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Ch. 19 - Amines
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook
All textbooksWade 9th EditionCh. 19 - AminesProblem 56
Chapter 19, Problem 56

The following spectra for A and B correspond to two structural isomers. The NMR singlet at δ1.16 in spectrum A disappears when the sample is shaken with D2O. The singlet at δ0.6 ppm in the spectrum of B disappears on shaking with D2O. Propose structures for these isomers, and show how your structures correspond to the spectra. Show what cleavage is responsible for the base peak at m/z 44 in the mass spectrum of A and the prominent peak at m/z 58 in the mass spectrum of B.
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Step 1: Analyze the NMR data for compound A. The singlet at δ1.16 ppm disappears when shaken with D2O, indicating the presence of an exchangeable proton, such as an -OH or -NH group. This suggests that compound A contains a hydroxyl or amine functional group.
Step 2: Analyze the NMR data for compound B. The singlet at δ0.6 ppm also disappears upon shaking with D2O, which similarly indicates the presence of an exchangeable proton. This suggests that compound B also contains a hydroxyl or amine functional group, but the chemical shift difference implies a different chemical environment compared to A.
Step 3: Examine the mass spectrum of compound A. The base peak at m/z 44 suggests a specific fragmentation pattern. This could correspond to the loss of a neutral molecule such as CO2 or CH2O, depending on the structure of A. Consider how the functional groups and molecular structure of A could lead to this fragmentation.
Step 4: Examine the mass spectrum of compound B. The prominent peak at m/z 58 suggests a different fragmentation pattern compared to A. This could correspond to the loss of a neutral fragment or the formation of a stable carbocation. Analyze how the structure of B could lead to this specific peak.
Step 5: Propose structures for A and B based on the NMR and mass spectral data. Ensure that the proposed structures are consistent with the observed chemical shifts, exchangeable protons, and fragmentation patterns. For A, consider a structure with a hydroxyl group in a specific environment, and for B, consider a structure with a hydroxyl or amine group in a different environment. Verify that the proposed structures are structural isomers of each other.

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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 provides information about the number and environment of hydrogen atoms in a molecule, indicated by chemical shifts (δ values). The appearance of singlets in the spectrum suggests the presence of protons in equivalent environments, while changes upon treatment with D2O indicate the presence of exchangeable protons, such as those in alcohols or amines.
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Isomerism

Isomerism refers to the phenomenon where two or more compounds have the same molecular formula but different structural arrangements or spatial orientations. Structural isomers differ in the connectivity of their atoms, which can lead to variations in physical and chemical properties, including NMR and mass spectra. Understanding isomerism is crucial for proposing the correct structures based on spectral data.
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Mass Spectrometry

Mass spectrometry is an analytical technique used to measure the mass-to-charge ratio of ions. It provides information about the molecular weight and structure of compounds. The base peak represents the most abundant ion, while specific peaks correspond to fragmentation patterns. In this case, the peaks at m/z 44 and 58 can be linked to specific cleavage pathways in the respective isomers, helping to elucidate their structures.
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Related Practice
Textbook Question

Pyrrole undergoes electrophilic aromatic substitution more readily than benzene, and mild reagents and conditions are sufficient. These reactions normally occur at the 2-position rather than the 3-position, as shown in the following example.

b. Explain why pyrrole reacts more readily than benzene, and also why substitution occurs primarily at the 2-position rather than the 3-position.

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Textbook Question

Show why p-nitroaniline is a much weaker base (3 pKb units weaker) than aniline.

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Textbook Question

Guanidine (shown) is about as strong a base as hydroxide ion. Explain why guanidine is a much stronger base than most other amines.

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Explain why N,N,2,6-tetramethylaniline (shown) is a much stronger base than N,N-dimethylaniline.

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Textbook Question

Using any necessary reagents, show how you can accomplish the following multistep syntheses.

(c)

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Textbook Question

Pyrrole undergoes electrophilic aromatic substitution more readily than benzene, and mild reagents and conditions are sufficient. These reactions normally occur at the 2-position rather than the 3-position, as shown in the following example.

a. Propose a mechanism for the acetylation of pyrrole just shown. You may begin with pyrrole and the acylium ion, CH3C≡O+. Be careful to draw all the resonance structures of the intermediate.

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