chapter 3 alkenes: structures, nomenclature, and an introduction to reactivity thermodynamics...
TRANSCRIPT
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Organic Chemistry 4th Edition
Paula Yurkanis Bruice Chapter 3
Alkenes:
Structures, Nomenclature, andan Introduction to Reactivity
Thermodynamicsand Kinetics
Irene LeeCase Western Reserve University
Cleveland, OH©2004, Prentice Hall
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Hydrocarbons containing double bonds
C C
double bond
the functional group
center of reactivity
Alkenes
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Noncyclic alkene: CnH2n
Cyclic alkene: CnH2n–2
Molecular Formula of Alkene
CH3CH2=CH2
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Systematic Nomenclature of Alkenes
•Longest continuous chain containing the functional group
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•Cite the substituents in alphabetical order
•Name with the lowest functional group number and then the lowest substituent numbers
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•No numbering of the functional group is needed in a cyclic alkene
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Special Nomenclatures
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Structure of Alkene
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Isomers of Alkene
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Dipole Moments of Alkene Isomers
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Conversion of alkene isomers requires breaking ofthe π bond between the two sp2 carbons
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Cis-Trans Interconversion in Vision
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E and Z isomers
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Naming by the E,Z SystemRule 1: Consider the atomic number of the
atoms bonded directly to a specific sp2 carbon.
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Rule 2: If there is a tie, consider the atoms attached to the tie.
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Rule 3: Multiple bonds are treated as attachment of multiple single bonds.
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Rule 4: Rank the priorities by mass number in isotopes.
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An alkene is an electron-rich molecule
Nucleophile: an electron-rich atom or molecule that shares electrons with electrophiles
Examples of Nucleophiles
A nucleophile
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Nucleophiles are attracted to electron-deficient atoms or molecules (electrophiles)
Examples of Electrophiles
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Electrophilic Addition of HBr to Alkene
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Curved Arrows in Reaction Mechanisms
Movement of a pair of electrons
Movement of one electron
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Utilization of Curved Arrows
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Rules for Use of Curved Arrows
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A Reaction Coordinate Diagram
Transition states have partially formed bonds
Intermediates have fully formed bonds
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Thermodynamics describes the properties of a system at equilibrium
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Thermodynamic Parameters
Gibbs standard free energy change (∆G°)
Enthalpy (∆H°): the heat given off or absorbed during a reaction
Entropy (∆S°): a measure of freedom of motion
∆G° = ∆H° – T∆S°
If ∆S° is small compared to ∆H°, ∆G° ~ ∆H°
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Exergonic Reaction–∆G°
Endergonic Reaction
+∆G°
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∆H° can be calculated from bond dissociation energies
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Solvation: the interaction between a solvent and a molecule (or ion) in solution
Solvation can affect ∆H° and/or ∆S°, which ultimately affects ∆G°
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Kinetics deals with the rate of chemical reactions and the factors that affect those rates
The rate-limiting step controls the overall rate of the reaction
Rate of a reaction =
number of collisionsper unit time
fraction withsufficient energy
fraction withproper orientationx x
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The free energy between the transition state and the reactants
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∆G‡: (free energy of transition state) – (free energy of reactants)
∆G‡ = ∆H‡ – T∆S‡
∆H‡:(enthalpy of transition state) – (enthalpy of reactants)
∆S‡:(entropy of transition state) – (entropy of reactants)
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Rates and Rate Constants
First-order reaction
A B
rate = k[A]
Second-order reaction
A + B C + D
rate = k[A][B]
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The Arrhenius Equation
k = Ae–Ea/RT
Ea = ∆H‡ + RT
Rate Constants and the Equilibrium Constant
A Bk1
k–1
Keq = k1/k–1 = [B]/[A]
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Transition State versus Intermediate
Transition states have partially formed bonds
Intermediates have fully formed bonds
intermediate
intermediate
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Electrophilic Addition of HBr to 2-Butene
The rate-limiting step controls the overall rate of the reaction