Deck 16: Infrared Spectroscopy

Question 1

Arrange the following list of compounds in order of increasing intensity of the C=C stretching absorption (least-intense first), assuming identical molar concentrations in the sample cell:
(a) CH₃CH=CHCH₂CH₃
(b) CH₂=CHCH₂CH₃
(c) CH₂=CCl₂

Answer 1

a < b < c, based on increasing bond moment of C=C.

Question 2

The infrared spectrum of a student's sample shows a weak absorption band at 3710 cm^-1, yet the student is positive that the compound is not an alcohol or amine. Explain.

Answer 2

If the student's sample is a carbonyl compound, the absorption probably arises from a carbo- nyl vibrational overtone. The absorption could also arise from dirty NaCl plates.

Question 3

A double peak around 3300 cm^-1 always indicates the presence of -NH₂.
Explain your answer.

Answer 3

False

Question 4

A student runs the infrared spectrum of cyclopentanone using chloroform as the solvent. The infrared spectrum shows a double carbonyl peak. Explain.

Question 5

The infrared spectrum (thin-film) of a compound with the molecular formula C₇H₅N shows weak absorption at 3100 cm^-1, moderately strong absorption at 2230 cm^-1, and three peaks between 1400-1500 cm^-1 as the principal absorption. Suggest a structure for this compound.

Question 6

Tell how you would distinguish between each of the following pairs of compounds by their infrared spectra alone.