Question

In a molecule containing a carboxylic acid group, what would be expected in a \(^{1} \mathrm{H}-\mathrm{NMR}\) spectrum? A. A deshielded hydrogen peak for the hydroxyl hydrogen, shifted left B. A deshielded hydrogen peak for the hydroxyl hydrogen, shifted right C. A shielded hydrogen peak for the hydroxyl hydrogen, shifted left D. A shielded hydrogen peak for the hydroxyl hydrogen, shifted right

Short answer

A. A deshielded hydrogen peak for the hydroxyl hydrogen, shifted left

Step-by-step solution

Understanding Carboxylic Acid Group

A carboxylic acid group consists of a carbonyl (C=O) and a hydroxyl (OH) group attached to the same carbon atom, typically written as -COOH.

Hydrogen in Hydroxyl Group

The hydrogen atom in the hydroxyl (OH) group is expected to be deshielded because it experiences the electron-withdrawing effects of the electronegative oxygen atom.

Deshielding Effect in NMR

Deshielded hydrogens appear at a higher chemical shift (to the left) in a \(^{1} \mathrm{H}- \mathrm{NMR}\) spectrum.

Conclusion

Given this information, the hydroxyl hydrogen peak in a carboxylic acid group will show up as a deshielded peak, shifted left in the \(^{1} \mathrm{H}- \mathrm{NMR}\) spectrum.

Key concepts

chemical shift

Understanding the chemical shift in NMR (Nuclear Magnetic Resonance) is crucial for analyzing molecules. The chemical shift is a measure of how the environment around a hydrogen atom affects the resonance frequency of its nuclear spin. This shift provides insights into the chemical structure because different environments cause different shifts. In a carboxylic acid group, the unique environment created by the -COOH group impacts the hydrogen atoms nearby. The chemical shift is expressed in parts per million (ppm) and helps to identify the location of hydrogen atoms in a molecule.
Hydrogens in different chemical environments will absorb energy at different frequencies. Hydrogens attached to carbon atoms near electronegative atoms or groups, such as oxygen in a carboxyl group, will be shifted differently compared to hydrogens attached to carbon atoms in a less electronegative environment.
When interpreting an NMR spectrum, peaks at different ppm values point to hydrogens in various environments within the molecule.

deshielding effect

In NMR terminology, deshielding refers to the effect that occurs when the electron density around a nucleus is reduced. This reduction exposes the nucleus to the external magnetic field more strongly. In our example of a carboxylic acid, the hydroxyl hydrogen is subjected to the deshielding effect.
Oxygen is highly electronegative, pulling electron density away from the hydrogen. This results in deshielding that shifts the resonance frequency of the hydrogen to a higher value (upfield shift) in the NMR spectrum.
So, a deshielded hydrogen will appear further to the left on an NMR spectrum, usually between 10-12 ppm for carboxylic acids.

hydroxyl hydrogen

The hydroxyl hydrogen in a carboxylic acid (-COOH) is a special case due to its environment. This hydrogen is attached to an oxygen atom, which is connected to a carbonyl group (C=O).
Due to the high electronegativity of oxygen, the hydrogen experiences a strong deshielding effect. This results in a significant chemical shift to the left in the NMR spectrum.
In practical terms, when analyzing a molecule that contains a carboxylic acid group using NMR, you should look for a peak between 10-12 ppm. This peak corresponds to the hydroxyl hydrogen and confirms the presence of a carboxylic acid group in the structure.