Resonance forms give us a quick way to assess where a

This preview shows page 20 - 25 out of 39 pages.

Resonance forms give us a quick way to assess where a compound’s or specieselectrophilic and nucleophilic character lies. We do this with a conventional arrow-pushing (electron-pushing) notation. For the allyl cation: HHHHHHHHHHthe hybrid structure is HHHHH123++Just like in drawing mechanisms, a double-headed arrow indicates movement of 2 electrons and a single-headed arrow indicates movement of 1 electron. This symbol: indicates that two structures are resonance forms. It is not implying a chemical reaction or an equilibrium in any way! Some important points: -Highly localized charge (+ or -) is not as stabilizing as “delocalized” charge –that is, charge spread out over more than one atom -Resonance spreads out charge distribution this is almost always a highly stabilizing factor -The more a charge is spread out (i.e., the more resonance forms), the more stable is the ion or molecule -All resonance forms of a given molecule have the same charge -The relative positions of atoms in resonance forms do not move Some rules regarding the writing of resonance forms: 1.Structures with a maximum number of octets are “most important” (contribute the most to the hybrid’s structure)2.Charges are best accommodated on atoms with appropriate electronegativity characteristics 3.Less separation of charge is more important than greater separation of charge
Chemistry 140A Summer 2012 (K. Albizati) 21 5.Reactive Intermediates a.Radicals A neutral species with an unpaired electron: -sp2hybridization -Trigonal planar geometry -The carbon is not charged -Electron can be delocalized via resonance -Reactive intermediate in radical halogenations of alkanes -Order of Stability is 3o> 2o> 1o> CH3Main Modes of Reactivity for Radicals
Chemistry 140A Summer 2012 (K. Albizati) 22 b.Carbonium Ions A positively charged species (cation) with an empty carbon 2p orbital: -sp2hybridization -Trigonal planar geometry -The carbon bears a +1 charge -The charge can be delocalized via resonance -Reactive intermediate in many chemical reactions -Order of Stability is 3o> 2o> 1o> CH3Main Modes of Reactivity for Carbonium Ions
Chemistry 140A Summer 2012 (K. Albizati) 23 c.Alkyl oxonium and Dialkyl oxonium ions A positively charged species (cation) with a protonated alcohol or ether group: -sp3hybridization on oxygen -Tetrahedral geometry of oxygen -The oxygen bears a +1 charge -The charge cannot be delocalized via resonance -Reactive intermediate in chemical reactions -Requires a strong acid to protonate oxygen Main Modes of Reactivity for Alkyl oxonium and Dialkyl oxonium Ions
Chemistry 140A Summer 2012 (K. Albizati) 24 6.Classes of Compounds a.Alkanes Hydrocarbons made up of only C and H no multiple bonds CnH2n+2Prefer to “stretch out” in a zigzag form:Very unreactive Used as fuels and solvents for the most part b.Cycloalkanes Cyclic compounds made up of only C and H CnH2n Small rings (3 and 4-membered) are angle strained and torsionally strained 5- and 6-membered rings can assume conformations to minimize angle and torsional strain 5-membered envelope form 6-membered chair form

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture