The infrared spectrum shows the frequencies of infrared light absorbed by a sample of butyraldehyde that was found to be contaminated. Which of the following could be the contaminant?

Question

The infrared spectrum shows the frequencies of infrared light absorbed by a sample of butyraldehyde that was found to be contaminated.  Which of the following could be the contaminant?

Quizplus · Accepted Answer

11ecba2d_12b7_4d65_a3bf_bd2f614e2781_MD0008_00 In infrared (IR) spectroscopy, a sample is irradiated with IR light, and a spectrometer detects and records the percentage of radiation that passes through the sample over a range of frequencies. Different functional groups absorb IR radiation at different frequencies and intensities. Therefore, the signal from each group appears in a particular region of the IR spectrum. Absorption frequencies depend on the type of bonds present in a particular functional group. Characteristic functional group absorptions include: 3650-3200 cm⁻¹ (O-H stretch) 3300 cm⁻¹ (sp C-H stretch) 3100 cm⁻¹ (sp² C-H stretch) 3000 cm⁻¹ (sp³ C-H stretch) 1810-1650 cm⁻¹ (C=O stretch) Aldehyde carbonyls (C=O) have a characteristic strong absorption between 1740 and 1720 cm⁻¹. The sample IR spectrum shows a strong absorption at 1730 cm⁻¹, which corresponds to the carbonyl of butyraldehyde. Alcohols have a broad signal between 3650 and 3200 cm⁻¹, so the absorption at 3350 cm⁻¹ most likely corresponds to an O-H stretch. 2-propanol is the only choice that contains an OH group, and therefore it is the most likely contaminant. (Choice A) Propene is an alkene, which has a characteristic sp² C-H stretch at 3100 cm⁻¹. The sample IR spectrum does not have signals indicative of a compound with a carbon-carbon double bond. (Choices B and D) Propanone contains an internal carbonyl (ketone), which has a strong C=O stretch between 1725 and 1705 cm⁻¹. Propyl ethanoate is an ester, and the ester C=O absorbs IR light between 1750 and 1735 cm⁻¹. The carbonyl stretch in the sample IR spectrum does not correspond to a ketone or an ester but rather to butyraldehyde. Educational objective: In infrared spectroscopy, a sample is irradiated with infrared light and the percentage transmission is detected and recorded over a variety of frequencies. Absorption frequencies depend on the type of bonds present in a particular functional group. Characteristic functional group absorption ranges include 3650-3200 cm⁻¹ (O-H stretch), 3300 cm⁻¹ (sp C-H stretch), 3100 cm⁻¹ (sp² C-H stretch), 3000 cm⁻¹ (sp³ C-H stretch), and 1810-1650 cm⁻¹ (C=O stretch).

The Infrared Spectrum Shows the Frequencies of Infrared Light Absorbed