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l20 - CH 203 O R G A N I C C H E M I S T R Y I...

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Rearrangements and elimination ! Bruno I. Rubio 1 CH 203 O R G A N I C C H E M I S T R Y I Rearrangements and elimination The product expected from a nucleophilic substitution reaction is sometimes not achieved because of the intervention of (1) carbocation rearrangements and (2) eliminations. In the laboratory, carbocation rearrangements and eliminations can either be unwanted nuisances or valuable reactions that lead to useful and interesting products. We discussed carbocation rearrangements in an earlier lecture. For now, we will focus on elimination reactions. Elimination reactions In previous lectures we presented the characteristics of the substrate and of the nucleophile that maximize the likelihood of the SN1 and SN2 reactions. To summarize: • SN1 substrate: (best) resonance stabilized, 3º > 2º > 1º > 0º (worst) • SN2 substrate: (best) 0º > 1º > 2º > 3º (worst) • SN1 nucleophile: weakly basic and neutral (e.g., H 2 O, ROH, X ) • SN2 nucleophile: anionic (e.g., HO , RO , CN ) An elimination reaction takes place when the nucleophile acts like a base in- stead of like a nucleophile, that is, when the nucleophile removes a proton from the substrate instead of attacking the carbon that bears the leaving group. An elimination reaction produces an alkene as the final product. The scheme below shows that a new double bond forms between the carbon that was origi- nally bonded to the leaving group (called the " carbon) and a neighboring sp 3 hybridized carbon (called the # carbon): C LG C + LG H ! " C C ! " Base + + H Base The base removes a proton (called the # proton) bonded to the # carbon and, as usual, the leaving group leaves with a pair of electrons.
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Rearrangements and elimination ! Bruno I. Rubio 2 Elimination mechanisms: E2 There are several elimination mechanisms. We will discuss only the E2 mecha- nism and the E1 mechanism. The E2 mechanism competes with SN2 and the E1 mechanism competes with SN1. The E2 mechanism, like the SN2 mechanism, is a one-step process: the leaving group leaves, the new double bond forms, and the base removes the # proton all at once: C LG C + LG H ! " C C !
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