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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 12
Chapter 13, Problem 12

Draw a splitting tree, similar to Figures 13-32 and 13-33, for proton Hc in styrene. What is the chemical shift of proton Hc?

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Identify the proton Hc in styrene. Styrene is an aromatic compound with a vinyl group attached to a benzene ring. Proton Hc is typically one of the protons in the vinyl group adjacent to the benzene ring.
Determine the coupling partners for proton Hc. In styrene, proton Hc will couple with the other protons in the vinyl group (Ha and Hb). Analyze the number of neighboring protons and their spin states to predict the splitting pattern.
Construct the splitting tree for proton Hc. Start with the initial peak for Hc, then split it based on the coupling with Ha (typically a doublet due to one neighboring proton). Next, split each peak of the doublet further due to coupling with Hb (another doublet, resulting in a doublet of doublets).
Estimate the chemical shift of proton Hc. The chemical shift depends on the electronic environment of Hc. Since Hc is part of the vinyl group adjacent to the benzene ring, it will experience deshielding due to the electron-withdrawing effect of the aromatic ring. This places its chemical shift in the range of approximately 5-6 ppm in a typical NMR spectrum.
Verify the splitting pattern and chemical shift using experimental data or reference spectra for styrene. Compare the predicted splitting tree and chemical shift with known values to ensure accuracy.

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Key Concepts

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

Chemical Shift

Chemical shift refers to the change in the resonant frequency of a nucleus in a magnetic field, influenced by the electronic environment surrounding it. In NMR spectroscopy, it is measured in parts per million (ppm) and provides insight into the molecular structure and the electronic effects of nearby atoms or groups. For protons, the chemical shift can indicate the presence of electronegative atoms or the degree of substitution in aromatic systems.
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Splitting Patterns

Splitting patterns in NMR arise from the interaction of non-equivalent neighboring protons, which causes the resonance signals to split into multiple peaks. This phenomenon, known as spin-spin coupling, provides information about the number of adjacent protons and their arrangement. Understanding these patterns is crucial for interpreting the NMR spectrum and deducing the structure of the compound.
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Aromatic Protons

Aromatic protons are those attached to carbon atoms in an aromatic ring, such as styrene. These protons typically exhibit unique chemical shifts due to the delocalized π-electrons in the aromatic system, which influence their electronic environment. The presence of substituents on the aromatic ring can further modify the chemical shifts and splitting patterns of these protons, making their analysis essential for structural determination.
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Related Practice
Textbook Question

The spectrum of trans-hex-2-enoic acid follows.

(a) Assign peaks to show which protons give rise to which peaks in the spectrum.

(b) Draw a tree to show the complex splitting of the vinyl proton centered around 7 ppm. Estimate the values of the coupling constants.

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Textbook Question

Propose a structure that corresponds to each spectrum.

(a) <IMAGE>

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Textbook Question

If the imaginary replacement of either of two protons forms enantiomers, then those protons are said to be enantiotopic. The NMR is not a chiral probe, and it cannot distinguish between enantiotopic protons. They are seen to be “equivalent by NMR.”

(a) Use the imaginary replacement technique to show that the two allylic protons (those on C3) of allyl bromide are enantiotopic.

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Textbook Question

Predict the theoretical number of different NMR signals produced by each compound, and give approximate chemical shifts. Point out any diastereotopic relationships.

a. 2-bromobutane

b. cyclopentanol

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Textbook Question

An unknown compound (C3H2NCl) shows moderately strong IR absorptions around 1650 cm–1 and 2200 cm–1. Its NMR spectrum consists of two doublets (J = 14 Hz) at δ5.9 and δ7.1. Propose a structure consistent with these data.

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Textbook Question

Draw the NMR spectra you expect for the following compounds.

(b)

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