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Chapter 13: Problem 13

How many kinds of electronically nonequivalent protons are present in each of the following compounds, and thus how many NMR absorptions might you expect in each? (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Br}\) (b) \(\mathrm{CH}_{3} \mathrm{OCH}_{2} \mathrm{CH}\left(\mathrm{CH}_{3}\right)_{2}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{NO}_{2}\) (d) Methylbenzene (e) 2 -Methyl-1-butene (f) cis-3-Hexene

Short Answer

Expert verified
(a) 2, (b) 4, (c) 3, (d) 2, (e) 5, (f) 4 distinct kinds of protons.

Step by step solution

01

Understanding NMR and Protons

Nuclear Magnetic Resonance (NMR) spectroscopy is a technique used to observe the local magnetic fields around atomic nuclei. In organic compounds, different protons in different chemical environments absorb at different frequencies. This results in distinct NMR signals for each type of chemically unique (nonequivalent) proton.
02

Analyze Compound (a) \\( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Br} \\\)

In bromoethane, * Protons in the \( \text{CH}_3 \) group are equivalent to each other due to rotation about the C鈥揅 bond.* Protons in the \( \text{CH}_2 \) group are equivalent to each other, but different from the \( \text{CH}_3 \) group.* There are 2 distinct sets of protons.
03

Analyze Compound (b) \\( \mathrm{CH}_{3} \mathrm{OCH}_{2} \mathrm{CH}\left(\mathrm{CH}_{3}\right)_{2} \\\)

In this compound:* The \( \text{CH}_3 \) group attached to oxygen is in a unique environment.* The protons in the \( \text{CH}_2\) group next to oxygen are another set.* The \( \text{CH}\left(\text{CH}_3\right)_2 \) produces three unique sets: the \( \text{CH} \) and two different \( \text{CH}_3 \) on the same carbon (because of proximity to \( \text{CH, \text{OH}, \text{CH}_3 \} \)).* There are 4 distinct sets of protons.
04

Analyze Compound (c) \\( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{NO}_{2} \\\)

In nitropropane:* The \( \text{CH}_3 \) group is unique.* The \( \text{CH}_2 \) group adjacent to \( \text{CH}_3 \) is in a different environment from both \( \text{CH}_3 \) and \( \text{CH}_2 \) next to \( \text{NO}_2 \).* There are 3 distinct sets of protons.
05

Analyze Compound (d) Methylbenzene (Toluene)

In methylbenzene:* The methyl group (\( \text{CH}_3 \)) is in a unique environment.* The aromatic \( \text{CH} \) protons are equivalent due to the symmetry of the phenyl group.* There are 2 distinct sets of protons.
06

Analyze Compound (e) 2-Methyl-1-butene

In 2-methyl-1-butene:* \( \text{CH}_3 \) on \( \text{C} \) with the double bond is distinct.* \( \text{CH}_3 \) at end is different from other \( \text{CH}_3 \) because of position.* The \( \text{CH}_2 \), \( \text{CH} \), and \( \text{CH} \) protons have unique environments due to the position relative to the double bond.* There are 5 distinct sets of protons.
07

