Problem 26
Question
If the \(-\mathrm{NH}_{2}\) protons of 2 -aminoethanol, \(\mathrm{NH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\), have a shift of \(1.1 \mathrm{ppm}\) and the \(-\mathrm{OH}\) proton has a shift of \(3.2 \mathrm{ppm}\), what will be the observed average \(\left(-\mathrm{NH}_{2},-\mathrm{OH}\right)\) proton shift if exchange is very fast?
Step-by-Step Solution
Verified Answer
The observed average proton shift is 2.15 ppm.
1Step 1: Understand the Concept of Proton Exchange
Proton exchange in NMR (Nuclear Magnetic Resonance) refers to the rapid movement of protons between different environments, such as between the \(-\mathrm{NH}_2\) and \(-\mathrm{OH}\) groups in this exercise. When exchange is very rapid, the distinct chemical shifts of these different protons will average out.
2Step 2: Identify the Proton Shifts Involved
The chemical shift for the \(-\mathrm{NH}_2\) protons is given as \(1.1\ \text{ppm}\), and the shift for the \(-\mathrm{OH}\) proton is \(3.2\ \text{ppm}\). These values will be used to calculate the average chemical shift.
3Step 3: Calculate the Average Chemical Shift
To find the average chemical shift when exchange is very fast, calculate the average by adding the two chemical shifts and dividing by the number of shifts. The formula for the average is: \\[\text{Average shift} = \frac{(\text{shift of } -NH_2 + \text{shift of } -OH)}{2}\]Substitute the given values: \[\text{Average shift} = \frac{(1.1 + 3.2)}{2} = \frac{4.3}{2} = 2.15 \text{ppm}\].
4Step 4: Conclusion of Calculation
The observed average proton shift for the \(-\mathrm{NH}_2\) and \(-\mathrm{OH}\) protons, assuming very fast exchange, is \(2.15\ \text{ppm}\). This average reflects the blended environment of the protons due to rapid exchange.
Key Concepts
Chemical ShiftsNuclear Magnetic ResonanceAminoethanol
Chemical Shifts
Chemical shifts are a fundamental concept in nuclear magnetic resonance (NMR) spectroscopy. These shifts refer to variations in the resonant frequency of a nucleus relative to a standard reference. In simple terms, chemical shifts can help us understand the environment surrounding certain atoms in a molecule.
In NMR, the nuclei of atoms like hydrogen are placed in a magnetic field, and their response is measured. This response changes depending on their electronic environment, which causes shifts in the NMR signals.
For instance, the \(-\mathrm{NH}_2\) protons in aminoethanol resonate at a shift of 1.1 ppm, while the \(-\mathrm{OH}\) proton resonates at a shift of 3.2 ppm. These different values reflect the different environments of the \(-\mathrm{NH}_2\) and \(-\mathrm{OH}\) groups. The \(-\mathrm{OH}\) group, being electronegative, deshields the proton more, causing its signal to appear at a higher chemical shift compared to the \(-\mathrm{NH}_2\) group.
This concept of chemical shifts allows us to infer where protons are situated within a molecule and the type of chemical environment they are in, providing invaluable insight into molecular structure.
In NMR, the nuclei of atoms like hydrogen are placed in a magnetic field, and their response is measured. This response changes depending on their electronic environment, which causes shifts in the NMR signals.
For instance, the \(-\mathrm{NH}_2\) protons in aminoethanol resonate at a shift of 1.1 ppm, while the \(-\mathrm{OH}\) proton resonates at a shift of 3.2 ppm. These different values reflect the different environments of the \(-\mathrm{NH}_2\) and \(-\mathrm{OH}\) groups. The \(-\mathrm{OH}\) group, being electronegative, deshields the proton more, causing its signal to appear at a higher chemical shift compared to the \(-\mathrm{NH}_2\) group.
This concept of chemical shifts allows us to infer where protons are situated within a molecule and the type of chemical environment they are in, providing invaluable insight into molecular structure.
Nuclear Magnetic Resonance
Nuclear Magnetic Resonance (NMR) is a technique that allows scientists to deduce the structure of organic compounds by observing the magnetic properties of atomic nuclei. When placed in a magnetic field, certain nuclei resonate at specific frequencies. These frequencies are influenced by the chemical environment.
NMR is widely used for analyzing complex organic molecules because:
In the context of our exercise, rapid proton exchange between different functional groups shifts the average signals. In aminoethanol, these shifts reveal both the chemical nature and the proton exchange between the \(-\mathrm{NH}_2\) and \(-\mathrm{OH}\) groups.
NMR is widely used for analyzing complex organic molecules because:
- It is non-destructive, allowing the sample to be recovered afterward.
- It provides detailed information regarding the structure, dynamics, reaction state, and chemical environment.
In the context of our exercise, rapid proton exchange between different functional groups shifts the average signals. In aminoethanol, these shifts reveal both the chemical nature and the proton exchange between the \(-\mathrm{NH}_2\) and \(-\mathrm{OH}\) groups.
Aminoethanol
Aminoethanol, also known as ethanolamine, is a molecule with both an amino (H\(_2\)) and a hydroxyl (OHgroup. Its formula is NH\(_2\)CH\(_2\)CH\(_2\)OH. This compound is not just an interesting subject in NMR, it's also important in many industrial applications.
The presence of both amino and hydroxyl groups makes aminoethanol amphiphilic, meaning it can interact with both water and fats. This characteristic makes it useful in applications such as detergents and surfactants. It is also used as an intermediate in chemical syntheses and as a buffering agent in various solutions.
In NMR studies, aminoethanol's protons provide a clear example of rapid proton exchange, especially between the \(-\mathrm{NH}_2\) and the \(-\mathrm{OH}\) environments. This rapid exchange of protons leads to observable average chemical shifts, simplifying the interpretation of NMR data by averaging the signals of involved protons.
Understanding aminoethanol in this context further exemplifies how chemical properties influence molecular behavior and how NMR can be used to study such behaviors.
The presence of both amino and hydroxyl groups makes aminoethanol amphiphilic, meaning it can interact with both water and fats. This characteristic makes it useful in applications such as detergents and surfactants. It is also used as an intermediate in chemical syntheses and as a buffering agent in various solutions.
In NMR studies, aminoethanol's protons provide a clear example of rapid proton exchange, especially between the \(-\mathrm{NH}_2\) and the \(-\mathrm{OH}\) environments. This rapid exchange of protons leads to observable average chemical shifts, simplifying the interpretation of NMR data by averaging the signals of involved protons.
Understanding aminoethanol in this context further exemplifies how chemical properties influence molecular behavior and how NMR can be used to study such behaviors.
Other exercises in this chapter
Problem 22
In nmr experiments, structural inferences sometimes are drawn from differences in resonance frequencies as small as \(1 \mathrm{~Hz}\). What difference in energ
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a. Identify the protons with different chemical shifts in each of the structures shown. Use letter subscripts \(\mathrm{H}_{A}, \mathrm{H}_{B}\), and so on, to
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In reasonably concentrated solution in water, ethanoic acid (acetic acid) acts as a weak acid (less than \(1 \%\) dissociated). Ethanoic acid gives two proton n
View solution Problem 29
Sketch the proton chemical shifts in ppm and \(\mathrm{Hz}\) as well as the integral you would expect for each of the following substances at \(60 \mathrm{MHz}\
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