Unformatted text preview: 6-2 6(c) continued nucleophilic attack after carbocation rearrangement H H H y)
H rearranged product Comments on 6-26(c) (1) The hydride shift to a 20 carbocation generates an allylic, resonance-stabilized 20 carbocation. (2) The double-bonded oxygen o f acetic acid is more nucleophilic because of the resonance forms it can have after attack. ( See Solved Problem 1-5 and Problem 1- 16 in the text.) (3) Attack on only one carbon of the allylic carbocation is shown. In reality, both positive carbons would be attacked in equal amounts, but they would give the identical product in this case. In other compounds, however, attack on the different carbons might give different products. ALWAYS CONSIDER ALL P OS SIBILITIES. H HH H H Cj H �-C-CH3H C D D
.. ---....� hydride shift H H c;? H O : C: H
- allylic-resonance-stabilized 3 H : 0: II :0 : I ..f-----.. . -- O.. II - (removes as on p. 107) H+ CH 3 H H �==C-CH3
� :I .. O- H I plus two other resonance forms as shown on p. 107 (d) ct
H 0 CH2-I
---l"� The 1 0 carbocation initially formed is very unstable; some chemists believe that rearrangement occurs at the same time as the leaving group leaves. At most, the 10 carbocation has a very short lifetime. r+CH2 N CH :�� CH2CH,;
hydride shift followed by nucleophilic attack V ---l"� V '----'
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