Which region of the infrared (IR) spectrum (in cm⁻¹) is most commonly used to identify the presence of a carbonyl (C=O) functional group in a molecule with the molecular formula C4H8O2?

• A. 1000–1300 cm⁻¹
• B. 1700–1750 cm⁻¹
• C. 3200–3600 cm⁻¹
• D. 2100–2260 cm⁻¹

Answer: (B)

Verified step by step guidance

1. Step 1: Understand the problem. The question asks which region of the infrared (IR) spectrum is most commonly used to identify the presence of a carbonyl (C=O) functional group in a molecule with the molecular formula C4H8O2. Infrared spectroscopy is a technique used to identify functional groups based on their characteristic absorption frequencies.
2. Step 2: Recall the characteristic IR absorption range for a carbonyl (C=O) functional group. Carbonyl groups typically exhibit a strong and sharp absorption band in the range of 1700–1750 cm⁻¹ due to the stretching vibration of the C=O bond.
3. Step 3: Analyze the given options. The options provided are: 1000–1300 cm⁻¹, 1700–1750 cm⁻¹, 3200–3600 cm⁻¹, and 2100–2260 cm⁻¹. Compare these ranges to the known absorption range for carbonyl groups.
4. Step 4: Eliminate irrelevant options. The range 1000–1300 cm⁻¹ is typically associated with C-O stretching vibrations, not C=O. The range 3200–3600 cm⁻¹ corresponds to O-H stretching vibrations, and 2100–2260 cm⁻¹ is associated with C≡C or C≡N triple bonds. Therefore, these ranges are not relevant for identifying a carbonyl group.
5. Step 5: Confirm the correct range. The range 1700–1750 cm⁻¹ is the most commonly used region to identify the presence of a carbonyl (C=O) functional group in IR spectroscopy. This is due to the strong and distinct absorption caused by the stretching vibration of the C=O bond.