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Chapter 3 - Alkenes

Chapter 3 - Alkenes. Naming Alkenes. Change the suffix from ane to ene. Naming Alkenes. The longest chain must include the double bond Chain is numbered to give the double bond carbons the lowest numbers Numbers indicating location of double bond are placed before chain length .

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Chapter 3 - Alkenes

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  1. Chapter 3 - Alkenes

  2. Naming Alkenes Change the suffix from ane to ene

  3. Naming Alkenes The longest chain must include the double bond Chain is numbered to give the double bond carbons the lowest numbers Numbers indicating location of double bond are placed before chain length

  4. Naming Alkenes Double bond gets precedence over substituents

  5. Naming Alkenes For more than one double bond use diene, triene etc

  6. Naming Alkenes Double bond in a cyclic always get numbers 1 and 2

  7. Naming Subtleties

  8. Terms • Vinylic carbons – sp2 carbons • Allylic carbons – ones next to the vinylic carbons

  9. Alkenes Double bond locks the molecule and there is no rotation about the double bond Energy for single bond 2.9 kcal/mol for double 62 kcal/mol

  10. Lowest Energy The Second Bond Must Break (Promote Electrons to the Antibonding Orbital). Then it Can Rotate

  11. Different Molecules The connections are the same, but that’s about all!!!

  12. Cis/Trans – Just Not Good Enough

  13. Configuration - E,Z

  14. Increasing priority E/Z Rules Assign priority based on the higher atomic number

  15. E/Z Rules If priority cannot be assigned on the basis of the atoms bonded to the CARBON, look to the next set of atoms. Priority is assigned at the first point of difference.

  16. E/Z Rules When a double or tripple bond shows up, count it as though there are two or three of the same atom

  17. E/Z nomenclature

  18. Thermodynamics and KineticsHow Far and How Fast • Thermodynamics • Description of a reaction at equilibrium(Keq) • Kinetics • Description of rates of a chemical reaction rate = k [A] [B]2 / [C]0

  19. For The Following Energy Discussion, Consider this Reaction

  20. Energy considerations • Mechanisms describe bond breaking-bond making

  21. Energy considerations • Exergonic vs. Endergonic Reactions ( Note: The more stable the species, the lower its energy. ) Ok, OK, OK, - What is exergonic and endergonic? Isn’t that supposed To be exothermic and endothermic???? Next slide!!!!!!!!

  22. Energy Terms • Endergonic and Exergonic describe the free energy used or expelled from a reaction. • Endothermic and exothermic describe the heat ΔH used or expelled from a reaction DGo = DHo - TDSo G and H are often close to the same unless you have a large temperature value For almost all normal processes the +/- value of G and H will be the same

  23. Energy considerations • DGo , Gibbs free energy change… • Predicts whether a reaction will happen “spontaneously” • Exergonic reactions = - DGo (spontaneous) • Endergonic reactions = + DGo

  24. DHo = energy bonds broken - energy bonds formed

  25. Calculate ΔH Notice the table doesn’t list the breaking of the pie bond without the breaking of the sigma bond. This value is 62 kcal/mol. DHo = energy bonds broken - energy bonds formed

  26. Where are Calculations Valid • Just in the Gas Phase • The solvent makes a difference! • Solvents can have a large effect on DH • Water can solvate cations +++ or anions - - -

  27. Rate of Reaction • Rate depends on: • The number of collisions/time • Fraction of collisions with sufficient energy • Fraction of collisions with proper orientation

  28. Kinetics • Energy of Activation, Ea (or DG**) • Fast vs. Slow reactions:

  29. Organic Chemistry Lingo • Kinetic Product: formed most rapidly • Thermodynamic Product: most stable product Many times the Kinetic Product is the Thermodynamic Product!

  30. Thermodynamic vs. Kinetic Control • If the kinetic product and thermodynamic product differ, the major product will depend on reaction conditions. • If carried out at mild (low-temp) conditions, the reaction will be irreversible and the kinetic product will be favored.

  31. Thermodynamic vs. Kinetic Control • If carried out at sufficiently vigorous (hi-temp) conditions, the reaction will be reversible and the thermodynamic product will be favored.

  32. Thermodynamic vs. Kinetic Control • Kinetic product and thermodynamic product: Formed fastest Most stable

  33. Transition state Transition state Reaction Coordinate Diagrams • Intermediate = product of one step, becomes reactant of next step • Transition state = intermediate that cannot be isolated (partially formed bonds, highest energy structures!) • Rate determining step = step with highest energy

  34. Reaction Coordinate Diagrams • 1) How many intermediates are there? • 2) Is the first formed intermediate more apt to revert to reactants or go on to form products? • 3) Which step in the reaction is the rate determining? • 4) Is the reaction endergonic or exergonic? What is the sign of Go? • 5) Which step has the greatest free energy of activation? • 6) Where are the transition states?

  35. Reaction Order • First Order Reaction – reaction is proportional to the concentration of one reactant • Second Order Reaction – reaction that is proportional to the concentration of two reactants

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