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Chapter 3 Alkenes: Structures, Nomenclature and an Introduction to Reactivity Thermodynamics

Chapter 3 Alkenes: Structures, Nomenclature and an Introduction to Reactivity Thermodynamics and Kinetics. Adapted from Profs. Turro & Breslow, Columbia University and Prof. Irene Lee, Case Western Reserve University. Ever put an apple into a bag with green tomatoes or a green banana?.

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Chapter 3 Alkenes: Structures, Nomenclature and an Introduction to Reactivity Thermodynamics

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  1. Chapter 3 Alkenes: Structures, Nomenclature and an Introduction to Reactivity Thermodynamics and Kinetics Adapted from Profs. Turro & Breslow, Columbia University and Prof.Irene Lee, Case Western Reserve University

  2. Ever put an apple into a bag with green tomatoes or a green banana? Natural Products: CH2=CH2 What might account for the difference between lemon and orange in the limonene structure?

  3. Molecular Formulas of Alkenes Saturated vs. Unsaturated: Missing Hydrogens Alkanes are completely “saturated” i.e. only single bonds Each double bond has 1 degree of unsaturation. Each ring has 1 degree of unsaturation. Each triple bond is 2 degrees of unsaturation. Compare a molecular formula to an alkane’s: every TWO Hydrogens less = I degree of unsaturation

  4. Molecular Formulas of Alkenes Saturated vs. Unsaturated: Missing Hydrogens Noncyclic alkene: CnH2n 1 degree of unsaturation Cyclic alkene: CnH2n-2 (Same as an alkyne; 2 degrees of unsaturation)

  5. Systematic Nomenclature of Alkenes • Follows alkane rules; treats double bond as a function: • Think of alcohols

  6. Substituents in alphabetical order with lowest numbers

  7. Cyclic alkenes:

  8. Important Special Terms Vinyl Hs: bonded to the double bond. Allylic Hs: on sp3 carbons next to the double bond.

  9. Structure of Alkenes

  10. Isomers of Alkene

  11. Dipole Moments of Alkene Isomers

  12. Cis-Trans Interconversion in Vision

  13. E and Z isomers Priority is in order of atomic number. Rank H vs. Br and C vs. Cl.

  14. Naming using E, Z E (entgegen:opposite): Z (zusammen; same) • Consider the atomic number of the • atoms bonded directly to a specific sp2 • carbon 1,1 on same side = Z 1,1 on opposite sides = E 1 1 1 2 2 1

  15. If the atoms are the same, eg. the carbon atoms: then consider the other atoms that are attached to them. 1 1 1 1 O vs C

  16. C | C- C C-C-C | C • Multiple bonds are treated as attachments • of multiple single bonds

  17. C | C- C C- C Rank the priorities by mass number in isotopes C |

  18. Examples of nucleophiles An alkene is an electron rich molecule, a nucleophile. “nucleophile”- likes nuclei (likes protons: H+) Nucleophiles: electron-rich atoms or molecules that react with electrophiles. “electrophile”- likes electrons (likes minus: e- and anions)

  19. Examples of Electrophiles Nucleophiles are attracted to electron-deficient atoms or molecules (electrophiles)

  20. Electrophilic Addition of HBr to Alkene A two step reaction. Mechanistic path of a reaction: how reactants form products. How can a mechanism be illustrated? i.e. bond making & bond breaking

  21. Movement of a pair of electrons: START arrows from electrons pointing to electrophile Use 1/2 arrow for the movement of one electron Using Curved Arrows in Reaction Mechanisms

  22. Using Curved Arrows

  23. A Reaction Coordinate Diagram Transition states have partially formed bonds Intermediates have fully formed bonds

  24. Gibbs standard free energy change (DGo) Enthalphy (DHo): the heat given off or absorbed during a reaction Entropy (DSo): a measure of freedom of motion Thermodynamic Parameters DGo = DHo - TDSo If DSo is small compared to DHo, DGo ~ DHo

  25. Endergonic Reaction Exergonic Reaction +DGo -DGo

  26. DH o for any reaction can be calculated from bond dissociation energies        

  27. Rate of a reaction = number of collision per unit time fraction with sufficient energy fraction with proper orientation X X Kinetics deals with the rate of chemical reactions and the reaction mechanism The rate-limiting step controls the overall rates of the reaction

  28. The free energy of activation & the transition state and the reactants

  29. DG‡ = DH‡ -TDS‡ DG‡ : (free energy of transition state)- (free energy of reactants) DH‡ : (enthalpy of transition state) - (enthalpy of reactants) DS‡ : (entropy of transition state) - (entropy of reactants)

  30. Second-order reaction First-order reaction A B A + B C + D rate = k [A] rate = k [A][B] Rates and Rate Constants

  31. The Arrhenius Equation -Ea/RT k = Ae Ea = DH‡ +RT k1 k-1 Rate Constants and the Equilibrium Constant Keq = k1/k-1 = [B]/[A]

  32. Transition State Versus Intermediate intermediate intermediate Transition states have partially formed bonds Intermediates have fully formed bonds

  33. Electrophilic Addition of HBr to 2-Butene G0 ‡ The rate-limiting step controls the overall rates of the Reaction. It has the highest activation energy.

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