C, CN and NO2 are the most conspicuous and readily identified. Do not try to analyse in detail the C-H absorptions (other than are they above or below 3000 cm-1).
Working through the following checklist in sequence should provide enough information for you to identify what is present in your compound.
| A | Is there a carbonyl group present? | Look for strong absorption in the region 1820-1660 cm-1. This peak is often the strongest in the spectrum and of medium width. |
| If a carbonyl group is present - Go to B. If no carbonyl group is present - Go to C | ||
| B | Carbonyl group present. Check also for the presence of: | |
| Acids | Look for a broad absorption in the region 3400-2400 cm-1 | |
| Amides | Look for a medium absorption near 3400 cm-1; sometimes this will be a double peak with two equivalent halves, e.g. R-NH2 | |
| Esters | Look for a strong absorption in the region 1300-1100 cm-1 | |
| Anhydrides | Look for two absorptions near 1810 and 1760 cm-1 | |
| Aldehydes | Look for two weak absorptions near 2850 and 2750 cm-1 - but beware, the higher peak is often obscured by other saturated C-H bands | |
| Ketones | May be present if all the above carbonyl derivaties have been eliminated | |
| C | Carbonyl group absent. Check also for the presence of: | |
| Alcohols | Look for a broad absorption near 3400-3300 cm-1 and bands near 1300-1000 cm-1; Note: non H-bonded O-H appears at 3600 cm-1 | |
| Phenols | As for alcohols, but check for presence of an aromatic ring - see D | |
| Amines | Look for a medium absorption near 3400 cm-1 | |
| Ethers | Look for a band near 1300-1000 cm-1 and the absence of one at around 3400 cm-1 | |
| D | Double bonds and/or aromatic rings: | |
| Alkene | Look for a weak absorption near 1650 cm-1 and for C-H bands between 3100-3000 cm-1 | |
| Aromatic | Look for medium/strong bands in the region 1600-1450 cm-1 and for C-H bands between 3100-3000 cm-1 | |
| E | Triple bonds: | |
| Nitriles | Look for a medium sharp at about 2250 cm-1 | |
| Alkynes | Look for a weak sharp absorption at about 2150 cm-1 | |
| F | Others: | |
| Nitro groups | Look for two strong absorptions in the regions 1600-1530 cm-1 and 1390-1300 cm-1 | |
| Hydrocarbons | Very simple spectrum with major absorptions in the 2850-3000 cm-1 region, with the only other absorptions appearing near 1460 cm-1 and 1375 cm-1 | |