Ch. 13 - Nuclear Magnetic Resonance Spectroscopy

Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook

Wade 9th Edition

Ch. 13 - Nuclear Magnetic Resonance Spectroscopy

Problem 26b

Chapter 13, Problem 26b

(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?

Verified step by step guidance

Step 1: Identify the structure of butan-2-one. Butan-2-one is a ketone with the molecular formula C4H8O. Its structure consists of a ketone group (C=O) at the second carbon, flanked by methyl groups on either side.

Step 2: Analyze the proton environments in butan-2-one. There are two distinct proton environments: (1) the methyl group adjacent to the carbonyl group (CH3-C=O), which experiences deshielding due to the electron-withdrawing effect of the carbonyl group, and (2) the terminal methyl group (CH3-CH2), which is less affected by the carbonyl group.

Step 3: Predict the chemical shifts for the proton NMR spectrum. The protons in the CH3-C=O group will appear downfield (higher ppm) due to deshielding, typically around 2.0-2.5 ppm. The protons in the CH3-CH2 group will appear upfield (lower ppm), typically around 0.9-1.2 ppm.

Step 4: Consider the '15 to 20 times as large' rule of thumb for predicting carbon chemical shifts. Carbon chemical shifts are generally 15 to 20 times larger than proton chemical shifts. For example, if the proton chemical shift for CH3-C=O is around 2.5 ppm, the corresponding carbon chemical shift for the carbonyl carbon might be approximately 37.5-50 ppm.

Step 5: Compare the predicted proton and carbon chemical shifts. Using the rule of thumb, verify whether the predicted carbon chemical shifts align with the expected values based on the proton chemical shifts. This comparison helps validate the relationship between proton and carbon environments in NMR spectroscopy.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Proton NMR Spectroscopy

Proton Nuclear Magnetic Resonance (NMR) spectroscopy is a technique used to determine the structure of organic compounds by analyzing the magnetic environment of hydrogen atoms in a molecule. In the spectrum, peaks correspond to different hydrogen environments, and their chemical shifts provide information about the electronic surroundings of the protons, allowing for the identification of functional groups and molecular structure.

Chemical Shifts

Chemical shifts in NMR spectroscopy refer to the resonance frequency of a nucleus relative to a standard reference frequency, typically measured in parts per million (ppm). The position of a peak in the NMR spectrum indicates the electronic environment of the protons, influenced by factors such as electronegativity and hybridization. Understanding chemical shifts is crucial for interpreting the NMR spectrum and predicting the structure of the compound.

The '15 to 20 Times' Rule

The '15 to 20 times as large' rule is a heuristic used in organic chemistry to estimate carbon chemical shifts based on proton chemical shifts. This rule suggests that the carbon chemical shift will typically be 15 to 20 times greater than the corresponding proton chemical shift due to the differences in the magnetic environment and the number of protons influencing the carbon atom. This relationship helps chemists predict carbon shifts when analyzing NMR data.

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Textbook Question

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.

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Textbook Question

Five proton NMR spectra are given here, together with molecular formulas. In each case, propose a structure that is consistent with the spectrum.

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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

Give the spectral assignments for the protons in isobutyl alcohol (Solved Problem 13-4). For example, H^a is a singlet, area = 1, at δ2.4.

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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

(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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