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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 56
Chapter 13, Problem 56

Justify the carbonyl stretching frequencies indicated for benzaldehyde and 4-methoxybenzaldehyde.
Benzaldehyde and 4-methoxybenzaldehyde structures with carbonyl stretching frequencies at 1696 cm⁻¹ and 1688 cm⁻¹, respectively.

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Identify the functional groups present in benzaldehyde and 4-methoxybenzaldehyde. Benzaldehyde contains a carbonyl group (C=O) attached to a benzene ring, while 4-methoxybenzaldehyde has a methoxy group (OCH₃) at the para position relative to the carbonyl group.
Understand the concept of electronic effects in aromatic compounds. Substituents on a benzene ring can influence the electron density of the ring and adjacent functional groups through resonance and inductive effects.
Analyze the effect of the methoxy group in 4-methoxybenzaldehyde. The methoxy group is an electron-donating group through resonance, which increases the electron density on the benzene ring and can affect the carbonyl group.
Consider how increased electron density affects the carbonyl stretching frequency. In general, increased electron density on the carbonyl group can lead to a decrease in the carbonyl stretching frequency in IR spectroscopy, as the bond becomes less polar and weaker.
Compare the carbonyl stretching frequencies of benzaldehyde and 4-methoxybenzaldehyde. Due to the electron-donating effect of the methoxy group, the carbonyl stretching frequency in 4-methoxybenzaldehyde is expected to be lower than that in benzaldehyde.

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Key Concepts

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

IR Spectroscopy

Infrared (IR) spectroscopy is a technique used to identify and study chemical substances based on their absorption of infrared light. Molecules absorb specific frequencies that correspond to the vibrations of their bonds, providing a fingerprint that can be used to identify functional groups, such as carbonyls, within a compound.
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General Features of IR Spect

Carbonyl Stretching Frequency

The carbonyl stretching frequency in IR spectroscopy refers to the specific range of wavenumbers where the C=O bond absorbs infrared light, typically around 1700 cm⁻¹. This frequency can be influenced by the electronic environment around the carbonyl group, such as the presence of electron-donating or electron-withdrawing substituents.
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Common IR Frequencies

Substituent Effects on Benzene Ring

Substituents on a benzene ring can affect the electronic distribution across the ring, influencing the properties of attached functional groups. Electron-donating groups, like methoxy, can increase electron density, potentially lowering the carbonyl stretching frequency, while electron-withdrawing groups can have the opposite effect, raising the frequency.
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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

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

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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