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14 Delocalized Pi Systems: Investigation by Ultraviolet and Visible Spectroscopy This chapter covers an assortment of topics derived from a single concept: conjugation. Conjugation refers to overlap of three or more p orbitals on adjacent atoms in a molecule. The allyl systems are the simplest (one bond plus a third p orbital), and conjugated dienes (two adjacent bonds = 4 p orbitals) are next in line. As you will see, conjugation affects the properties of the involved orbital systems, giving rise to modified elec- tronic characteristics, stability, chemical reactivity, and spectroscopy. Introductory aspects of all of these are presented here. Outline of the Chapter 14-1 The Allyl System An introduction to the system created by overlap of three p orbitals. 14-2, 14-3, 14-4 Chemistry of the Allyl System Consequences of conjugation on reaction types you've already seen. 14-5, 14-6 Conjugated Dienes The system made up of four p orbitals. 14-7 Extended Conjugation and Benzene 14-8, 14-9 Special Reactions of Conjugated π Systems A new set of mechanisms for ring-forming reactions. 14-10 Polymerization of Conjugated Dienes 14-11 Electronic Spectra: Ultraviolet and Visible Spectroscopy Keys to the Chapter 14-1. The Allyl System Delocalization generally results in stabilization. The experimental results cited in Section 14-1 illustrate the relative ease of generating allylic radicals, cations, and anions, compared with ordinary 1° radicals, cations, or anions. The origins of allylic stabilization are presented in two different but equivalent ways: using resonance and using molecular orbitals. Both viewpoints offer useful insights into the allyl system. You should pay spe- cial attention to the electrostatic consequences of conjugation as implied by these resonance and molecular- orbital pictures. Electrons can move freely through conjugated TT either toward an electron-deficient atom or away from an electron-rich one. This delocalization obviously is electrostatically desirable and, again, results in overall stabilization. 280