MCAT® ExamChemical and Physical Foundations of Biological SystemsHard
A researcher is studying the properties of an unknown organic compound. Elemental analysis reveals the compound contains carbon, hydrogen, and oxygen. Infrared (IR) spectroscopy shows a strong, broad absorption band around 3300 cm⁻¹ and a strong absorption around 1710 cm⁻¹. Based on these spectroscopic data, which of the following functional groups is MOST likely present in the compound?
- AAlkene
- BEther
- CCarboxylic Acid
- DAldehyde
Show answer & explanationAnswer & explanation
Correct answer: C. Carboxylic Acid
A strong, broad absorption band around 3300 cm⁻¹ is characteristic of an O-H stretch, typically broadened by hydrogen bonding. A strong absorption around 1710 cm⁻¹ is characteristic of a C=O stretch. The presence of both a broad O-H and a C=O stretch strongly indicates a carboxylic acid functional group (R-COOH).
Why the other options are wrong
- A. Alkenes would show C=C stretches (around 1600-1680 cm⁻¹) and C-H stretches for sp² carbons (above 3000 cm⁻¹), but not a broad O-H or strong C=O.
- B. Ethers would have C-O stretches (around 1000-1300 cm⁻¹) but lack both the characteristic broad O-H and C=O stretches.
- D. Aldehydes would show a C=O stretch (around 1700-1725 cm⁻¹) and characteristic aldehyde C-H stretches (around 2700-2800 cm⁻¹), but not a broad O-H stretch.
IR Spectroscopy Functional Group Analysis
Infrared (IR) spectroscopy identifies functional groups in organic molecules by measuring the absorption of infrared radiation at specific wavelengths, corresponding to molecular vibrations.
- O-H stretch (alcohols): broad band 3200-3600 cm⁻¹.
- O-H stretch (carboxylic acids): very broad band 2500-3300 cm⁻¹ (overlaps C-H).
- C=O stretch (ketones, aldehydes, esters, carboxylic acids): strong band 1680-1750 cm⁻¹.
- C-H stretch (alkanes): 2850-2960 cm⁻¹; (alkenes): 3000-3100 cm⁻¹; (alkynes): 3300 cm⁻¹.
- N-H stretch (amines): 3300-3500 cm⁻¹ (1 or 2 sharp peaks).
Memory trick: Wavelengths tell the story of functional groups!