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Ch. 14 - Structural Identification I: Infrared Spectroscopy and Mass Spectrometry
Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition
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All textbooksMullins 1st EditionCh. 14 - Structural Identification I: Infrared Spectroscopy and Mass SpectrometryProblem 54
Chapter 13, Problem 54

Under acidic conditions, alkene A can be isomerized to the more stable alkene B. How could IR spectroscopy be used to distinguish between A and B? [There are a few correct answers.]
Chemical reaction showing isomerization of alkene A to alkene B using H2SO4.

Verified step by step guidance
1
Identify the structural differences between alkene A and alkene B. Typically, isomerization involves changes in the position of the double bond or the configuration around it.
Understand that IR spectroscopy is used to identify functional groups based on characteristic absorption bands. Alkenes typically show C=C stretching vibrations.
Determine the IR absorption range for C=C stretching vibrations, which is usually around 1600-1680 cm-1. The exact position can vary depending on the substitution pattern of the alkene.
Consider that the stability of alkenes can affect the C=C stretching frequency. More substituted alkenes (like alkene B, if it is more stable) might show a shift in the C=C stretching frequency compared to less substituted alkenes (like alkene A).
Use IR spectroscopy to compare the C=C stretching frequencies of alkene A and alkene B. Look for shifts or changes in intensity that could indicate differences in substitution or conjugation, helping to distinguish between the two isomers.

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Key Concepts

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

Alkene Isomerization

Alkene isomerization involves the rearrangement of atoms within an alkene molecule, often resulting in a more stable configuration. Under acidic conditions, this process can shift the position of double bonds or alter the geometry, leading to a more thermodynamically favorable isomer. Understanding the stability of alkenes is crucial for predicting the outcome of isomerization reactions.
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Infrared (IR) Spectroscopy

IR spectroscopy is a technique used to identify functional groups in organic compounds by measuring the absorption of infrared light. Different bonds absorb specific frequencies, creating a spectrum that can be analyzed to determine molecular structure. For alkenes, the C=C stretching vibration typically appears around 1600-1680 cm⁻¹, allowing differentiation between isomers based on subtle shifts in these peaks.
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Alkene Stability

Alkene stability is influenced by factors such as the degree of substitution and the geometry of the double bond. More substituted alkenes are generally more stable due to hyperconjugation and the electron-donating effects of alkyl groups. Additionally, trans alkenes are often more stable than cis due to reduced steric hindrance. Recognizing these stability trends helps predict which isomer will be favored under certain conditions.
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Related Practice
Textbook Question

Would you expect the stretching band of the carbonyl to appear at a higher frequency for cyclohexanecarbaldehyde or benzaldehyde? Explain.

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Textbook Question

A carboxylic acid can be converted to an ester using the conditions shown. Explain how a comparison of an IR spectrum of the reactant(s) and product can be used to determine whether the reaction was successful or not.

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Textbook Question

What functional groups might be present in the IR spectra for the molecules with the given molecular formulas. [Be sure to use the molecular formula in your analysis.]

(b) C₃H₄O₄

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Textbook Question

Justify the carbonyl stretching frequencies indicated for benzaldehyde and 4-methoxybenzaldehyde.

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Textbook Question

Hoping to make the following diene, a chemist treated the diol shown with acid. Based on the IR spectrum, was the reaction successful? If not, what compound was made instead?

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Textbook Question

Justify the carbonyl stretching frequencies for a series of methoxybenzaldehydes. Specifically, why are the 2- and 4-methoxy derivatives similar to each other but different from the 3-methoxy derivative?

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