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 38

Chapter 14, Problem 38

What coupling constant would you expect between Hₐ and H₆ in cyclopent-2-en-1-one?

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Identify the structure of cyclopent-2-en-1-one. It is a five-membered ring with a double bond between the second and third carbon atoms and a ketone group at the first carbon.

Locate the hydrogen atoms Hₐ and H₆ on the cyclopent-2-en-1-one structure. Hₐ is typically the hydrogen on the carbon adjacent to the ketone group, and H₆ is on the carbon involved in the double bond.

Understand that the coupling constant, J, is influenced by the dihedral angle between the two hydrogen atoms and the electronic environment. In a conjugated system like cyclopent-2-en-1-one, the coupling constant is affected by the π-electron system.

Consider the typical range of coupling constants for vicinal hydrogens in alkenes, which is usually between 6-12 Hz. The exact value depends on the dihedral angle and the presence of conjugation.

Use the Karplus equation, which relates the coupling constant to the dihedral angle between the hydrogens:

J = A + B cos (θ) + C cos (2 θ)

, where A, B, and C are constants determined experimentally. Estimate the dihedral angle based on the molecular geometry to predict the coupling constant.

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Key Concepts

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

Coupling Constant

The coupling constant, denoted as J, is a measure of the interaction between nuclear spins in NMR spectroscopy. It reflects the energy difference between spin states and is influenced by the number of bonds separating the nuclei and the geometry of the molecule. Understanding J values helps predict splitting patterns in NMR spectra.

Cyclopent-2-en-1-one Structure

Cyclopent-2-en-1-one is a five-membered ring with a double bond between the second and third carbon atoms and a ketone group at the first carbon. The structure affects the electronic environment and spatial arrangement of hydrogen atoms, influencing their NMR coupling constants. Recognizing the positions of Hₐ and H₆ is crucial for predicting their interaction.

Vicinal Coupling

Vicinal coupling refers to the interaction between hydrogen atoms on adjacent carbon atoms, typically separated by three bonds. In cyclopent-2-en-1-one, the coupling constant between Hₐ and H₆ is expected to be influenced by the ring strain and the presence of the double bond, which can alter typical vicinal coupling values.

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Related Practice

Textbook Question

Draw the expected signal for a hydrogen with the following coupling constants.

(a) Hₐ : δ 5.34 (Jₐ꜀ = 12 , Jₐ₆ = 2)

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Textbook Question

Draw a spectrum for each of the following molecules, being sure to indicate the multiplicity, integration, and chemical shift of each peak. Label each signal based on the set of equivalent hydrogens to which it corresponds.

(b)

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Textbook Question

A chemist made what was thought to be compound A or B. How could coupling constants between H_a and H_b be used to distinguish between the two isomers? [Hint: Draw a chair conformation of each.]

(c)

Alkene hydrogens usually appear at similar chemical shifts between 5 and 6 ppm. The alkene hydrogens in the structure shown are very different. Rationalize the difference in chemical shifts between Hₐ and H₆ in the structure shown.

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Textbook Question

Draw the expected signal for a hydrogen with the following coupling constants.

(b) Hₐ : δ 3.34 (Jₐ꜀ = 9 , Jₐ₆ = 4 )

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