Ch. 13 - Nuclear Magnetic Resonance Spectroscopy

Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook

Wade 9th Edition

Ch. 13 - Nuclear Magnetic Resonance Spectroscopy

Problem 49

Chapter 13, Problem 49

The three isomers of dimethylbenzene are commonly named ortho-xylene, meta-xylene, and para-xylene. These three isomers are difficult to distinguish using proton NMR, but they are instantly identifiable using ¹³C NMR.

(a) Describe how carbon NMR distinguishes these three isomers.
(b) Explain why they are difficult to distinguish using proton NMR.

Verified step by step guidance

Step 1: Analyze the structural differences between ortho-xylene, meta-xylene, and para-xylene. These isomers differ in the relative positions of the two methyl groups on the benzene ring: ortho (1,2-), meta (1,3-), and para (1,4-). This affects the symmetry of the molecule and the environment of the carbon atoms.

Step 2: In 13C NMR spectroscopy, each unique carbon environment produces a distinct signal. The symmetry of the molecule determines the number of unique carbon environments. For example, ortho-xylene has fewer symmetrical carbons compared to para-xylene, which has the highest symmetry and fewer unique carbon signals.

Step 3: Proton NMR spectroscopy focuses on the hydrogen environments. In the case of xylene isomers, the methyl groups and aromatic hydrogens produce overlapping signals due to similar chemical environments, making it difficult to distinguish the isomers.

Step 4: The splitting patterns in proton NMR are influenced by the coupling between hydrogens. However, the proximity of the methyl groups to the aromatic hydrogens in ortho-, meta-, and para-xylene results in similar coupling patterns, further complicating differentiation.

Step 5: Summarize the key point: 13C NMR is more effective for distinguishing xylene isomers because it provides clear information about the number of unique carbon environments, which are directly influenced by the molecular symmetry. Proton NMR lacks this clarity due to overlapping signals and similar coupling patterns.

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Key Concepts

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

Isomerism

Isomerism refers to the phenomenon where compounds have the same molecular formula but different structural arrangements. In the case of dimethylbenzene, the three isomers—ortho, meta, and para—differ in the positions of the methyl groups on the benzene ring. Understanding isomerism is crucial for distinguishing between these compounds, as their unique structures lead to different chemical and physical properties.

Nuclear Magnetic Resonance (NMR) Spectroscopy

NMR spectroscopy is a powerful analytical technique used to determine the structure of organic compounds. In carbon-13 NMR, the different environments of carbon atoms in a molecule lead to distinct chemical shifts, allowing for the identification of isomers like ortho-, meta-, and para-xylene. Proton NMR, however, can be less effective in distinguishing these isomers due to overlapping signals from equivalent protons in similar environments.

Chemical Shift

Chemical shift is a key concept in NMR spectroscopy that describes the resonance frequency of a nucleus relative to a standard in a magnetic field. It is influenced by the electronic environment surrounding the nucleus. In carbon-13 NMR, the chemical shifts of the carbon atoms in ortho-, meta-, and para-xylene differ due to their distinct spatial arrangements, making it easier to differentiate between the isomers compared to proton NMR, where the shifts may overlap.

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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

Different types of protons and carbons in alkanes tend to absorb at similar chemical shifts, making structure determination difficult. Explain how the ¹³C NMR spectrum, including the DEPT technique, would allow you to distinguish among the following four isomers.

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Textbook Question

Hexamethylbenzene undergoes free-radical chlorination to give one monochlorinated product (C₁₂H₁₇Cl) and four dichlorinated products (C₁₂H₁₆Cl₂). These products are easily separated by GC-MS, but the dichlorinated products are difficult to distinguish by their mass spectra. Draw the monochlorinated product and the four dichlorinated products, and explain how ¹³C NMR would easily distinguish among these compounds.

(a) Draw all six isomers of formula C₄H₈ (including stereoisomers).

(b) For each structure, show how many types of H would appear in the proton NMR spectrum.

(d) If an unknown compound of formula C₄H₈ shows two types of H and three types of C, can you determine its structure from this information?

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Textbook Question

When 2-chloro-2-methylbutane is treated with a variety of strong bases, the products always seem to contain two isomers (A and B) of formula C₅H₁₀. When sodium hydroxide is used as the base, isomer A predominates. When potassium tert-butoxide is used as the base, isomer B predominates. The ¹H and ¹³C NMR spectra of A and B are given below.

(a) Determine the structures of isomers A and B.

(b) Explain why A is the major product when using sodium hydroxide as the base and why B is the major product when using potassium tert-butoxide as the base.

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Textbook Question

(A true story.) A major university was designated as a national nuclear magnetic resonance center by the National Science Foundation. Several large superconducting instruments were being installed when a government safety inspector appeared and demanded to know what provisions were being made to handle the nuclear waste produced by these instruments. Assume you are the manager of the NMR center, and offer an explanation that could be understood by a nonscientist.

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