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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 30
Chapter 13, Problem 30

An inexperienced graduate student was making some 4-hydroxybutanoic acid. He obtained an excellent yield of a different compound, whose 13C NMR spectrum is shown here.
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(a) Propose a structure for this product.
(b) Assign the peaks in the 13C NMR spectrum to the carbon atoms in the structure.

Verified step by step guidance
1
Step 1: Analyze the molecular formula provided, C₄H₆O₂. This indicates the compound contains 4 carbons, 6 hydrogens, and 2 oxygens. The degree of unsaturation can be calculated using the formula: (2C + 2 - H)/2. For C₄H₆O₂, the degree of unsaturation is 1, suggesting the presence of one double bond or ring.
Step 2: Examine the 13C NMR spectrum. The peak at approximately 180 ppm corresponds to a carbonyl carbon (C=O), which is characteristic of carboxylic acids or esters. This suggests the compound contains a carbonyl group.
Step 3: The peaks between 20-60 ppm correspond to sp³ hybridized carbons, likely part of alkyl chains. The presence of multiple CH₂ groups suggests a chain structure with methylene groups.
Step 4: Based on the molecular formula and NMR data, propose a structure. The compound is likely γ-butyrolactone, a cyclic ester (lactone) formed from 4-hydroxybutanoic acid. The carbonyl group at 180 ppm corresponds to the ester carbon, and the CH₂ groups correspond to the alkyl chain carbons in the ring.
Step 5: Assign the peaks in the 13C NMR spectrum: (1) The peak at 180 ppm is the ester carbonyl carbon. (2) The peaks at 20-60 ppm correspond to the CH₂ groups in the ring. Each CH₂ group has a slightly different chemical environment, leading to distinct peaks.

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Key Concepts

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

NMR Spectroscopy

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 different carbon environments in a molecule, as well as their relative positions. In a 13C NMR spectrum, each peak corresponds to a distinct carbon atom or group of equivalent carbons, allowing chemists to deduce the molecular structure based on the chemical shifts observed.
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Functional Groups

Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. In the case of 4-hydroxybutanoic acid, the hydroxyl (-OH) and carboxylic acid (-COOH) groups are key functional groups that influence the compound's reactivity and properties. Understanding these groups is essential for predicting the structure of the product formed during synthesis.
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Isomerism

Isomerism refers to the phenomenon where two or more compounds have the same molecular formula but different structures or arrangements of atoms. In organic chemistry, isomers can differ in connectivity (structural isomers) or spatial arrangement (stereoisomers). Recognizing potential isomers is crucial when analyzing the product obtained from a reaction, as it helps in proposing the correct structure based on the NMR data.
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Related Practice
Textbook Question

A bottle of allyl bromide was found to contain a large amount of an impurity. A careful distillation separated the impurity, which has the molecular formula C3H6O. The following 13C NMR spectrum of the impurity was obtained:

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(a) Propose a structure for this impurity.

(b) Assign the peaks in the 13C NMR spectrum to the carbon atoms in the structure.

(c) Suggest how this impurity arose in the allyl bromide sample.

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Textbook Question

The standard 13C NMR spectrum of phenyl propanoate is shown here. Predict the appearance of the DEPT-90 and DEPT-135 spectra.

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Textbook Question

A laboratory student was converting cyclohexanol to cyclohexyl bromide by using one equivalent of sodium bromide in a large excess of concentrated sulfuric acid. The major product she recovered was not cyclohexyl bromide, but a compound of formula C6H10 that gave the following 13C NMR spectrum:

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(a) Propose a structure for this product.

(b) Assign the peaks in the 13C NMR spectrum to the carbon atoms in the structure.

(c) Suggest modifications in the reaction to obtain a better yield of cyclohexyl bromide.

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Textbook Question

An unknown compound has the molecular formula C9H11Br. Its proton NMR spectrum shows the following absorptions:

singlet, δ7.1, integral 44 mm

singlet, δ2.3, integral 130 mm

singlet, δ2.2, integral 67 mm

Propose a structure for this compound.

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Textbook Question

Predict the multiplicity (the number of peaks as a result of splitting) and the chemical shift for each shaded proton in the following compounds.

(a)

(b)

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Textbook Question

(b) Draw the proton NMR spectrum you would expect for butan-2-one. How well do the proton chemical shifts predict the carbon chemical shifts using the "15 to 20 times as large" rule of thumb?

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