1559T.eh14259-27511/3/059:27Page266 266 chapter 14 DELOCALIZED Pi SYSTEMS:INVESTIGATION BY ULTRAVIOLET AND VISIBLE SPECTROSCOPY 42.We figured we'd ask you this question nowso you coud take your time and figure out the right Have a lookat Figure 14-in the text.At high temperature.an equilibrium mixture exists because there is for any ocationon the reaction to any other ocation interchangin rapidlyand at any given time the relative of each are govemed by their relative thermodynamic stabilities hat being the 43.CH,-CH-CH-CH-CH,CH,-CH-CH-CH-CH, (1)Conjugated diene CH,=CH-CH:-CH-CH2 CH,=CH-CH:-CH-CH, (3)Isolated diene (4)Ordinary secondary cation (1)is more stable than (3).and (2)is more stable than (4). Reaction coordinate Reaction(1)+H(2)is faster and leads to the more stable cation.Note:When the text says that allylic their stability and their ease of formation,as you might expect. 44. )m -dw-8.642. We figured we’d ask you this question now, so you could take your time and figure out the right answer instead of maybe getting it wrong on an exam. Have a look at Figure 14-8 in the text. At high temperature, an equilibrium mixture exists because there is enough energy for molecules to “move” from any location on the reaction coordinate to any other location on it. In other words, all three species—the two products and the intermediate allylic cation—are interchanging rapidly, and at any given time the relative quantities of each are governed by their relative thermodynamic stabilities. That being the case, if the temperature were to drop, the interconversion processes would slow down because fewer molecules would contain sufficient energy to pass over the activation barriers. This would mainly affect conversion of the two product molecules into the intermediate carbocation because those processes possess the highest activation barriers. The result is that the thermodynamic ratio of products originally established at high temperature would remain pretty much unchanged (frozen) upon cooling of the reaction mixture. It will not revert to the kinetic ratio! 43. (1) is more stable than (3), and (2) is more stable than (4). Reaction (1)  H n (2) is faster and leads to the more stable cation. Note: When the text says that allylic and secondary cations are similar in energy, it is referring to the ease of formation of the simplest allylic cation,  CH2OCHPCH2, which is primary at each end. Additional alkyl groups on allylic cations increase their stability and their ease of formation, as you might expect. 44. Expect 1,2- and 1,4-addition to occur in each case. Note that the 1,2-additions in (b) and (c) might be expected to show anti stereochemistry, similar to additions to ordinary alkenes. (a) I 1, 2 and 1, 4 products are the same! I H H I CH2 CH CH2 CH2 (3) Isolated diene (4) Ordinary secondary cation CH CH2 CH CH2 CH3  CH H CH2 CH CH CH3 (1) Conjugated diene (2) Allylic cation, secondary at each end CH CH3 CH CH3  CH CH H 266 • Chapter 14 DELOCALIZED Pi SYSTEMS: INVESTIGATION BY ULTRAVIOLET AND VISIBLE SPECTROSCOPY 1559T_ch14_259-275 11/3/05 9:27 Page 266