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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 18c,d
Chapter 13, Problem 18c,d

Predict the theoretical number of different NMR signals produced by each compound, and give approximate chemical shifts. Point out any diastereotopic relationships.
c. Ph—CHBr—CH2Br
d. vinyl chloride

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Step 1: Analyze the molecular structure of the compound Ph—CHBr—CH2Br. Identify the unique hydrogen environments by considering symmetry and connectivity. Note that the phenyl group (Ph) is a distinct group, and the two bromine atoms attached to the carbon chain can create different environments for the hydrogens.
Step 2: Determine if any hydrogens are equivalent due to symmetry. For example, in Ph—CHBr—CH2Br, the hydrogens on the CH2 group may or may not be equivalent depending on the spatial arrangement and the presence of the bromine atom on the adjacent carbon.
Step 3: Identify diastereotopic hydrogens. Diastereotopic hydrogens are non-equivalent hydrogens on the same carbon atom that are in different spatial environments due to the presence of a chiral center or other stereochemical factors. In this case, the CH2 group hydrogens may be diastereotopic because of the CHBr group.
Step 4: For vinyl chloride, analyze the structure (CH2=CHCl). Identify the unique hydrogen environments on the double bond. Recall that hydrogens on a double bond are not equivalent due to restricted rotation and the presence of the chlorine atom, which creates an electron-withdrawing effect.
Step 5: Approximate the chemical shifts for each unique hydrogen environment. For Ph—CHBr—CH2Br, hydrogens on the phenyl group typically appear around 7-8 ppm, the CHBr hydrogen may appear around 4-5 ppm, and the CH2 hydrogens may appear around 3-4 ppm. For vinyl chloride, the hydrogens on the double bond typically appear around 5-6 ppm, with slight differences due to the electron-withdrawing effect of the chlorine atom.

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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 unique environments in a molecule. Each unique environment produces a distinct signal in the NMR spectrum, allowing chemists to infer structural details.
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General NMR Features

Chemical Shifts

Chemical shifts in NMR refer to the resonance frequency of a nucleus relative to a standard reference, typically tetramethylsilane (TMS). They are measured in parts per million (ppm) and provide insight into the electronic environment surrounding the nuclei. Different functional groups and molecular environments influence chemical shifts, allowing for the identification of specific types of hydrogen or carbon atoms in a compound.
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1H NMR Chemical Shifts

Diastereotopic Protons

Diastereotopic protons are non-equivalent protons in a molecule that are in different environments due to the presence of chiral centers or other stereochemical factors. They can produce distinct NMR signals, which is crucial for predicting the number of signals in a spectrum. Identifying diastereotopic relationships helps in understanding the stereochemistry of a compound and can influence the interpretation of NMR data.
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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

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

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

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

(a) acid catalysis.

(b) base catalysis.

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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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