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Chapter 14: Problem 3

How many \({ }^{1} \mathrm{H}\) NMR signals does each compound show? a. \(\mathrm{CH}_{3} \mathrm{CH}_{3}\) c. \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\) e. \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CO}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\) g. \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OCH}_{2} \mathrm{CH}_{3}\) b. \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3}\) d. \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCH}\left(\mathrm{CH}_{3}\right)_{2}\) f. \(\mathrm{CH}_{3} \mathrm{OCH}_{2} \mathrm{CH}\left(\mathrm{CH}_{3}\right)_{2}\) h. \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\)

Short Answer

Expert verified
a. 1 signal, c. 2 signals, e. 4 signals, g. 2 signals, b. 2 signals, d. 3 signals, f. 3 signals, h. 4 signals.

Step by step solution

01

Understanding NMR Signals

In proton NMR ( H NMR), each distinct hydrogen environment in a molecule corresponds to a different NMR signal. To determine the number of such signals, we identify groups of protons that are in chemically equivalent environments and will resonate at the same frequency.
02

Compound a: Ethane \\(\mathrm{CH}_{3} \mathrm{CH}_{3}\\)

Ethane contains two equivalent methyl groups, where all protons are chemically equivalent. Therefore, it shows one signal in its H NMR spectrum.
03

Compound c: Butane \\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\\)

Butane has two sets of chemically equivalent protons: terminal methyl groups ( CH鈧�) and two methylene groups ( CH鈧�). These give rise to two distinct signals, one for each proton environment.
04

Compound e: Ethyl propanoate \\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CO}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\\)

This molecule contains four different sets of protons: methyl ( CH鈧�) attached to a methylene, methylene next to carbonyl, methylene next to oxygen, and the methyls on both ends. Here, the four different environments lead to four different signals.
05

Compound g: Diethyl ether \\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OCH}_{2} \mathrm{CH}_{3}\\)

In diethyl ether, we have two methyl groups (chemically equivalent), two equivalent methylene groups adjacent to the oxygen. Therefore, we expect two signals corresponding to these two environments.
06

Compound b: Propane \\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3}\\)

Propane displays two types of proton environments: the terminal methyl groups which are equivalent and a central methylene group. These define two signals in its H NMR spectrum.
07

Compound d: 2,2,4-Trimethylpentane \\(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCH}\left(\mathrm{CH}_{3}\right)_{2}\\)

This compound includes three types of different environments: the isopropyl methine hydrogen, the isopropyl methyl groups, and the additional CH鈧� groups attached to CH. Thus, it creates three signals in the H NMR spectrum.
08

Compound f: tert-Butyl methyl ether \\(\mathrm{CH}_{3} \mathrm{OCH}_{2} \mathrm{CH}\left(\mathrm{CH}_{3}\right)_{2}\\)

The three types of protons include those in the tert-butyl group, the methylene next to oxygen, and the methyl directly attached to oxygen, leading to three distinct signals in H NMR.
09

Compound h: Butanol \\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\\)

Butanol features four different environments: the terminal methyl group, two different methylene groups, and an hydroxyl proton. These produce four signals in the H NMR.

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Key Concepts

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

chemical shift
In NMR Spectroscopy, a **chemical shift** describes the resonant frequency of a nucleus relative to a standard in a magnetic field. It is primarily a measure of the local magnetic environment of the nuclear spins. Chemical shifts are expressed in parts per million (ppm) and are influenced by the electron density surrounding the nuclei. Higher electron density shields the nucleus, leading to an upfield shift (lower ppm), while deshielding decreases electron density, causing a downfield shift (higher ppm).
  • Electronegative elements like oxygen and nitrogen often induce downfield shifts, as they remove electron density.
  • Electron-donating groups such as alkyl groups typically result in upfield shifts.
Understanding chemical shifts helps chemists deduce the structure of a molecule and identify functional groups within its framework.
proton NMR signals
In proton NMR spectroscopy, each distinct hydrogen atom or group's protons can give rise to a unique **proton NMR signal**. The goal is to predict how many separate signals one would observe for a given molecule. The presence of different proton environments in a molecule dictates the number of signals in an NMR spectrum. Different environments mean protons experience varied local magnetic fields, thus resonating at different frequencies.
  • A compound like ethane ( CH鈧僀H鈧�) yields one signal as all protons share the same environment.
  • More complex molecules like butane ( CH鈧僀H鈧侰H鈧侰H鈧�) exhibit multiple signals due to distinct proton environments.
Numbers of signals provide insights into the symmetry and diversity of chemical environments present in the molecule.
equivalent protons
**Equivalent protons** are those that are indistinguishable by NMR and resonate at the same frequency due to similar local environments. Recognizing equivalent protons is critical for predicting and interpreting NMR spectra. For instance, in the compound ethane ( CH鈧僀H鈧�), all six protons are equivalent, resulting in a single signal. Similarly, in propane ( CH鈧僀H鈧侰H鈧�), the terminal methyl groups have equivalent protons, while the central methylene does not.
  • Symmetry in the molecule often leads to proton equivalence, as seen in symmetrical molecules like ethane.
  • Understanding equivalence can simplify complex NMR spectra, reducing the total number of signals.
By identifying equivalent protons, one can quickly determine molecular symmetry and make more accurate structural deductions.
proton environments
A **proton environment** refers to the specific surroundings of a hydrogen atom within a molecule, influencing its chemical shift and resonance in an NMR spectrum. Different environments lead to distinct signals. The environment of a proton is shaped by:
  • The type and number of nearby atoms, especially electronegative ones like oxygen or halogens.
  • Bonding context, such as being part of a methyl, methylene, or methine group.
  • Proximity to functional groups, like alcohols, carbonyls, or ethers, which affect the electronic surroundings.
For example, in diethyl ether ( CH鈧僀H鈧侽CH鈧侰H鈧�), the methyl ( CH鈧�) and methylene ( CH鈧�) groups experience different environments, resulting in two signals. Understanding these environments allows chemists to predict the number and type of signals in an NMR spectrum, aiding in molecular structure analysis.

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Most popular questions from this chapter

Explain why the \({ }^{13} \mathrm{C}\) NMR spectrum of 3 -methyl-2-butanol shows five signals.

Draw the four constitutional isomers having molecular formula \(\mathrm{C}_{4} \mathrm{H}_{9} \mathrm{Br}\) and indicate how many different kinds of carbon atoms each has.

Identify the carbon atoms that give rise to the signals in the \({ }^{13} \mathrm{C}\) NMR spectrum of each compound. a. \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH} ;{ }^{13} \mathrm{C}\) NMR: \(14,19,35\), and \(62 \mathrm{ppm}\) b. \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCHO} ;{ }^{13} \mathrm{C}\) NMR: 16,41 , and \(205 \mathrm{ppm}\) c. \(\mathrm{CH}_{2}=\mathrm{CHCH}(\mathrm{OH}) \mathrm{CH}_{3} ;{ }^{13} \mathrm{C}\) NMR: \(23,69,113\), and \(143 \mathrm{ppm}\)

Draw the structures for the four isomeric dihalides with molecular formula \(\mathrm{C}_{3} \mathrm{H}_{6} \mathrm{Cl}_{2}\) and label each different type of proton.

Which compounds give a \({ }^{1} \mathrm{H}\) NMR spectrum with two signals in a ratio of \(2: 3 ?\) a. \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl}\) b. \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3}\) c. \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OCH}_{2} \mathrm{CH}_{3}\) d. \(\mathrm{CH}_{3} \mathrm{OCH}_{2} \mathrm{CH}_{2} \mathrm{OCH}_{3}\)
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