It is not implying a chemical reaction or an

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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 Spring 2013 (K. Albizati) 21 5.Reactive Intermediates a.Carbenes A neutral species with two ligands and a non-bonded pair -Hybridization unclear -Non-linear geometry -The carbon is not charged -Reactive intermediate in cyclopropanation of alkenes 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> CH3
Chemistry 140A Spring 2013 (K. Albizati) 22 Main Modes of Reactivity for Carbonium Ions 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
Chemistry 140A Spring 2013 (K. Albizati) 23 -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 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:
Chemistry 140A Spring 2013 (K. Albizati) 24 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
Chemistry 140A Spring 2013 (K. Albizati) 25 c.Alkyl halides Contain a polar C-halogen bond responsible for reactivity Endows R-X with electrophilic properties (that is, R-X is an electrophile) Nucleophilic Substitution is a main reactivity mode SN2 concerted mechanism where leaving group departs as the nucleophile start to form a bond SN1 two step mechanism where a carbonium ion is produced by departure of the leaving group followed by capture of the carbonium ion by a nucleophile Elimination is a main reactivity mode E2 concerted mechanism where the hydrogen is removed by a base while the C-X bond is breaking E1

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