Textbook Question
Draw the important resonance contributors for the following cations, anions, and radicals.
(c)
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Ch. 15 - Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy
Wade9th EditionOrganic ChemistryISBN: 9780135213728
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Table of contents
- Ch.1 - Structure and Bonding107
- Ch. 2 - Acids and Bases; Functional Groups115
- Ch.3 - Structure and Stereochemistry of Alkanes100
- Ch.4 - The Study of Chemical Reactions99
- Ch.5 - Stereochemistry93
- Ch.6 - Alkyl Halides; Nucleophilic Substitution118
- Ch. 7 - Structure and Synthesis of Alkenes; Elimination158
- Ch.8 - Reactions of Alkenes171
- Ch.9 - Alkynes91
- Ch.10 - Structure and Synthesis of Alcohols143
- Ch.11 - Reactions of Alcohols104
- Ch. 12 - Infrared Spectroscopy and Mass Spectrometry22
- Ch. 13 - Nuclear Magnetic Resonance Spectroscopy45
- Ch. 14 - Ethers, Epoxides, and Thioethers72
- Ch. 15 - Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy89
- Ch. 16 - Aromatic Compounds74
- Ch. 17 - Reactions of Aromatic Compounds126
- Ch. 18 - Ketones and Aldehydes193
- Ch. 19 - Amines129
- Ch. 20 - Carboxylic Acids77
- Ch. 21 - Carboxylic Acid Derivatives141
- Ch. 22 - Condensations and Alpha Substitutions of Carbonyl Compounds175
- Ch. 23 - Carbohydrates and Nucleic Acids93
- Ch. 24 - Amino Acids, Peptides, and Proteins54
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Wade 9th EditionCh. 15 - Conjugated Systems, Orbital Symmetry, and Ultraviolet SpectroscopyProblem 28
Wade 9th EditionCh. 15 - Conjugated Systems, Orbital Symmetry, and Ultraviolet SpectroscopyProblem 28Chapter 15, Problem 28
A solution was prepared using 0.0010 g of an unknown steroid (of molecular weight around 255) in 100 mL of ethanol. Some of this solution was placed in a 1-cm cell, and the UV spectrum was measured. This solution was found to have λmax = 235 nm, with A = 0.74.
(a) Compute the value of the molar absorptivity at 235 nm.
(b) Which of the following compounds might give this spectrum?
Verified step by step guidance1
Step 1: Use the Beer-Lambert Law to calculate the molar absorptivity (ε). The Beer-Lambert Law is expressed as A = ε × c × l, where A is the absorbance, ε is the molar absorptivity, c is the concentration in mol/L, and l is the path length in cm. Rearrange the formula to solve for ε: ε = A / (c × l).
Step 2: Calculate the concentration (c) of the steroid solution. The molecular weight of the steroid is approximately 255 g/mol, and the mass of the steroid used is 0.0010 g. Use the formula c = (mass / molecular weight) / volume, where the volume is 100 mL (converted to liters: 0.1 L).
Step 3: Substitute the calculated concentration (c), absorbance (A = 0.74), and path length (l = 1 cm) into the Beer-Lambert Law equation to compute ε.
Step 4: Analyze the UV spectrum data and the λmax value (235 nm). This wavelength corresponds to the π → π* electronic transition, which is characteristic of conjugated double bonds or aromatic systems. Compare this information with the structures provided in the image.
Step 5: Examine the chemical structures in the image. Identify which compound has conjugated double bonds or aromatic systems that could absorb at 235 nm. The presence of conjugation in the steroid structure is key to matching the UV spectrum data.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Beer-Lambert Law
The Beer-Lambert Law relates the absorbance of light by a solution to the concentration of the absorbing species and the path length of the light. It is expressed as A = εlc, where A is absorbance, ε is molar absorptivity, l is the path length in cm, and c is the concentration in mol/L. This law is fundamental for calculating molar absorptivity from absorbance measurements.
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The Beer-Lambert Law Concept 2
Molar Absorptivity (ε)
Molar absorptivity, also known as molar extinction coefficient, is a measure of how strongly a chemical species absorbs light at a given wavelength. It is expressed in units of L/(mol·cm) and is specific to each compound and wavelength. A higher ε indicates a greater ability of the compound to absorb light, which is crucial for identifying compounds based on their UV spectra.
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02:02Major IR absorptions
UV-Vis Spectroscopy
UV-Vis spectroscopy is an analytical technique used to measure the absorbance of UV or visible light by a sample. It provides information about the electronic transitions in molecules, allowing for the identification of compounds based on their characteristic absorption wavelengths. The λmax indicates the wavelength at which maximum absorbance occurs, which is essential for comparing the unknown compound to known standards.
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Related Practice
Textbook Question
The diene lactone shown in part (a) has one electron-donating group (-OR) and one electron-withdrawing group (C=O). This diene lactone is sufficiently electron-rich to serve as the diene in a Diels–Alder reaction.
b. The Diels–Alder product A is not very stable. Upon mild heating, it reacts to produce CO2 gas and methyl benzoate (PhCOOCH3), a very stable product. Explain how this strongly exothermic decarboxylation takes place. (Hint: Under the right conditions, the Diels–Alder reaction can be reversible.)
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Textbook Question
Draw the important resonance contributors for the following cations, anions, and radicals.
(d)
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Textbook Question
Predict the products of the following proposed Diels–Alder reactions. Include stereochemistry where appropriate.
(a)
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Textbook Question
Draw the important resonance contributors for the following cations, anions, and radicals.
(e)
(f)
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Textbook Question
Predict the products of the following proposed Diels–Alder reactions. Include stereochemistry where appropriate.
(d)
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