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Ch. 13 - Nuclear Magnetic Resonance Spectroscopy
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook
All textbooksWade 9th EditionCh. 13 - Nuclear Magnetic Resonance SpectroscopyProblem 22
Chapter 13, Problem 22

Draw the expected NMR spectrum of methyl propionate, and point out how it differs from the spectrum of ethyl acetate.

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Identify the molecular structure of methyl propionate (CH3CH2COOCH3) and ethyl acetate (CH3COOCH2CH3). Both are esters, but their structural differences will influence their NMR spectra.
Analyze the proton environments in methyl propionate. For methyl propionate, there are three distinct proton environments: (1) the methyl group attached to the oxygen (OCH3), (2) the methylene group (CH2) adjacent to the carbonyl group, and (3) the terminal methyl group (CH3) attached to the methylene group.
Analyze the proton environments in ethyl acetate. For ethyl acetate, there are also three distinct proton environments: (1) the methyl group attached to the carbonyl group (CH3CO), (2) the methylene group (CH2) attached to the oxygen, and (3) the terminal methyl group (CH3) attached to the methylene group.
Compare the chemical shifts and splitting patterns. In methyl propionate, the OCH3 group will appear as a singlet at a higher chemical shift due to the electron-withdrawing effect of the oxygen. The CH2 group will show a quartet due to coupling with the adjacent CH3 group, and the terminal CH3 group will appear as a triplet due to coupling with the CH2 group. In ethyl acetate, the CH3CO group will appear as a singlet at a lower chemical shift compared to the OCH3 group in methyl propionate. The CH2 group will show a quartet, and the terminal CH3 group will appear as a triplet, similar to methyl propionate.
Summarize the key differences. The primary difference between the NMR spectra of methyl propionate and ethyl acetate lies in the chemical shifts of the methyl groups attached to the oxygen (OCH3 in methyl propionate) versus the carbonyl group (CH3CO in ethyl acetate). The splitting patterns for the CH2 and CH3 groups will be similar in both compounds, but their exact chemical shifts may vary slightly due to the different electronic environments.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Nuclear Magnetic Resonance (NMR) Spectroscopy

NMR spectroscopy is a powerful analytical technique used to determine the structure of organic compounds. It relies on the magnetic properties of certain nuclei, primarily hydrogen (1H) and carbon (13C), to provide information about the number of hydrogen atoms, their environment, and the connectivity of atoms in a molecule. The resulting spectrum displays peaks that correspond to different chemical environments, allowing chemists to deduce structural information.
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Chemical Shifts

Chemical shifts in NMR spectroscopy refer to the variation in resonance frequency of a nucleus due to its electronic environment. Measured in parts per million (ppm), these shifts provide insight into the types of functional groups present and their relative positions in a molecule. For example, protons in methyl groups typically resonate at different chemical shifts compared to those in ethyl groups, which is crucial for distinguishing between compounds like methyl propionate and ethyl acetate.
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Integration and Peak Area

In NMR spectra, the area under each peak is proportional to the number of protons contributing to that signal, known as integration. This allows chemists to determine the relative number of hydrogen atoms in different environments within a molecule. By comparing the integration of peaks in the NMR spectrum of methyl propionate to that of ethyl acetate, one can identify differences in the number and types of hydrogen atoms present, aiding in structural analysis.
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Related Practice
Textbook Question

Draw the NMR spectrum expected from ethanol that has been shaken with a drop of D2O.

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Textbook Question

Propose chemical structures consistent with the following NMR spectra and molecular formulas. In spectrum (a), explain why the peaks around δ1.65 and δ3.75 are not clean multiplets, but show complex splitting.

(a) <IMAGE>

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Textbook Question

Five proton NMR spectra are given here, together with molecular formulas. In each case, propose a structure that is consistent with the spectrum.

(b) <IMAGE>

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Textbook Question

Give the spectral assignments for the protons in isobutyl alcohol (Solved Problem 13-4). For example, Ha is a singlet, area = 1, at δ2.4.

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Textbook Question

Propose mechanisms to show the interchange of protons between ethanol molecules under

(a) acid catalysis.

(b) base catalysis.

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Textbook Question

(a) Show which carbon atoms correspond with which peaks in the 13C NMR spectrum of butan-2-one (Figure 13-45).

<IMAGE>

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