Question

Assign the resonances in the \({ }^{13} \mathrm{C}\) NMR spectrum of methyl propanoate, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CO}_{2} \mathrm{CH}_{3}\).

Short answer

Assign peaks: ester carbonyl (~170 ppm), ester-linked methyl (~50-60 ppm), methylene (~25 ppm), terminal methyl (~10-25 ppm).

Step-by-step solution

Identify Carbon Types

In methyl propanoate, there are four distinct carbon environments, corresponding to the different types of carbon atoms present in the molecule: the ester carbonyl carbon (\(\mathrm{C}=\mathrm{O}\)), the ester-linked methyl carbon (\(\mathrm{CH}_3\)), the middle methylene carbon (\(\mathrm{CH}_2\)), and the terminal methyl carbon (\(\mathrm{CH}_3\).).

Assign the Ester Carbonyl Carbon

The ester carbonyl carbon \( (\mathrm{C}=\mathrm{O}) \) typically resonates in the range of 160-180 ppm in a \(\mathrm{{}^{13}C}\) spectrum due to deshielding effects and the high oxidized state. We identify this peak as the most downfield signal, often around 170 ppm.

Assign the Ester-linked Methyl Carbon

The methyl carbon connected to the oxygen of the ester (\(-\mathrm{OCH}_3\) ) usually appears in the range of 50-60 ppm. This placement is due to the deshielding effect of the nearby electronegative oxygen atom.

Assign the Middle Methylene Carbon

The methylene carbon (\(-\mathrm{CH}_2-\) ) that's not directly attached to a functional group like oxygen typically resonates in the range of 20-40 ppm. For methyl propanoate, this signal will appear around 25 ppm.

Assign the Terminal Methyl Carbon

The terminal methyl group (\(\mathrm{CH}_3\)) in methyl propanoate, which is furthest from electronegative oxygen, shows up at an upfield position, usually around 10-25 ppm in the \(\mathrm{{}^{13}C}\)spectrum.

Key concepts

Methyl Propanoate

Methyl propanoate is an ester derived from propanoic acid and methanol.
Methyl propanoate plays a role in chemistry mainly as an example of an ester-like compound.
Its chemical formula is \(\text{CH}_3\text{CH}_2\text{CO}_2\text{CH}_3\), where the ester group \(-\text{CO}_2\text{CH}_3\) forms a central structure in this molecule.

Understanding methyl propanoate is vital in organic chemistry due to its typical chemical reactions and interactions explained by its structure.
The molecular structure consists of a propanoate framework with a methyl group attached, which influences its physical and chemical behavior.

• The propanoate skeleton offers stability.
• The ester functional group shows typical reactivity patterns with nucleophiles.

With a moderate boiling point and a pleasant smell, methyl propanoate can also serve as an aroma compound in industrial applications.

Carbon Environments

In the context of \(\text{^{13}C}\) NMR, carbon environments refer to the unique electronic surroundings experienced by each carbon atom in a molecule.
These environments arise from different types of chemical bonds and nearby atoms, which influence the electronic shielding of the carbon nuclei.

In methyl propanoate, there are four distinct carbon environments to consider:

• Ester Carbonyl Carbon \(\text{C} = \text{O}\): Heavily influenced by its connection to an oxygen atom, resulting in significant deshielding and giving a characteristic low-field shift.
• Ester-linked Methyl Carbon \(-\text{OCH}_3\): This carbon feels the effect of the electronegative oxygen it's bonded to.
• Middle Methylene \(-\text{CH}_2-\): Represents an intermediate level of shielding due to its position.
• Terminal Methyl \(\text{CH}_3\): The most shielded environment, often more upfield in the spectrum.

Recognizing these environments helps predict and interpret \(\text{^{13}C}\) NMR spectra accurately.

Chemical Shift Assignments

Chemical shift assignments involve determining the position of different peaks in the \(\text{^{13}C}\) NMR spectrum based on the unique environments of carbon atoms.
Nuclei experience different levels of magnetic shielding, resulting in varying shifts visible in the spectrum.

The placement of these shifts provides insight into the molecular structure.

• Ester Carbonyl Carbon: Appears typically between 160-180 ppm, influenced by deshielding effects.
• Ester-linked Methyl Carbon: Shifts in the 50-60 ppm range due to its neighboring oxygen.
• Middle Methylene Carbon: Shows a more moderate shift between 20-40 ppm.
• Terminal Methyl Carbon: Lies in the 10-25 ppm range.

Understanding these assignments is crucial for analyzing the structure of organic compounds through NMR spectrometry.

Ester Carbonyl Carbon

The ester carbonyl carbon has a significant impact in the field of NMR spectroscopy, primarily due to its distinct electronic environment.
In the \(\text{^{13}C}\) NMR, the carbonyl carbon resonates at lower fields, approximately 160 to 180 ppm.
This low-field resonance is due to the electron-withdrawing property of the oxygen atom in the \(-\text{CO} \cdots\) group, which causes deshielding.

Besides chemical shift, the ester carbonyl carbon's signal might also appear as a singular peak, reflecting its unique electronic density.

• The deshielding is potent enough to cause a notable downfield shift.
• Identifying this signal in spectra helps confirm the presence of ester groups.
• This assignment is key for analyzing complex organic molecules with ester functionalities.

The distinctive resonance of the ester carbonyl carbon allows scientists to verify ester connectivity in molecular structures.

Nuclear Magnetic Resonance

Nuclear Magnetic Resonance (NMR) spectroscopy is an analytical tool predominantly used for determining the structure of organic compounds.
This technique exploits the magnetic properties of certain atomic nuclei.

In \(\text{^{13}C}\) NMR spectroscopy, the focus is on carbon atoms. This method provides data about the number of unique carbon environments present within a molecule.

• Basic Principle: NMR leverages the fact that nuclei in a magnetic field absorb electromagnetic radiation at a frequency characteristic of the isotope.
• Output Understanding: The \^{13}C\ NMR spectrum shows peaks at specific chemical shifts corresponding to each unique carbon environment in a compound.
• Interpretation Skills: Interpreting these peaks allows chemists to deduce structural information about the molecule.

NMR is fundamental in organic chemistry and biochemistry, due to its ability to reveal molecular structure details that are not easily accessible by other methods.