Spectroscopic Features of Aldehydes

IR Spectroscopy

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Like ketones, identifying aldehydes starts with observing a carbonyl stretch (1650-1800); normally this is the strongest peak in the spectrum. Aldehydes tend to come at slightly higher frequencies than ketones (H is less electron-donating than an alkyl group), but conjugation can also lower the frequency. In addition, there is a characteristic double peak at 2700 and 2800 cm^-1. This originates from the aldehyde C-H stretch; the doubling is an odd effect from interaction of this primary frequency with a close overtone band. The physics is complex, but this doublet is a nice way to distinguish an aldehyde from a ketone.

¹H NMR Spectroscopy

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The standout signal is the aldehyde proton; this occurs between 9-10 ppm. It couples to any protons on the alpha carbon.
As with ketones, there is a deshielded signal for any proton on the alpha carbon; between 2-2.5 ppm and will couple normally to its neighbors. The electronegativity of the aldehyde group provides progressive deshielding as you work your way down the chain.

¹³C NMR Spectroscopy

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The carbonyl carbon is in the same region as that for a ketone (190 ppm or further downfield), so the ¹³C spectrum may not be as useful as the ¹H spectrum in distinguishing the two groups. There is a similar electronegativity effect on the alpha and beta carbons, but because of beta and gamma effects this principle is less useful than 2D NMR in assigning carbons.

Mass spectrometry