Portmess Homework Q&S 121

Portmess Homework Q&S 121 - 115 STRUCTURE AND...

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The alkyl groups move closer together (van der Waals strain increases) as the transition state for for- mation of cis -4-nonene is approached. No comparable increase in strain is involved in going to the transition state for formation of the trans isomer. 5.41 Begin by writing chemical equations for the processes speci f ed in the problem. First consider re- arrangement by way of a hydride shift: Rearrangement by way of a methyl group shift is as follows: A hydride shift gives a tertiary carbocation; a methyl migration gives a secondary carbocation. It is reasonable to expect that rearrangement will occur so as to produce the more stable of these two car- bocations because the transition state has carbocation character at the carbon that bears the migrat- ing group. We predict that rearrangement proceeds by a hydride shift rather than a methyl shift, since the group that remains behind in this process stabilizes the carbocation better. 5.42
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