Two Isomers of C2H4O2: IR and NMR Evidence
A school laboratory has two unlabelled bottles, both labelled C2H4O2 (M = 60 g mol−1). One contains ethanoic acid (the acid in vinegar); the other contains methyl methanoate, a volatile ester. The infrared spectrum of sample X has a strong broad absorption from 2500 to 3300 cm−1 and a strong sharp absorption at 1710 cm−1. Its 1H NMR spectrum has two signals: a singlet at δ 2.1 (3H) and a very broad singlet at δ 11.5 (1H). Sample Y has a strong absorption at 1740 cm−1 and no broad absorption above 3000 cm−1. Its 1H NMR spectrum has two singlets: δ 3.7 (3H) and δ 8.0 (1H). A student says, 'Both compounds contain a C=O group, so infrared spectroscopy cannot tell them apart.' The teacher asks the class to assign X and Y and to evaluate that claim. X boils at 118 °C; Y boils at 32 °C.
1. Assign structures to X and Y. Justify each assignment using the IR and 1H NMR data.
Read the worked answer
X is ethanoic acid, CH3COOH. The broad IR absorption at 2500–3300 cm−1 is the O−H stretch of a carboxylic acid, and the 1710 cm−1 peak is the C=O stretch. The 1H NMR has a 3H singlet at δ 2.1 for CH3 next to C=O and a broad 1H singlet at δ 11.5 for the acidic COOH proton, which exchanges with water and is broad.
Y is methyl methanoate, HCOOCH3. It has no O−H absorption, and its C=O stretch is at 1740 cm−1, typical of an ester. The 1H NMR has a 3H singlet at δ 3.7 for the OCH3 group and a 1H singlet at δ 8.0 for the aldehydic formate proton, HCO−. The integration (3H:1H) and chemical shifts match these assignments.
2. Evaluate the student's claim that IR cannot distinguish the two compounds. Refer to specific absorptions and explain why the NMR data are also needed.
Read the worked answer
The claim is false. IR can distinguish them: X has a broad O−H absorption from 2500 to 3300 cm−1, which Y lacks, and the C=O positions differ (1710 cm−1 for the carboxylic acid and 1740 cm−1 for the ester). The O−H absorption alone is strong evidence for a carboxylic acid. The C=O difference is real but can be affected by hydrogen bonding and sample conditions, so it is less reliable on its own.
NMR confirms the assignment. X has an exchangeable acidic proton at δ 11.5, while Y has an aldehydic proton at δ 8.0 and an OCH3 at δ 3.7. Using both techniques is better than relying on a single peak, because IR identifies functional groups and NMR shows the number and environment of hydrogens.
3. Explain the large difference in boiling point between X (118 °C) and Y (32 °C) using intermolecular forces.
Read the worked answer
Ethanoic acid molecules form strong intermolecular hydrogen bonds through the COOH group: the O−H can donate a hydrogen bond to the C=O of another molecule, and dimer formation is common. Breaking these interactions requires a lot of energy, so the boiling point is high (118 °C).
Methyl methanoate has a C=O and an OCH3 group, so it has dipole–dipole attractions and dispersion forces, but it has no O−H bond, so it cannot donate hydrogen bonds. Its intermolecular forces are much weaker, and less energy is needed to separate the molecules, giving a much lower boiling point (32 °C). The difference is therefore mainly due to hydrogen bonding in the carboxylic acid.