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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 36
Chapter 13, Problem 36

The following proton NMR spectrum is of a compound of molecular formula C3H8O.
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(a) Propose a structure for this compound.
(b) Assign peaks to show which protons give rise to which signals in the spectrum.

Verified step by step guidance
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Step 1: Analyze the molecular formula C3H8O. This formula suggests the compound contains three carbon atoms, eight hydrogen atoms, and one oxygen atom. The degree of unsaturation is zero, indicating no double bonds, triple bonds, or rings are present.
Step 2: Examine the proton NMR spectrum. The spectrum shows multiple peaks at different chemical shifts (δ values). Peaks labeled 'a', 'b', 'c', 'd', and 'e' correspond to different proton environments. The integration of these peaks will help determine the number of protons in each environment.
Step 3: Assign chemical shifts to functional groups. The peak at δ ≈ 4 ppm (labeled 'e') is characteristic of protons attached to a carbon bonded to an oxygen atom (likely a hydroxyl group or an alcohol). The peak at δ ≈ 1 ppm (labeled 'a') corresponds to protons in a methyl group (-CH3). Peaks at δ ≈ 2-3 ppm (labeled 'b', 'c', and 'd') are likely from protons in methylene groups (-CH2-) adjacent to the oxygen atom.
Step 4: Propose a structure based on the molecular formula and NMR data. A plausible structure for C3H8O is isopropanol (CH3CHOHCH3). This structure matches the molecular formula and the expected proton environments: a hydroxyl group (-OH), two methyl groups (-CH3), and one methine group (-CH).
Step 5: Assign peaks to specific protons in the structure. The peak at δ ≈ 4 ppm ('e') corresponds to the hydroxyl proton (-OH). The peak at δ ≈ 1 ppm ('a') corresponds to the protons in the two methyl groups (-CH3). The peaks at δ ≈ 2-3 ppm ('b', 'c', and 'd') correspond to the methine proton (-CH) and the splitting pattern due to coupling with adjacent protons.

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Key Concepts

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

Proton NMR Spectroscopy

Proton Nuclear Magnetic Resonance (NMR) spectroscopy is a powerful analytical technique used to determine the structure of organic compounds. It provides information about the number of hydrogen atoms (protons) in a molecule and their environment, allowing chemists to infer connectivity and functional groups. Peaks in the NMR spectrum correspond to different types of protons, with their chemical shifts indicating the electronic environment around them.
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Chemical Shift

Chemical shift is a key parameter in NMR spectroscopy that indicates the resonance frequency of a nucleus relative to a standard reference. It is measured in parts per million (ppm) and reflects the electronic environment surrounding the protons. Different functional groups and molecular structures influence the chemical shift, allowing for the identification of specific types of protons in a compound.
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Integration and Splitting Patterns

Integration in NMR refers to the area under a peak, which correlates to the number of protons contributing to that signal. Splitting patterns arise from the interaction of neighboring protons, providing insight into the number of adjacent protons (n+1 rule). Understanding these patterns helps in deducing the connectivity and arrangement of protons in the molecular structure, which is crucial for accurately proposing a compound's structure.
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Related Practice
Textbook Question

Predict the approximate chemical shifts of the protons in the following compounds.

(c) CH3–O–CH2CH2CH2Cl

(d) CH3CH2–C≡C–H

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Textbook Question

An unknown compound has the molecular formula C9H11Br. Its proton NMR spectrum shows the following absorptions:

singlet, δ7.1, integral 44 mm

singlet, δ2.3, integral 130 mm

singlet, δ2.2, integral 67 mm

Propose a structure for this compound.

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Textbook Question

Sketch your predictions of the proton NMR spectra of the following compounds.

(b)

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Textbook Question

Predict the multiplicity (the number of peaks as a result of splitting) and the chemical shift for each shaded proton in the following compounds.

(a)

(b)

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Textbook Question

Sketch your predictions of the proton NMR spectra of the following compounds.

(a) CH3–O–CH2CH3

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Textbook Question

Using a 60-MHz spectrometer, a chemist observes the following absorption: doublet, J = 7 Hz, at δ4.00

(a) What would the chemical shift (δ) be in the 300-MHz spectrum?

(b) What would the splitting value J be in the 300-MHz spectrum?

(c) How many hertz from the TMS peak is this absorption in the 60-MHz spectrum? In the 300-MHz spectrum?

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