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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 39a
Chapter 13, Problem 39a

Sketch your predictions of the proton NMR spectra of the following compounds.
(a) CH3–O–CH2CH3

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Step 1: Analyze the molecular structure of CH3-O-CH2CH3. This compound is an ether, with a methyl group (CH3) attached to an oxygen atom, which is then connected to an ethyl group (CH2CH3). Identify the distinct proton environments in the molecule.
Step 2: Determine the number of unique proton environments. In CH3-O-CH2CH3, there are three distinct environments: (1) the protons in the CH3 group attached to oxygen, (2) the protons in the CH2 group adjacent to oxygen, and (3) the protons in the terminal CH3 group of the ethyl chain.
Step 3: Predict the chemical shifts for each proton environment. (1) The CH3 group attached to oxygen will appear downfield due to the electronegativity of oxygen. (2) The CH2 group will appear slightly upfield compared to the CH3 group attached to oxygen but still downfield due to its proximity to oxygen. (3) The terminal CH3 group will appear upfield as it is furthest from the electronegative oxygen atom.
Step 4: Consider the splitting patterns for each proton environment. (1) The CH3 group attached to oxygen will appear as a singlet because it has no neighboring protons. (2) The CH2 group will appear as a quartet due to coupling with the three protons of the adjacent CH3 group. (3) The terminal CH3 group will appear as a triplet due to coupling with the two protons of the adjacent CH2 group.
Step 5: Summarize the expected NMR spectrum. The spectrum will show three signals: a singlet for the CH3 group attached to oxygen, a quartet for the CH2 group, and a triplet for the terminal CH3 group. The relative integration of the signals will correspond to the number of protons in each environment (3:2:3).

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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 in different environments within a molecule, allowing chemists to infer connectivity and functional groups. Peaks in the NMR spectrum correspond to different types of hydrogen environments, influenced by factors such as electronegativity and molecular symmetry.
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Chemical Shift

Chemical shift refers to the position of a peak in an NMR spectrum, measured in parts per million (ppm). It indicates the electronic environment surrounding a hydrogen atom, which affects its resonance frequency. For example, hydrogens attached to electronegative atoms or in deshielded environments appear at lower ppm values, while those in shielded environments appear at higher values. Understanding chemical shifts is crucial for interpreting NMR spectra accurately.
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Integration and Splitting Patterns

Integration in NMR spectroscopy refers to the area under a peak, which correlates to the number of hydrogen atoms contributing to that signal. Splitting patterns arise from the interaction of neighboring hydrogen atoms, described by the n+1 rule, where n is the number of adjacent hydrogens. These patterns provide insights into the number of neighboring protons and help deduce the molecular structure, making them essential for predicting NMR spectra.
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Related Practice
Textbook Question

Tell precisely how you would use the proton NMR spectra to distinguish between the following pairs of compounds.

(a) 1-bromopropane and 2-bromopropane

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Textbook Question

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.

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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

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

(b)

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Textbook Question

Tell precisely how you would use the proton NMR spectra to distinguish between the following pairs of compounds.

(b)

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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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