Question

aldehydes, ketones, carboxylic acids, and esters all give strong carbonyl stretching absorptions in the ir spectrum. how can you use other peaks in their ir spectra to distinguish among these four common functional groups?

Answer 1

In infrared spectroscopy, aldehydes, ketones, carboxylic acids, and esters can be distinguished by examining secondary peaks such as the broad O-H stretching in acids or specific C-H stretching in aldehydes, alongside the strong carbonyl absorption that all four share.

Step-by-step explanation:

In infrared spectroscopy, different functional groups have characteristic absorption peaks which allow us to identify them. To distinguish between aldehydes, ketones, carboxylic acids, and esters, one must look beyond the carbonyl stretching absorptions.

Aldehydes typically show a carbonyl peak around 1720-1740 cm¹ and a distinct peak around 2700-2800 cm¹ due to C-H stretching. Ketones have a carbonyl absorption slightly lower, generally around 1715 cm¹, without the aldehyde-associated C-H stretch.

Carboxylic acids present not only the carbonyl absorption, often closer to 1700 cm¹, but also a broad O-H stretching peak around 2500-3300 cm¹ due to hydrogen bonding. Esters show a carbonyl stretch near 1735-1750 cm¹ and lack the broad O-H stretch.

By examining these secondary peaks and patterns, alongside the characteristic strong carbonyl peaks, one can distinguish among aldehydes, ketones, carboxylic acids, and esters in IR spectroscopy.