Using infrared spectroscopy, how can salicylic acid be differentiated from acetylsalicylic acid (aspirin)? 1) Acetylsalicylic acid and salicylic acid cannot be differentiated via IR spectroscopy as they have all the same stretches present. 2) Acetylsalicylic acid will have two carbonyl (C=O) stretches at approximately 1700 cm-1. It will possess one C-O stretch at about 1680 cm for the carboxyl group, which is present in both, and one unique carbonyl stretch at 1725 cm for the ester functional group. 3) Salicylic acid will have 3 unique aliphatic sp3-hybridized C-H stretches at approximately 3150-3050 cm. 4) Salicylic acid will have two carbonyl (C-O) stretches at approximately 1700 cm. It will possess one C-O stretch at about 1680 cm for the carboxyl group, which is present in both, and one unique carbonyl stretch at 1725 cm for the ester functional group.
The Correct Answer and Explanation is:
The correct answer is Option 2: Acetylsalicylic acid will have two carbonyl (C=O) stretches at approximately 1700 cm-1. It will possess one C-O stretch at about 1680 cm for the carboxyl group, which is present in both, and one unique carbonyl stretch at 1725 cm for the ester functional group.
Explanation:
Infrared (IR) spectroscopy is a valuable tool for identifying functional groups in compounds based on their vibrational frequencies. Salicylic acid and acetylsalicylic acid (aspirin) are structurally similar, with acetylsalicylic acid being an ester derivative of salicylic acid. However, their IR spectra show key differences due to the presence of different functional groups.
- Carboxyl Group (C=O and C-O stretches):
Both salicylic acid and acetylsalicylic acid have a carboxyl group (–COOH), which contributes to similar IR features in both compounds. This includes a broad C-O stretch around 1680 cm-1 due to the carboxyl group. In both spectra, the C=O stretch of the carboxyl group typically appears around 1700 cm-1. However, acetylsalicylic acid will show a second C=O stretch around 1725 cm-1 because of the ester group (–COOCH3) in its structure, which is absent in salicylic acid. This ester group introduces a distinct stretch that is absent in salicylic acid, making the two compounds distinguishable via IR. - C-H Stretches:
Salicylic acid has aliphatic C-H stretches in the 3000-3150 cm-1 range, corresponding to the aromatic ring and the –OH group. While these stretches are also present in acetylsalicylic acid, they do not provide a clear difference between the two compounds. - Conclusion:
The key differentiating factor in the IR spectra of these two compounds is the additional ester C=O stretch at 1725 cm-1 found in acetylsalicylic acid. This ester functionality is responsible for the unique IR feature that allows acetylsalicylic acid to be distinguished from salicylic acid.
