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CHEM2213 Topic 3: Reaction Of Alkenes & Alkynes Notes.docx

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Department
Chemistry
Course
Chemistry 1027A/B
Professor
Mark Workentin
Semester
Fall

Description
Topic 3 Reaction Of Alkenes & Alkynes What Are The Characteristics Reactions Of Alkenes?  Most characteristic reaction of alkenes is addition to the C-C double bond in such a way that the pi bond is broken and in its place sigma bonds are formed to two new atoms / groups  Many alkenes form polymers by the addition of monomers to a growing polymer chain What Is A Reaction Mechanism?  Reaction Mechanism: step-by-step description of how a chemical reaction occurs o Describes which bonds break and which new ones form as well as the order and relative rates of the various bond-breaking and bond- forming steps o Reaction mechanism describes role of solvent or catalyst  Bond breaking corresponds to an increase in energy and bond forming corresponds to a decrease in energy  Energy Diagram: graph showing the changes in energy that occur during a chemical reaction  Reaction Coordinate: measure of the progress of a reaction o Indicates how far the reaction has progressed, from no reaction to a completed reaction  Heat Of Reaction: difference in energy between reactants and products ΔH  Exothermic Reaction: energy of the products is lower than the energy of the reactants, heat is liberated  Endothermic Reaction: energy of the products is higher than the energy of the reactants, heat is absorbed  Transition State: unstable species of maximum energy formed during the course of a reaction  Activation Energy: difference in energy between reactants and the transition state o Minimum energy required for a reaction to occur o Considered an energy barrier for the reaction  Reaction Intermediate: unstable species that lies in an energy minimum between two transition states  Rate-Determining Step: step in a reaction sequence that crosses the highest energy barrier (slowest step in multistep reaction)  There are many patterns that occur frequently in organic reaction mechanisms: adding a proton, taking a proton away, reaction of a nucleophile and electrophile to form a new bond, rearrangement of a bond What Are The Mechanisms Of Electrophilic Additions To Alkenes?  Nucleophile: nucleus-loving, seeks a region of low electron density  Electrophile: electron-poor species that can accept a pair of electrons to form a new covalent bond (Lewis Acid)  Addition of hydrogen halides produce haloalkanes (alkyl halides)  Regioselective Reaction: one direction of bond forming / breaking occurs in preference to all other directions  Markovnikov’s Rule: in the addition of HX or H O2to an alkene, H adds to the C of the double bond with the greater number of H’s  Electrophilic Addition – add a proton  reaction of an electrophile and a nucleophile to form a new covalent bond  Step 1 results in the formation of an organic cation o Carbocation: species containing a C atom with only 3 bonds to it and bearing a + charge o All carbocations are Lewis acids and electrophiles o Electron-deficient C of a carbocation uses its sp hybrid orbitals to form sigma bonds to 3 groups  Reaction of HX and alkene can give 2 different carbocation intermediates + depending on which of the double-bonded C atoms forms a bond with H o More stable carbocation intermediate forms faster than a less stable carbocation intermediate (3° > 2° > 1°)  The proton of H-X adds to the less substituted carbon of a double bond because this mode of addition produces the more stable carbocation intermediate  System bearing a charge is more stable if the charge is delocalized o Electron-releasing ability of alkyl groups bonded to a cationic C is accounted for by the inductive effect o Electron deficiency of the C atom bearing a + charge exerts an electron-withdrawing inductive effect that polarizes electrons from adjacent sigma bonds towards it o As the number of alkyl groups bonded to the cationic C increases, the stability of the cation increases as well  In the presence of an acid catalyst, water adds to the C-C double bond of an alkene to give an alcohol  Hydration: addition of water  Oxonium Ion: ion that contains an O atom that bears a + charge and 3 bonds to O  Acid-Catalyzed Hydration – add a proton  reaction of a nucleophile and an electrophile to form a new covalent bond  take a proton away  Cl2and Br r2act with alkenes by addition of halogen atoms to the two C atoms of the double bond, forming two new carbon-halogen bonds  Stereoselective Reaction: one stereoisomer is formed / destroyed in
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