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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 57
Chapter 13, Problem 57

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?
IR spectra of methoxybenzaldehydes: 4- and 2-methoxy at 1688-1689 cm⁻¹, 3-methoxy at 1702 cm⁻¹.

Verified step by step guidance
1
Understand the concept of electronic effects in benzene rings: Substituents on a benzene ring can influence the electronic environment of other substituents through resonance and inductive effects. This can affect properties such as IR stretching frequencies.
Identify the position of the methoxy group: In methoxybenzaldehydes, the methoxy group can be in the ortho (2-), meta (3-), or para (4-) position relative to the aldehyde group.
Analyze the resonance effects: The methoxy group is an electron-donating group through resonance. In the 2- and 4-methoxy derivatives, the methoxy group can participate in resonance with the carbonyl group, stabilizing the structure and affecting the carbonyl stretching frequency.
Consider the meta position: In the 3-methoxy derivative, the methoxy group is in the meta position, which does not allow for resonance interaction with the carbonyl group. This results in a different electronic environment compared to the 2- and 4-methoxy derivatives.
Relate to IR spectroscopy: The differences in electronic environments due to the position of the methoxy group lead to variations in the carbonyl stretching frequencies observed in IR spectroscopy. The 2- and 4-methoxy derivatives have similar frequencies due to similar resonance effects, while the 3-methoxy derivative differs due to the lack of resonance interaction.

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Key Concepts

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

Electronic Effects of Substituents

Substituents on a benzene ring can influence the electronic distribution through resonance and inductive effects. Ortho (2-) and para (4-) positions allow for resonance interaction with the ring, affecting electron density at the carbonyl group. This can alter the carbonyl stretching frequency in IR spectroscopy, as seen in methoxybenzaldehydes.
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Infrared (IR) Spectroscopy

IR spectroscopy is a technique used to identify functional groups in a molecule by measuring the absorption of infrared light, which causes molecular vibrations. The carbonyl group typically shows a strong absorption band, and its frequency can shift based on electronic effects from substituents, providing insights into molecular structure.
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Resonance and Inductive Effects

Resonance involves the delocalization of electrons across adjacent atoms, while inductive effects involve the transmission of charge through sigma bonds. In methoxybenzaldehydes, the methoxy group can donate electrons through resonance, particularly at the ortho and para positions, affecting the carbonyl group's electron density and IR absorption frequency.
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Related Practice
Textbook Question

The first step of a reaction called electrophilic aromatic substitution is as follows:

If this step is rate-determining for the overall reaction, which benzene derivative would you expect to react most quickly? Which would react most slowly?

(a)

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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

Justify the carbonyl stretching frequencies indicated for 3-chlorobenzaldehyde and 3-fluorobenzaldehyde.

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Textbook Question

Rank the following acetophenone derivatives on the basis of the carbonyl stretching frequency (1 = highest ; 4 = lowest ).

(a)

(b)

(c)

(d)

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Textbook Question

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.]

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Textbook Question

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

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