually in 1000-1200 cm-1 range, unreliable because other things absorb there too |
| distinguishing an ether | Look for 1000-1200 cm-1 absorption and absence of 1700cm-1 and broad 3300/3000cm-1 absorptions |
| N-H, primary amine | 3300cm-1, 2 sharp spikes |
| N-H, secondary amine | 3300cm-1, 1 sharp spike |
| typical C=O | 1700cm-1, overtones usually seen around 3400 cm-1 |
| ketone C=O | 1710 cm-1 |
| aldehyde frequencies | C=O at 1725cm-1, 2 lowered C-H frequencies around 2700cm-1 and 2800 cm-1 |
| Carboxylic acid C=O | 1710 cm-1 |
| conjugated C=O | 1685 cm-1, resonance stabilization lowers typical C=O stretch. It is a good way to know there is a C=C even if the C=C stretch does not appear |
| C=O of amides | 1640-1680cm-1, exhibits resonance. DO NOT MISTAKE FOR AN ALKENE. It will be stronger and broader than alkene. |
| Distinguishing between a C=O of an amide and an Alkene | C=O of amide is stronger and broader. Amide has N-H stretches and spikes. Alkene will have C-H above 3000cm-1 |
| C=O of an ester | 1735cm-1 |
| strained cyclic ketones C=O (5 membered or smaller) | Will absorb above 1725 b/c of increased e- density |
| C-N | 1200cm-1, rarely useful |
| C=N | 1660cm-1, much stronger than C=C although same region |
Created by:
HugsAndKisses
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