16: Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy
- Page ID
- 424320
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)After reading this chapter and completing ALL the exercises, a student can be able to
- construct & interpret MO diagrams of ethene, butadiene and allylic systems (refer to section 16.1)
- recognize reactions that are enhanced by resonance stabilization of the allylic intermediate (refer to section 16.2)
- predict the products and specify the reagents for electrophilic addition reactions (EAR) of conjugated dienes (refer to section 16.3)
- specify reaction conditions to promote thermodynamic or kinetic control of the reaction mechanism; correlate these conditions to reaction energy diagrams (section 16.4)
- predict the products and specify the reagents for bimolecular substitution reactions (SN2) of allylic halides (refer to section 16.5)
- predict the products of Diels-Alder reactions with stereochemistry, including the orientation of cycloaddition with asymmetrical reagents (refer to sections 16.6 and 16.7)
- develop mechanisms to explain the observed products of 1,2- & 1,4- addition reactions, including the resonance forms of the stabilized intermediates (refer to section 16.6)
- use MO theory to predict whether cycloaddition reactions will be thermally or photochemically allowed (refer to section 16.6 and 16.7)
- recognize the effect of conjugation on UV absorption (refer to section 16.9 and 16.10)
- use Beer’s Law in UV absorption calculations (refer to section 16.9 and 16.10)
- explain how light, the conjugation of double bonds, and the stereochemistry of double bonds contribute to visualizing color
- 16.1: Stability of Conjugated Dienes - Molecular Orbital Theory
- The stability of conjugated dienes can be explained using both resonance and Molecular Orbital (MO) Theory. The MO Theory is explored in greater detail in this sections. A brief review of MO Theory can be found in Chapter 2 Section 2 of this LlibreText.
- 16.2: Allylic Cations
- Allylic carbocations and radicals are important reactive intermediates. Resonance and MO Theory can be used to explain the stability of allyic carbocations.
- 16.3: Electrophilic Additions to Conjugated Dienes
- Conjugated dienes can undergo 1,2-Electrophilic Addition and 1,4-Electrophilic Addition reactions because of the conjugated pi-bond system.
- 16.4: Kinetic versus Thermodynamic Control
- Low reaction temperatures favor kinetically controlled reactions. High temperatures favor thermodynamically controlled reactions. Some reactions are neither kinetically nor thermodynamically controlled.
- 16.5: SN2 Reactions of Allylic Halides and Tosylates
- Allylic halides and tosylates are excellent electrophiles for bimolecular nucleophilic substitution reactions (SN2).
- 16.6: The Diels-Alder (4 + 2) Cycloaddition Reaction
- The Diels-Alder reaction is a (4 + 2) cycloaddition reaction between a diene and a dieophile that a forms 6-membered ring and requires heat.
- 16.7: Diels-Alder Stereochemistry
- The stereochemistry of the Diels-Alder reaction is studied in more detail.
- 16.8: Diene Polymers - Natural and Synthetic Rubbers
- Conjugated dienes can be polymerized to form important compounds like rubber.
- 16.9: Structure Determination in Conjugated Systems - Ultraviolet Spectroscopy
- The ultraviolet (UV) region of the electromagnetic spectrum corresponds to conjugated bond energies. Molecular Orbital Theory describes this bond energy using electron transitions from the HOMO and LUMO. The most useful UV region of the electromagnetic spectrum has a wavelength between 200 and 400 nm.
- 16.10: Interpreting Ultraviolet Spectra - The Effect of Conjugation
- The ultraviolet absorption maximum of a conjugated molecule is dependent upon the extent of conjugation.
- 16.11: Conjugation, Color, and the Chemistry of Vision
- Eyes receive light energy then transfer and passing the energy into neural impulses to brain.
- 16.12: Additional Exercises
- This section has additional exercises for the key learning objectives of the chapter.
- 16.13: Solutions to Additional Exercises
- This section has the solutions to the additional exercises from the previous section.