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14.E: Conjugated Compounds and Ultraviolet Spectroscopy (Exercises)

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    61719
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    These are the homework exercises to accompany the Textmap for McMurry's Organic Chemistry textbook.

    14.1 Stability of Conjugated Dienes: Molecular Orbital Theory

    Exercises 14.1

    Questions

    Q14.1.1

    The heat of hydrogenation for allene is about 300 kJ/mol. Order a conjugated diene, a non-conjugated diene, and allene in increasing stability.

    Solutions

    S14.1.1

    allene < non-conjugated diene < conjugated diene (most stable)

     

    14.2 Electrophilic Additions to Conjugated Dienes: Allylic Carbocations

    Exercises 14.2

    Questions

    Q14.2.1

    Give the 1,2 and the 1,4 products of the addition of one equivalent of HBr to 1,3-hexa-diene.

    Q14.2.2

    Look at the previous addition reaction of HBr with a diene. Consider the transition states, predict which of them would be the major products and which will be the minor.

    Solutions

    S14.2.1

    S14.2.2

    The products i-iii all show a secondary cation intermediate which is more stable than primary. Therefore those would be major products and the iv product would be the minor product.

    14.3 Kinetic vs. Thermodynamic Control of Reactions

    Exercises 14.3

    Questions

    Q14.3.1

    Consider the reaction with 1,3-buta-diene reacting with HCl. Propose a mechanism for the reaction.

    Q14.3.2

    Predict why the 1,4 adduct is the major product in this reaction compared to the 1,2.

    Solutions

    S14.3.1

    14.3.png

    S14.3.2

    Even though the cation would prefer to be in a secondary position in the transition state, the final product is less stable with a terminal alkene. Therefore the major product will be the 1,4 adduct.

    14.5 Characteristics of the Diels-Alder Reaction

    Exercises 14.5

    Questions

    Q14.5.1

    Of the following dienes, which are S-trans and which are s-cis? Of those that are s-trans, are they able to rotate to become s-cis?

    Q14.5.2

    Predict the product of the following reaction.

    Solutions

    S14.5.1

    A) s-trans, unable to rotate to become s-cis

    B) s-cis

    C) s-trans, can rotate to become s-cis.

    S14.5.2

     

    14.6 Diene Polymers: Natural and Synthetic Rubbers

    Exercises 14.6

    Questions

    Q14.6.1

    Draw a segment for the polymer that may be made from 2-tert-butyl-1,3-butadiene.

    Q14.6.2

    Propose the mechanism for the acid catalyzed polymerization of 2-methyl-1,3-butadiene.

    Solutions

    S14.6.1

    S14.6.2

    The initial step is an addition of a hydrogen from the acid, followed by the polymerization.

    14.7 Structure Determination in Conjugated Systems: Ultraviolet Spectroscopy

    Exercises 14.7

    Questions

    Q14.7.1

    What is the energy range for 300 nm to 500 nm in the ultraviolet spectrum? How does this compare to energy values from NMR and IR spectroscopy?

    Solutions

    S14.7.1

    E = hc/λ

    E = (6.62 × 10−34 Js)(3.00 × 108 m/s)/(3.00 × 10−7 m)

    E = 6.62 × 10−19 J

    The range of 3.972 × 10-19 to 6.62 × 10-19 joules. This energy range is greater in energy than the in NMR and IR.

    14.8 Interpreting Ultraviolet Spectra: The Effect of Conjugation

    Exercises 14.8

    Questions

    Q14.8.1

    Which of the following would show UV absorptions in the 200-300 nm range?

    Solutions

    S14.8.1

    B and D would be in that range.