Analyze Compound (f) cis-3-Hexene

In cis-3-hexene:* The methyl groups (\( \text{CH}_3 \)) at ends are distinct.* The methylene groups next to \( \text{CH}_3 \) and the methyl on the cis configuration differ.* Each set near the double bond is unique due to their different positions.* There are 4 distinct sets of protons.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Electronically Nonequivalent Protons
In NMR spectroscopy, protons that exist in different magnetic environments within a molecule are called electronically nonequivalent protons. These protons absorb at different frequencies because their chemical environments are unique. Even within a single group of protons, if the surrounding electronic environments vary, they can become nonequivalent and produce different NMR signals. Understanding the electronic environment around protons helps in identifying how many unique proton signals can be observed in an NMR spectrum, aiding in the structural analysis of the molecule.
Chemical Environments
The chemical environment of a proton in a molecule describes its surrounding atomic and electronic configuration. This is crucial in NMR spectroscopy since it determines the resonance frequency of a proton. Protons in distinct chemical environments will experience different electronic surroundings, causing them to have unique chemical shifts in the NMR spectrum. Factors such as the presence of electronegative atoms, the type of adjacent bonds, or molecular symmetry can heavily influence the chemical environment. Recognizing these environments helps in predicting and interpreting NMR data effectively.
Proton NMR Signals
Proton NMR signals arise from the interaction of protons with an external magnetic field in NMR spectroscopy. When protons are exposed to this field, they align either with or against the field at specific frequencies determined by their electronic environments. These frequencies are captured as peaks or signals in an NMR spectrum. Each signal corresponds to a set of equivalent protons, while the number of signals indicates the number of different proton-containing environments within the molecule. Careful examination of these signals can provide profound insights into molecular structure and dynamics.
Organic Compounds Analysis
NMR spectroscopy is a powerful tool for the analysis of organic compounds, enabling the identification of molecular structures by examining hydrogen atoms in different environments. By interpreting the number and position of proton NMR signals, along with their splitting patterns, one can deduce the molecular architecture and functional groups present. This analysis is critical in fields like pharmaceuticals, materials science, and organic chemistry research, where understanding the intricate details of molecular structures can impact everything from product design to synthetic pathways.

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Most popular questions from this chapter

Propose structures for compounds with the following formulas that show only one peak in their \({ }^{1} \mathrm{H}\) NMR spectra: (a) \(\mathrm{C}_{5} \mathrm{H}_{12}\) (b) \(\mathrm{C}_{5} \mathrm{H}_{10}\) (c) \(\mathrm{C}_{4} \mathrm{H}_{8} \mathrm{O}_{2}\)

Propose structures for compounds that fit the following \({ }^{1} \mathrm{H}\) NMR data: (a) \(\mathrm{C}_{5} \mathrm{H}_{10} \mathrm{O}\) (b) \(\mathrm{C}_{3} \mathrm{H}_{5} \mathrm{Br}\) \(0.95 \delta\) (6 \(\mathrm{H}\), doublet, \(J=7 \mathrm{~Hz}\) ) \(\quad 2.32 \delta(3 \mathrm{H},\) singlet \()\) \(2.10 \delta(3 \mathrm{H},\) singlet \()\) \(5.35 \delta(1 \mathrm{H},\) broad singlet \()\) \(2.43 \delta(1 \mathrm{H}, \mathrm{multiplet})\) \(5.54 \delta(1 \mathrm{H},\) broad singlet \()\)

The following \({ }^{1} \mathrm{H}\) NMR absorptions were obtained on a spectrometer operating at \(200 \mathrm{MHz}\) and are given in hertz downfield from the TMS standard. Convert the absorptions to \(\delta\) units. (a) \(436 \mathrm{~Hz}\) (b) \(956 \mathrm{~Hz}\) (c) \(1504 \mathrm{~Hz}\)

Compound \(A\), a hydrocarbon with \(M^{+}=96\) in its mass spectrum, has the \({ }^{13} \mathrm{C}\) spectral data given below. On reaction with \(\mathrm{BH}_{3},\) followed by treatment with basic \(\mathrm{H}_{2} \mathrm{O}_{2}, \mathrm{~A}\) is converted into \(\mathrm{B},\) whose \({ }^{13} \mathrm{C}\) spectral data are also given below. Propose structures for \(\mathbf{A}\) and \(\mathbf{B}\).

The following \({ }^{1} \mathrm{H}\) NMR absorptions were obtained on a spectrometer operating at \(300 \mathrm{MHz}\). Convert the chemical shifts from \(\delta\) units to hertz downfield from TMS. (a) 2.18 (b) \(3.45 \delta\) (c) \(6.30 \delta\) (d) \(7.70 \delta\)
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