Table of contents

1. A Review of General Chemistry5h 5m
2. Molecular Representations1h 14m
3. Acids and Bases2h 46m
4. Alkanes and Cycloalkanes4h 17m
5. Chirality3h 39m
6. Thermodynamics and Kinetics1h 22m
7. Substitution Reactions1h 48m
8. Elimination Reactions2h 30m
9. Alkenes and Alkynes2h 9m
10. Addition Reactions3h 19m
11. Radical Reactions1h 58m
12. Alcohols, Ethers, Epoxides and Thiols2h 42m
13. Alcohols and Carbonyl Compounds2h 17m
14. Synthetic Techniques1h 26m
15. Analytical Techniques:IR, NMR, Mass Spect7h 3m
16. Conjugated Systems6h 13m
17. Ultraviolet Spectroscopy51m
18. Aromaticity2h 34m
19. Reactions of Aromatics: EAS and Beyond5h 1m
20. Phenols55m
21. Aldehydes and Ketones: Nucleophilic Addition4h 56m
22. Carboxylic Acid Derivatives: NAS2h 51m
23. The Chemistry of Thioesters, Phophate Ester and Phosphate Anhydrides1h 10m
24. Enolate Chemistry: Reactions at the Alpha-Carbon1h 53m
25. Condensation Chemistry2h 9m
26. Amines1h 43m
27. Heterocycles2h 0m
28. Carbohydrates5h 53m
29. Amino Acids4h 20m
30. Peptides and Proteins2h 42m
31. Catalysis in Organic Reactions1h 30m
32. Lipids 2h 50m
33. The Organic Chemistry of Metabolic Pathways2h 52m
34. Nucleic Acids1h 32m
35. Transition Metals6h 14m
36. Synthetic Polymers1h 49m

15. Analytical Techniques:IR, NMR, Mass Spect

Infrared Spectroscopy

Organic Chemistry

15. Analytical Techniques:IR, NMR, Mass Spect

Infrared Spectroscopy

Previous problemNext problem

Problem 56

Textbook Question

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

Verified step by step guidance

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.

Verified video answer for a similar problem:

This video solution was recommended by our tutors as helpful for the problem above

Play a video:

Was this helpful?

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.

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.

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.

Watch next

Master General Features of IR Spect with a bite sized video explanation from Johnny

Start learning

Related Videos

Related Practice

Textbook Question

For each pair, choose the molecule that you expect to have the highest wavenumber for its C=O stretch.

(d)

Textbook Question

Based on Hooke's law, choose the bond in each pair that you expect to vibrate at a higher wavenumber.

(e) C=N vs C≡N

Textbook Question

Based on Hooke's law, choose the bond in each pair that you expect to vibrate at a higher wavenumber.

(f) C―S vs C=O

Textbook Question

Choose the bond in each pair that you expect to have the more intense stretching band

(b) C=O vs. C=N

Textbook Question

Choose the bond in each pair that you expect to vibrate at the higher wavenumber.

(a) C―N vs. C = N

Textbook Question

Choose the bond in each pair that you expect to vibrate at the higher wavenumber.

(b) C―H vs. C―O

Textbook Question

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

(a)

(b)

(c)

(d)

Textbook Question

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

Show more practices

Download the Mobile app

Privacy

Do not sell my personal information

Accessibility

Patent notice

Sitemap