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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 43
Chapter 13, Problem 43

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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Verified step by step guidance
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Step 1: Analyze the NMR spectrum provided. The spectrum shows two distinct peaks: one at approximately 4 ppm corresponding to 4 hydrogens, and another at approximately 2 ppm corresponding to 2 hydrogens. This suggests the presence of two different types of proton environments in the molecule.
Step 2: Consider the reaction conditions. The original reaction involved adding bromine across the double bond of but-2-ene to form 2,3-dibromobutane. However, due to elevated temperatures, side reactions likely occurred, leading to impurities. Elevated temperatures can promote elimination or rearrangement reactions.
Step 3: Interpret the chemical shifts. The peak at 4 ppm is characteristic of protons attached to carbons bonded to electronegative atoms like bromine. The peak at 2 ppm is typical of protons on carbons adjacent to a double bond or other electron-withdrawing groups.
Step 4: Propose a structure for the impurity. Based on the NMR data, the impurity likely contains a bromine atom bonded to a carbon, as indicated by the 4 ppm peak. The 2 ppm peak suggests the presence of a double bond or a similar electron-withdrawing group. A plausible structure could be 1-bromo-2-butene, formed via elimination of HBr from 2,3-dibromobutane.
Step 5: Assign the peaks to the protons in the proposed structure. In 1-bromo-2-butene, the 4 hydrogens at 4 ppm correspond to the two CH2 groups adjacent to the bromine atom. The 2 hydrogens at 2 ppm correspond to the CH group and the CH2 group near the double bond.

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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 works by measuring the magnetic properties of certain nuclei, typically hydrogen (1H) or carbon (13C), in a magnetic field. The resulting spectrum provides information about the number of hydrogen atoms in different environments, allowing chemists to deduce the molecular structure and identify functional groups.
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Chemical Shifts

Chemical shifts in NMR spectra indicate the environment of the hydrogen atoms in a molecule. They are measured in parts per million (ppm) and reflect the electronic environment surrounding the nuclei. For example, protons attached to carbons adjacent to electronegative atoms (like bromine) will appear downfield (higher ppm) due to deshielding effects, while protons in more shielded environments appear upfield (lower ppm).
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1H NMR Chemical Shifts

Integration of Peaks

The integration of peaks in an NMR spectrum corresponds to the number of hydrogen atoms contributing to each signal. The area under each peak is proportional to the number of protons in that environment. In the provided spectrum, the peak labeled '4 H' suggests that there are four equivalent protons, while the '2 H' peak indicates two equivalent protons, which helps in determining the molecular structure and confirming the presence of specific groups in the compound.
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Related Practice
Textbook Question

Tell precisely how you would use the proton NMR spectra to distinguish between the following pairs of compounds.

(a) 1-bromopropane and 2-bromopropane

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Textbook Question

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

Sketch your predictions of the proton NMR spectra of the following compounds.

(b)

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Textbook Question

Tell precisely how you would use the proton NMR spectra to distinguish between the following pairs of compounds.

(b)

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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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