CHEM 140B Chapter Notes - Chapter 14: Electronegativity, Stereochemistry, Stereospecificity

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Ochem textbook notes chapter 14: delocalized pi systems. Delocalized: electrons in 3 or more parallel p orbitals that participate in pi-type overlap; these electrons are shared by 3 or more atomic centers. Have more extended delocalization of pi electrons: have new modes of reactivity (thermal, photochemical cycloaddition, ring closures) 14. 1 overlap of 3 adjacent p orbitals: electron delocalization in the 2-propenyl (allyl) system. Effect of neighboring double bonds on the reactivity of a carbon center: observation 1: the primary c-h bond in propene is relatively weak. Weaker than a tertiary c-h bond, so 2-propenyl radical has special stability: observation 2: in contrast with saturated primary haloalkanes, 3-chloropropene dissociates relatively quickly under sn1 (solvolysis) conditions and undergoes rapid unimolecular substitution through a carbocation intermediate. Contradicts expectations (since primary carbons rarely react through sn1 (cid:271)e(cid:272)ause usually the (cid:272)ar(cid:271)o(cid:272)atio(cid:374) is(cid:374)"t sta(cid:271)le e(cid:374)ough); cation derived from 3- chloropropene is somehow more stable than other primary carbocations.