Ch. 15 - Structural Identification II: Nuclear Magnetic Resonance Spectroscopy

Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition

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Ch. 15 - Structural Identification II: Nuclear Magnetic Resonance Spectroscopy

Problem 23a

Chapter 14, Problem 23a

The methyl hydrogens in propane appear at a chemical shift of 0.9 ppm, whereas the methyl hydrogens of propene appear around 2.5 ppm. Explain.

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Identify the structural difference between propane and propene. Propane is a saturated alkane with the formula C3H8, while propene is an unsaturated alkene with the formula C3H6, containing a double bond.

Understand the concept of chemical shift in NMR spectroscopy. Chemical shift is influenced by the electronic environment surrounding the hydrogen atoms. It is measured in parts per million (ppm).

Consider the electronic environment of the methyl hydrogens in propane. In propane, the methyl groups are bonded to sp3 hybridized carbon atoms, which are saturated and have a relatively low electron-withdrawing effect, resulting in a chemical shift of around 0.9 ppm.

Analyze the electronic environment of the methyl hydrogens in propene. In propene, the methyl group is adjacent to a double bond, which involves sp2 hybridized carbon atoms. The double bond creates a deshielding effect due to the electron-withdrawing nature of the π-bond, leading to a higher chemical shift of around 2.5 ppm.

Conclude that the difference in chemical shift between the methyl hydrogens in propane and propene is due to the presence of the double bond in propene, which alters the electronic environment and increases the deshielding effect, resulting in a higher chemical shift.

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Key Concepts

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

Chemical Shift in NMR Spectroscopy

Chemical shift is a key concept in NMR spectroscopy that refers to the resonant frequency of a nucleus relative to a standard in a magnetic field. It provides information about the electronic environment surrounding the nucleus. In organic molecules, different functional groups and bonding environments cause variations in electron density, leading to different chemical shifts.

Shielding and Deshielding Effects

Shielding and deshielding are phenomena that affect the chemical shift in NMR. Shielding occurs when electron clouds around a nucleus reduce the external magnetic field's effect, leading to a lower chemical shift. Deshielding happens when electron density is reduced, often due to electronegative atoms or pi bonds, resulting in a higher chemical shift. This explains why methyl hydrogens in propene, affected by the deshielding effect of the double bond, appear at a higher ppm than in propane.

Effect of Pi Bonds on Chemical Shift

Pi bonds, such as those in alkenes, can significantly influence the chemical shift of nearby hydrogens. The electron cloud of a pi bond can create a local magnetic field that opposes the external field, leading to deshielding of adjacent hydrogens. This is why the methyl hydrogens in propene, which are near a pi bond, have a higher chemical shift compared to those in propane, which lacks such a bond.

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Related Practice

Textbook Question

For the molecules shown, give the number of signals expected and the relative ratio of the signal integrations.

(b)

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Textbook Question

An unknown compound (C₇H₆O) gives the IR spectrum shown here. At what chemical shifts would you expect to see signals in the ¹H NMR spectrum?

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Textbook Question

(a) Calculate the resonance frequency of an aldehydic proton ( δ 9.3 ppm) if it is detected on a 60-MHz NMR spectrometer.

(b) What if it were detected on a 300-MHz instrument?

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Textbook Question

In Figure 15.34, H_a was assumed to be 'up.' How does the analysis change if we assume instead that H_a is down?

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Textbook Question

Without worrying about the relative location of the signals (i.e., the chemical shift) or the splitting patterns, draw a spectrum of the following molecule, being sure to indicate the integration of each peak. Label each signal based on the set of equivalent hydrogens to which it corresponds. [We expand on this question in future assessments.]

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Textbook Question

Though Figure 15.34 was concerned with the appearance of H_a, how would H_b appear in the spectrum?

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