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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 39b
Chapter 13, Problem 39b

Sketch your predictions of the proton NMR spectra of the following compounds.
(b)

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1
Step 1: Analyze the molecular structure of the compound (CH3)2CH-C(O)-CH3. This molecule contains three distinct types of hydrogen environments: (a) the hydrogens on the two methyl groups attached to the isopropyl group, (b) the hydrogen on the methine group (CH), and (c) the hydrogens on the methyl group attached to the carbonyl group.
Step 2: Predict the chemical shifts for each type of hydrogen environment. (a) The hydrogens on the methyl groups attached to the isopropyl group will appear as a single peak due to their equivalent environment, likely in the range of 0.9-1.2 ppm. (b) The hydrogen on the methine group (CH) will appear downfield due to the electron-withdrawing effect of the carbonyl group, likely in the range of 2.0-2.5 ppm. (c) The hydrogens on the methyl group attached to the carbonyl group will also appear downfield, likely in the range of 2.1-2.4 ppm.
Step 3: Determine the splitting patterns for each type of hydrogen environment. (a) The hydrogens on the methyl groups attached to the isopropyl group will experience splitting due to the adjacent methine hydrogen, resulting in a doublet. (b) The methine hydrogen will experience splitting due to the six hydrogens on the adjacent methyl groups, resulting in a septet. (c) The hydrogens on the methyl group attached to the carbonyl group will not experience splitting as there are no adjacent hydrogens, resulting in a singlet.
Step 4: Predict the integration values for each peak. (a) The hydrogens on the methyl groups attached to the isopropyl group will integrate to 6 protons. (b) The methine hydrogen will integrate to 1 proton. (c) The hydrogens on the methyl group attached to the carbonyl group will integrate to 3 protons.
Step 5: Combine all predictions to sketch the proton NMR spectrum. The spectrum will include: (a) a doublet for the methyl groups attached to the isopropyl group, (b) a septet for the methine hydrogen, and (c) a singlet for the methyl group attached to the carbonyl group. Ensure the chemical shifts, splitting patterns, and integration values are consistent with the analysis.

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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, with their chemical shifts indicating the electronic environment around them.
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General NMR Features

Chemical Shifts

Chemical shifts in NMR spectroscopy refer to the position of the peaks in the spectrum, measured in parts per million (ppm). They provide insight into the electronic environment of hydrogen atoms, influenced by nearby electronegative atoms or functional groups. For example, hydrogens adjacent to a carbonyl group typically appear downfield (higher ppm) due to deshielding effects, which is crucial for predicting the NMR spectrum of the given compound.
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Integration and Splitting Patterns

Integration in NMR refers to the area under the peaks, 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. Understanding these patterns helps in deducing the number of hydrogen atoms in different environments and the overall structure of the compound.
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Related Practice
Textbook Question

A small pilot plant was adding bromine across the double bond of but-2-ene to make 2,3-dibromobutane. A controller malfunction allowed the reaction temperature to rise beyond safe limits. A careful distillation of the product showed that several impurities had formed, including the one having the NMR spectra that appear below. Determine its structure, and assign the peaks to the protons in your structure.

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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

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

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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