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Chapter 20: Problem 486

Explain why the chemical shift of the acidic proton of a carboxylic acid, dissolved in a nonpolar solvent like carbon tetrachloride, varies less with concentration than that of the OH proton of an alcohol under the same conditions.

Short Answer

Expert verified
The chemical shift of the acidic proton of a carboxylic acid dissolved in a nonpolar solvent like carbon tetrachloride varies less with concentration than that of the OH proton of an alcohol under the same conditions because carboxylic acids have a conjugated structure with delocalized electron density, resulting in relatively constant chemical shifts across different concentrations. In contrast, alcohols do not have a conjugated structure and exhibit greater variations in chemical shift with changing concentrations due to increased intermolecular interactions in concentrated solutions.

Step by step solution

01

Understand chemical shift

Chemical shift is the phenomenon by which the NMR resonance frequencies of different protons in a sample vary, depending on their local environment. It is measured in parts per million (ppm) and is directly related to the electron density around the proton.
02

Analyze the role of electron density

The chemical shift is mainly affected by the electron density around the proton. When the electron density around a proton increases, the chemical shift decreases, and vice versa. The functional groups that are involved (carboxylic acid and alcohol) play a significant role in determining the electron density around the acidic proton and the OH proton.
03

Analyze the acidic proton of a carboxylic acid

In a carboxylic acid, the acidic proton (H) is attached to a highly electronegative oxygen (O) atom. The electronegativity of oxygen causes the electron density around the proton to be relatively low. Moreover, carboxylic acids have a conjugated structure where the electron density is delocalized over the entire molecule, causing a relatively constant value for chemical shifts across different concentrations of carboxylic acids.
04

Analyze the OH proton of an alcohol

In an alcohol, the OH proton is attached to an oxygen atom, which also has a high electronegativity. However, unlike carboxylic acids, alcohols do not have a conjugated structure, and the electron density is not delocalized over the entire molecule. This causes a more significant variation in chemical shift across different concentrations of alcohols.
05

Analyze the effect of concentration on chemical shift

When the concentration of the solution increases, there is an increased probability of intermolecular interactions between the molecules, causing changes in the electron density around different protons. The degree of variation in chemical shift depends on the nature of the functional group and the local electronic environment.
06

Conclusion

The chemical shift of the acidic proton of a carboxylic acid dissolved in a nonpolar solvent like carbon tetrachloride varies less with concentration than that of the OH proton of an alcohol under the same conditions because: 1. Carboxylic acids have a conjugated structure where the electron density is delocalized over the entire molecule, leading to a relatively constant value for chemical shifts across different concentrations. 2. Alcohols do not have a conjugated structure and have more significant variations in chemical shift with changing concentrations due to the higher probability of intermolecular interactions in concentrated solutions.

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