Question

Which of the following groups MAY have no detectable IR absorption in the range usually associated with an absorption?

Answer 1

Here we have to get the functional group which may not be detected in IR spectrum.

The ketene and alkyne group may not be detectable by IR absorption.

The infrared (IR) spectrum is an important record which gives sufficient information about the structure of a compound. The absorption of infrared radiation causes the various bands in a molecule to stretch and bend with respect to one another. The most important region for an organic chemist is 4000 cm⁻¹ to 667 cm⁻¹. The region is called the IR region.

The IR spectra is an important tool to recognize the presence of a particular group in an organic group.

The IR region of the given functional group are shown as:

1. Alkyne -C≡C- 2100 cm⁻¹  to 2260 cm⁻¹

2. Ketene -C=C=O- 2100 cm⁻¹ to 2150 cm⁻¹

3. Aldehyde carbonyl -CH=O 1700 cm⁻¹ to 1725 cm⁻¹

4. nitrile -C≡N 2200 cm⁻¹ to 2250 cm⁻¹

5. Ether -C-O- 1000 cm⁻¹ to 1300 cm⁻¹.

Among the function groups the -C≡C- bond is hard to detect by IR spectrum. It is not found in symmetrical alkyne.

The ketene group also not clearly indicative in the IR region.