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Unformatted text preview: 6-59 continued (b) In order to undergo substitution and therefore inversion, HO- wouid have to be the leaving group, but H O- is never a leaving group in S N2. No reaction can occur. (c) Once the OH is protonated, it can leave as H20. Racemization occurs in the S N1 mechanism because of the planar, achiral carbocation intermediate which "erases" all stereochemistry of the starting material. Racemization occurs in the S N2 mechanism by establishing an equilibrium of Rand S enantiomers, as explained in 6-59(a). HO-C-H .. ::
-- \:::) H+ H O-C-H + H -C-OH 30 minor (b) trace amount The carbocation produced in this E l elimination is and will not rearrange. The product ratio follows the Zaitsev rule. racemic mixture trace-from unrearranged 2 0 carbocation from either unrearranged 2 0 carbocation or rearranged carbocation 30 The carbocation produced in this E l elimination is 20 and can either eliminate to give the first two alkenes , or can rearrange by from rearranged a hydride shift to a 30 carbocation which 30 carbocation would produce the last two products. The amounts of the last two products are not predictable as they are both trisubstituted, but the first product will certainly be the least. The carbocation produced in this E l elimination is 2 0 and can either eliminate to give the first alkene, or can rearrange by a methyl shift to a 30 carbocation which would produce the last two products. The middle product is major as it is tetrasubstituted versus disubstituted for the last structure and monosubstituted for the first structure. (c) trace-from unrearranged 20 carbocation ¥+�+Jy
major from rearranged 30 carbocation minor from rearranged 30 carbocation 124 ...
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