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:(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.

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 29

Chapter 13, Problem 29

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.

Verified step by step guidance

Analyze the molecular formula C3H6O. This suggests the impurity contains three carbons, six hydrogens, and one oxygen. The presence of oxygen indicates a functional group such as an alcohol, ether, or carbonyl group.

Examine the 13C NMR spectrum. There are three distinct peaks, indicating three unique carbon environments. The chemical shifts are approximately 140 ppm, 120 ppm, and 60 ppm. These shifts suggest the presence of unsaturated carbons and a carbon bonded to an electronegative atom.

Assign the peaks: The peak at ~140 ppm corresponds to a carbon in a double bond (likely a CH group in an alkene). The peak at ~120 ppm corresponds to another unsaturated carbon (likely a CH2 group in an alkene). The peak at ~60 ppm corresponds to a CH2 group bonded to an electronegative atom, such as oxygen.

Propose a structure: Based on the molecular formula and NMR data, the impurity is likely propen-2-ol (CH3-CH=CH-OH). This structure matches the molecular formula and the NMR data, with the hydroxyl group accounting for the oxygen atom and the unsaturated carbons accounting for the peaks at ~140 ppm and ~120 ppm.

Suggest how the impurity arose: The impurity could have formed via hydrolysis of allyl bromide (CH2=CH-CH2Br) in the presence of water or moisture, leading to the formation of propen-2-ol as a byproduct.

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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 unique carbon atom or group of equivalent carbons, allowing chemists to deduce the molecular structure based on the chemical shifts and splitting patterns observed.

Molecular Formula and Structure

The molecular formula of a compound indicates the types and numbers of atoms present, but it does not provide information about the arrangement of these atoms. For the impurity with the formula C3H6O, various structural isomers are possible, including alcohols, ethers, and carbonyl compounds. Understanding how to derive possible structures from a molecular formula is crucial for proposing a correct structure based on additional data, such as NMR spectra.

Formation of Impurities

Impurities in organic compounds can arise from various sources, including side reactions during synthesis, degradation of the main product, or contamination during handling. In the case of allyl bromide, the presence of an impurity with the formula C3H6O suggests that it may have formed through reactions such as hydrolysis or rearrangement. Understanding the reaction mechanisms and conditions that lead to such byproducts is essential for explaining the origin of impurities in organic samples.

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Related Practice

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.

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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

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.

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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?

Textbook Question

(a) Show which carbon atoms correspond with which peaks in the 13C NMR spectrum of butan-2-one (Figure 13-45).

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