The 1H NMR spectra of three isomers with molecular formula C4H9Br are shown here. Which isomer produces which spectrum? c.

Ch. 14 - NMR Spectroscopy

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Bruice 8th Edition

Ch. 14 - NMR Spectroscopy

Problem 54c

Chapter 15, Problem 54c

The ¹H NMR spectra of three isomers with molecular formula C₄H₉Br are shown here. Which isomer produces which spectrum?
c. <IMAGE>

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Analyze the molecular formula C4H9Br, which indicates the presence of four carbons, nine hydrogens, and one bromine atom. This suggests the possibility of structural isomers.

Examine the 1H NMR spectrum provided. The spectrum shows three distinct signals corresponding to different proton environments. The integration values are labeled as 2H, 2H, and 6H, which represent the relative number of protons in each environment.

Interpret the chemical shift values. The signal at approximately 3.5 ppm corresponds to protons near an electronegative atom, likely the bromine. The signal at around 1.8 ppm corresponds to protons in a slightly deshielded environment, such as those adjacent to a carbon-bromine bond. The signal at approximately 1.0 ppm corresponds to protons in a typical alkyl environment.

Consider the splitting patterns. The signal at 3.5 ppm appears as a multiplet, indicating coupling with adjacent protons. The signal at 1.8 ppm also shows splitting, suggesting coupling with nearby protons. The signal at 1.0 ppm appears as a singlet, indicating no coupling with adjacent protons.

Based on the integration, chemical shifts, and splitting patterns, deduce the structure of the isomer that matches this spectrum. The integration of 6H suggests the presence of two equivalent methyl groups, while the 2H signals correspond to protons in distinct environments near the bromine atom and the alkyl chain.

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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, primarily hydrogen (1H), to provide information about the number of hydrogen atoms in different environments within a molecule. The resulting spectrum displays peaks corresponding to these environments, allowing chemists to infer structural details.

Chemical Shift

Chemical shift refers to the position of a peak in an NMR spectrum, measured in parts per million (ppm). It indicates the electronic environment surrounding the hydrogen atoms, influenced by factors such as electronegativity and hybridization. Different isomers will exhibit distinct chemical shifts due to variations in their molecular structures, making this a key aspect for identifying isomers in NMR analysis.

Integration in NMR

Integration in NMR spectroscopy quantifies the area under each peak in the spectrum, which correlates to the number of hydrogen atoms contributing to that signal. This allows chemists to determine the relative number of protons in different environments, aiding in the identification of isomers. For example, if a peak integrates to 6H, it indicates that six hydrogen atoms are in that specific environment, providing crucial information for structural elucidation.

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