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Reactions of Alkenes . Addition of HOX; X: Cl, Br. CCl 4. Ozonolysis. Ozonolysis. Reaction Mechanism. A reaction mechanism describes how a reaction occurs which bonds are broken and which new ones are formed the order in which bond-breaking and bond-forming steps take place

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slide2

Addition of HOX; X: Cl, Br

CCl4

Ozonolysis

Ozonolysis

reaction mechanism
Reaction Mechanism
  • A reaction mechanism describes how a reaction occurs
    • which bonds are broken and which new ones are formed
    • the order in which bond-breaking and bond-forming steps take place
    • the role of the catalyst (if any is present)
    • the energy of the entire system during the reaction
energy diagram
Energy Diagram
  • Energy diagram: a graph showing the changes in energy that occur during a chemical reaction
  • Reaction coordinate:a measure of the change in position of atoms during a reaction
  • Heat of reaction, H:the difference in energy between reactants and products
    • exothermic:products are lower in energy than reactants; heat is released
    • endothermic:products are higher in energy than reactants; heat is absorbed
energy diagram1
Energy Diagram
  • Transition state:an energy maximum
    • represents an unstable species formed during the course of a reaction
  • Activation Energy, Ea:the difference in energy between reactants and the transition state
    • determines the rate of reaction
    • if Ea is large, only a few molecular collisions occur with sufficient energy to reach the transition state, and the reaction is slow
    • if Ea is small, many collisions generate sufficient energy to reach the transition state, and the reaction is fast
activation energy
Activation Energy
  • Activation energy
    • Is the difference in energy between the reactants and the transition state.
    • Is the minimum energy required to a reaction to occur or happen
    • This will determine the rate of the reaction
      • How fast the reaction occurs
energy diagram2
Energy Diagram
  • an energy diagram for a two-step reaction involving formation of an intermediate
slide8

Nucleophile(nucleus loving): An electron rich species that seeks a region of low electron density.

  • Electrophile (electron loving): A low electron-density species that seeks a region of high electron density.
addition of hx x halogens hydrohalogenation
Addition of HX (X=halogens)Hydrohalogenation
  • Carried out with pure reagents or in a polar solvent such as acetic acid
  • Addition is regioselective
    • regioselective reaction:a reaction in which one direction of bond-forming or bond-breaking occurs in preference to all other directions
    • Markovnikov’s rule: in additions of HX, H adds to the carbon with the greater number of hydrogens
hcl 2 butene
HCl + 2-Butene
  • A two-step mechanism
    • Step 1: formation of the sec-butyl cation, a 2° carbocation intermediate
    • Step 2: reaction of the sec-butyl cation (an electrophile) with chloride ion (a nucleophile) completes the reaction
carbocations
Carbocations
  • Carbocation:a species in which a carbon atom has only six electrons in its valence shell and bears positive charge
  • Carbocations are:
    • classified as 1°, 2°, or 3° depending on the number of carbons bonded to the carbon bearing the positive charge
    • electrophiles; that is, they are electron-loving
    • Lewis acids; that is, they are electron-pair acceptors
carbocations1
Carbocations
  • a 3° carbocation is more stable than a 2° carbocation, and requires a lower activation energy for its formation
  • a 2° carbocation is, in turn, more stable than a 1° carbocation, and requires a lower activation energy for its formation
  • methyl and 1° carbocations are so unstable that they are never observed in solution
addition of h 2 o
Addition of H2O
  • Addition of H2O is called hydration
    • acid-catalyzed hydration of an alkene is regioselective; hydrogen adds preferentially to the moresubstituted carbon of the double bond
slide14
Step 1: proton transfer to the alkene gives a carbocation

Step 2: a Lewis acid/base reaction gives an oxonium ion

Step 3: proton transfer to solvent gives the alcohol

carbon rearrangements
Carbon Rearrangements
  • Rearrangement: change in connectivity of atoms in a product compared to starting material
  • 1,2-shift
  • Hydride ion is migrating group
  • More stable carbocation is formed
addition of cl 2 and br 2
Addition of Cl2 and Br2
  • Carried out with either the pure reagents or in an inert solvent such as CH2Cl2
addition of cl 2 and br 21
Addition of Cl2 and Br2
  • addition is stereoselective
  • stereoselective reaction: a reaction in which one stereoisomer is formed or destroyed in preference to all others that might be formed or destroyed
  • addition to a cycloalkene, for example, gives only a trans product
addition of cl 2 and br 22
Addition of Cl2 and Br2
  • Step 1: formation of a bromonium ion intermediate
  • Step 2: halide ion opens the three-membered ring
addition of cl 2 and br 23
Addition of Cl2 and Br2
  • Anti coplanar addition to a cyclohexene corresponds to trans-diaxial addition
ozonolysis
Ozonolysis
  • Cleavage of a Carbon-Carbon double bond
  • Two Carbonyl groups in place
reduction of alkenes
Reduction of Alkenes
  • Alkenes react with H2 in the presence of a transition metal catalyst to give alkanes
    • the most commonly used catalysts are Pd, Pt, and Ni
    • the reaction is called catalytic reduction or catalytic hydrogenation
reduction of alkenes1
Reduction of Alkenes
  • the most common pattern is syn addition of hydrogens; both hydrogens add to the same side of the double bond