Infra Red Spectrum - 4

Infrared spectrum 4: propanal, CH₃CH₂CHOPropanal is an aldehyde. Like a ketone, it contains a C=O group, but the C=O carbon also has a hydrogen atom attached (the CHO group).Key absorptions• A strong, sharp C=O absorption at about 1730 cm^-1. Aldehydes absorb at 1720 to 1740 cm^-1, slightly higher than ketones (1700 to 1720 cm^-1), but the ranges are close, so this alone is not enough to tell them apart.• Two weaker absorptions at about 2720 and 2820 cm^-1. These come from the C-H bond of the CHO group. They are the clearest infrared evidence of an aldehyde, because ketones do not have them.• Normal C-H absorptions at about 2850 to 2980 cm^-1.What is missingThere is no broad O-H absorption, so the compound is not an alcohol or a carboxylic acid.Worked exampleA compound C₃H₆O shows a strong absorption at 1730 cm^-1, two weak absorptions at 2720 and 2820 cm^-1, and no absorption between 3200 and 3550 cm^-1. The C=O and the aldehyde C-H absorptions show that it is an aldehyde, so it is propanal and not its isomer propanone.