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

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3:16 minutes

Problem 13a,b,c

Textbook Question

Convert the following infrared wavelengths to cm^-1. (a) 6.24 𝜇m, typical for an aromatic C=C (b) 3.38 𝜇m, typical for a saturated C-H bond (c) 5.85 𝜇m, typical for a ketone carbonyl

Verified step by step guidance

Step 1: Understand the relationship between wavelength (𝜆) and wavenumber (𝜈̅). The wavenumber is the reciprocal of the wavelength and is expressed in cm⁻¹. The formula is:

ν_{̅} = \frac{1}{λ}

, where 𝜆 is the wavelength in cm.

Step 2: Convert the given wavelengths from micrometers (𝜇m) to centimeters (cm). Recall that 1 𝜇m = 10⁻⁴ cm. Multiply each wavelength by 10⁻⁴ to convert to cm.

Step 3: Apply the formula for wavenumber to each converted wavelength. For example, for part (a), substitute the converted wavelength value into the formula

ν_{̅} = \frac{1}{λ}

Step 4: Repeat the calculation for parts (b) and (c) using their respective converted wavelengths. Ensure that each calculation uses the correct wavelength value in cm.

Step 5: Verify the units of the final wavenumber values to ensure they are expressed in cm⁻¹, as this is the standard unit for infrared spectroscopy.

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Key Concepts

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

Infrared Spectroscopy

Infrared spectroscopy is a technique used to identify molecular structures by measuring the absorption of infrared light by a sample. Different functional groups absorb infrared radiation at characteristic wavelengths, allowing chemists to deduce the presence of specific bonds or groups in a molecule.

Wavelength and Wavenumber Conversion

Wavelength is the distance between successive peaks of a wave, typically measured in micrometers (𝜇m) in infrared spectroscopy. Wavenumber, expressed in cm-1, is the reciprocal of the wavelength and is commonly used in spectroscopy because it directly relates to energy levels. The conversion from 𝜇m to cm-1 involves taking the reciprocal of the wavelength in centimeters.

Functional Group Identification

Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. In infrared spectroscopy, different functional groups absorb infrared light at specific wavelengths, allowing for their identification. For example, C=C bonds, C-H bonds, and carbonyl groups each have distinct absorption features that can be correlated with their respective infrared wavelengths.

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Related Practice

Textbook Question

There are three carbon–oxygen bonds in methyl acetate.

a. What are their relative bond lengths?

b. What are the relative infrared (IR) stretching frequencies of these bonds?

Multiple Choice

How does IR work?

Textbook Question

A C-D (carbon–deuterium) bond is electronically much like a C-H bond, and it has a similar stiffness, measured by the spring constant, k. The deuterium atom has twice the mass (m) of a hydrogen atom, however.

(a) The infrared absorption frequency is approximately proportional to $\sqrt{\frac{k}{m}}$ , when one of the bonded atoms is much heavier than the other, and m is the lighter of the two atoms (H or D in this case). Use this relationship to calculate the IR absorption frequency of a typical C-D bond. Use 3000 cm^–1 as a typical C-H absorption frequency.

(b) A chemist dissolves a sample in deuterochloroform (CDCl₃) and then decides to take the IR spectrum and simply evaporates most of the CDCl₃. What functional group will appear to be present in this IR spectrum as a result of the CDCl₃ impurity?

Textbook Question

Which of the bonds shown in red are expected to have IR-active stretching frequencies?

Textbook Question

Which of the following compounds has a vibration that is infrared inactive?

1-butyne, 2-butyne, H₂, H₂O, Cl₂, ethene

Textbook Question

Assuming that the force constant is approximately the same for C–C, C–N, and C–O bonds, predict the relative positions of their stretching vibrations in an IR spectrum.

Textbook Question

Why is the C–O absorption band of 1-hexanol at a smaller wavenumber (1060 cm^–1) than the C–O absorption band of pentanoic acid (1220 cm^–1)?

Textbook Question

Which occurs at a larger wavenumber:

a. the C–O stretch of phenol or the C–O stretch of cyclohexanol?

b. the C=O stretch of a ketone or the C=O stretch of an amide?

c. the C–N stretch of cyclohexylamine or the C–N stretch of aniline?

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