Ch. 15 - Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy

Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook

Wade 9th Edition

Ch. 15 - Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy

Problem 22

Chapter 15, Problem 22

Match four of the following UV absorption maxima (λ_max) with the corresponding compounds: (1) 232 nm; (2) 256 nm; (3) 273 nm; (4) 292 nm; (5) 313 nm; (6) 353 nm.

Verified step by step guidance

Step 1: Analyze the chemical structures provided in the image. Each structure contains conjugated double bonds, which are responsible for UV absorption. The extent of conjugation influences the λmax (absorption maxima). Greater conjugation typically results in a higher λmax.

Step 2: Structure (i) contains three conjugated double bonds in a fused ring system. This level of conjugation suggests a moderate λmax value, likely in the range of 256 nm or 273 nm.

Step 3: Structure (ii) has an extended conjugation system compared to (i), with four conjugated double bonds. This increased conjugation suggests a higher λmax value, likely in the range of 313 nm or 353 nm.

Step 4: Structure (iii) has two conjugated double bonds in a cyclic system. This lower level of conjugation suggests a lower λmax value, likely in the range of 232 nm or 256 nm.

Step 5: Structure (iv) contains three conjugated double bonds in a fused ring system, similar to (i). However, the arrangement of the conjugation may slightly alter the λmax value, likely in the range of 273 nm or 292 nm.

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Key Concepts

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

UV-Vis Spectroscopy

UV-Vis spectroscopy is a technique used to measure the absorption of ultraviolet or visible light by a substance. The wavelengths at which a compound absorbs light (λmax) provide insights into its electronic structure, particularly the presence of conjugated systems. This method is essential for identifying and characterizing organic compounds based on their unique absorption patterns.

Conjugation and λmax

Conjugation refers to the overlap of p-orbitals across adjacent double bonds or lone pairs, allowing for delocalization of electrons. This delocalization lowers the energy gap between the ground and excited states of electrons, resulting in longer wavelengths of light being absorbed (higher λmax). Compounds with extensive conjugation typically exhibit higher λmax values due to this effect.

Structure-Activity Relationship (SAR)

Structure-Activity Relationship (SAR) is a principle in organic chemistry that correlates the chemical structure of a compound with its biological activity or physical properties, such as UV absorption. By analyzing how different structural features influence λmax, chemists can predict the behavior of similar compounds and design new molecules with desired properties.

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Show that the [4 + 4] cycloaddition of two butadiene molecules to give cycloocta-1,5-diene is thermally forbidden but photochemically allowed.

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Textbook Question

There is a different, thermally allowed cycloaddition of two butadiene molecules. Show this reaction, and explain why it is thermally allowed. (Hint: Consider the dimerization of cyclopentadiene.)

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Phenolphthalein is an acid–base indicator that is colorless below pH 8 and red above pH 8. Explain briefly why the first structure is colorless and the second structure is colored.

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Textbook Question

One milligram of a compound of molecular weight 160 is dissolved in 10 mL of ethanol, and the solution is poured into a 1-cm UV cell. The UV spectrum is taken, and there is an absorption at λ_max = 247 nm. The maximum absorbance at 247 nm is 0.50. Calculate the value of e for this absorption.

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Textbook Question

Classify the following dienes and polyenes as isolated, conjugated, cumulated, or some combination of these classifications.

(a) cycloocta-1,4-diene

(b) cycloocta-1,3-diene

(d) cycloocta-1,3,5,7-tetraene

(e) cyclohexa-1,3,5-triene (benzene)

(f) penta-1,2,4-triene

Textbook Question

Predict the products of the following reactions.

(a) allyl bromide + cyclohexyl magnesium bromide

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