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.

Ch. 13 - Nuclear Magnetic Resonance Spectroscopy

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

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Wade 9th Edition

Ch. 13 - Nuclear Magnetic Resonance Spectroscopy

Problem 51

Chapter 13, Problem 51

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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Step 1: Analyze the molecular structures of the four isomers (a, b, c, d). Each structure has a unique arrangement of carbons and hydrogens, which will result in distinct 13C NMR signals. For example, cyclic structures like cyclohexane (a) and cyclopentane (b) will have different chemical environments compared to branched structures like (c) and (d).

Step 2: Identify the number of unique carbon environments in each isomer. In 13C NMR, each chemically distinct carbon atom will produce a separate signal. For example, (a) has a symmetrical cyclohexane ring, while (b) has a cyclopentane ring with an ethyl group attached, leading to different carbon environments.

Step 3: Use DEPT (Distortionless Enhancement by Polarization Transfer) spectroscopy to distinguish between CH, CH2, and CH3 groups. DEPT-90 shows only CH carbons, DEPT-135 differentiates CH2 (negative peaks) and CH3/CH (positive peaks). This technique helps identify the types of carbons present in each isomer.

Step 4: Compare the splitting patterns and chemical shifts in the 13C NMR spectrum. Chemical shifts are influenced by the electronic environment around each carbon. For example, carbons in strained rings (like in (c) and (d)) will have different shifts compared to carbons in less strained rings (like (a) and (b)).

Step 5: Correlate the observed 13C NMR data with the molecular structures to distinguish the isomers. For instance, (c) has a highly branched structure with a quaternary carbon, which will appear as a unique signal in the spectrum. Similarly, (d) has a three-membered ring, which will produce distinct signals due to ring strain and unique carbon environments.

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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, such as carbon-13 ( 13C), to provide information about the environment surrounding these nuclei. In alkanes, different types of protons and carbons can absorb at similar chemical shifts, making it challenging to distinguish between isomers without advanced techniques.

Chemical Shifts

Chemical shifts in NMR spectroscopy refer to the resonant frequency of a nucleus relative to a standard reference frequency. They provide insight into the electronic environment of the nuclei, allowing chemists to infer structural information. In alkanes, the chemical shifts of protons and carbons can overlap, complicating the interpretation of the spectrum, especially when distinguishing between isomers with similar structures.

DEPT Technique

The DEPT (Distortionless Enhancement by Polarization Transfer) technique is a specialized NMR method that enhances the detection of specific types of carbon atoms in a molecule. It allows for the differentiation between CH, CH2, and CH3 groups by providing distinct signals for each type. This technique is particularly useful in analyzing complex mixtures or isomers, as it helps to clarify the carbon environment and improve the resolution of the NMR spectrum.

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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.

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.

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Textbook Question

(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

Each of these four structures has molecular formula C₄H₈O₂. Match the structure with its characteristic proton NMR signals. (Not all of the signals are listed in each case.)

(a) sharp 1H singlet at δ8.0 and 2H triplet at δ4.0

(b) sharp 3H singlet at δ2.0 and 2H quartet at δ4.1

(d) broad 1H singlet at δ11.5 and 2H triplet at δ2.3

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Textbook Question

How many signals would you expect to see in the 13C NMR of the following compounds? In each case, show which carbon atoms are equivalent in the ¹³C NMR.

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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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