Organic Chemistry MCAT Practice Exam

What does an NMR peak at 11 ppm typically indicate?

1. Aldehyde presence

2. Carboxylic acid presence

3. Aromatic system

4. Alkyl chain

The correct answer is: Carboxylic acid presence

An NMR peak at 11 ppm typically indicates the presence of a carboxylic acid. In proton NMR (1H NMR) spectroscopy, the chemical shift values provide information about the electronic environment of hydrogen atoms within a molecule. Carboxylic acids exhibit a characteristic peak that appears downfield (at higher ppm values) due to the strong electronegativity of the carboxyl group (-COOH). The hydrogen atom attached to this group is deshielded because it is involved in hydrogen bonding and interacts with the electron-withdrawing nature of the carbonyl (C=O) oxygen. This deshielding causes the proton to resonate at a higher frequency, corresponding to the 11 ppm range. In contrast, aldehydes typically resonate around 9-10 ppm, which is lower than carboxylic acids, while aromatic systems have peaks in the range of 6-8 ppm due to the aromatic protons experiencing different electronic environments. Alkyl chains generally produce signals around 0.5-5 ppm, much lower than 11 ppm. Thus, the 11 ppm peak is strongly indicative of a carboxylic acid's hydrogen.