Methyl carbocation, geometry

One possible explanation is that adamantyl cation, an intermediate in the reaction, is particularly unstable because it cannot accomodate a planar carbocation center (see Chapter 1, Problem 9). Examine the geometry of adamantyl cation. Does it incorporate a planar carbocation center Compare electrostatic potential maps of adamantyl cation and 2-methyl-2-propyl cation. Which cation better delocalizes the positive charge Assuming that the more delocalized cation is also the more stable cation, would you expect adamantyl tosylate to react slower or faster than tcrf-butyl tosylate Calculate the energy of the reaction. 

Simple application of either the Brown or the Y-T equation to the substituent effects on p/tR+ for a,a-diphenylethylenes [31(X,Y)] (in Table 9) is not necessarily successful (Goethals et ai, 1978). The difficulties arise mainly from stereochemical factors the RHF/6-31G optimization of the 1,1-diphenylethyl carbocation [31C ] provides a propeller geometry shown in Fig. 17. The phenyl rings are twisted from the plane of the sp carbocation centre by 23.4° and 34.0° (Fujio et ai, unpublished). This difference is probably due to the steric non-equivalence of the methyl group. 

Begin by performing an AMI geometry optimization on methyl, ethyl, isopropyl, and ferf-butyl carbocations. These carbocations are built as described in Part C of Experiment 20A. Don t forget to specify that each one has a positive charge. Also select a density surface for each one with the electrostatic potential mapped onto the surface. 

The structures, geometries, and relative stabilities of simple alkyl radicals are similar to those of alkyl carbocations. Methyl radical is planar, and all other radicals are nearly so, with bond angles near 120° about the carbon with the unpaired electron. This geometry indicates that carbon is sp hybridized and that the unpaired electron occupies the unhybridized 2p orbital. As mentioned, the order of stability of allyl radicals, like alkyl carbocations, is 3° > 2° > 1° methyl. 

PROBLEM 3.52 In this chapter, we developed a picture of the methyl cation ( CH3), a planar species in which carbon is hybridized. In Chapter 2, we briefly saw compounds formed from several rings, bicychc compounds. Explain why it is possible to form carbocation (a) in the bicychc compound shown below, but very difficult to form carbocation (b). It may be helpful to use models to see the geometries of these molecules. 

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