chapter 3 alkenes: structures, nomenclature, and an introduction to reactivity thermodynamics...
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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
Hydrocarbons containing double bonds
C C
double bond
the functional group
center of reactivity
Alkenes
Noncyclic alkene: CnH2n
Cyclic alkene: CnH2n–2
Molecular Formula of Alkene
CH3CH2=CH2
Systematic Nomenclature of Alkenes
•Longest continuous chain containing the functional group
•Cite the substituents in alphabetical order
•Name with the lowest functional group number and then the lowest substituent numbers
•No numbering of the functional group is needed in a cyclic alkene
Special Nomenclatures
Structure of Alkene
Isomers of Alkene
Dipole Moments of Alkene Isomers
Conversion of alkene isomers requires breaking ofthe π bond between the two sp2 carbons
Cis-Trans Interconversion in Vision
E and Z isomers
Naming by the E,Z SystemRule 1: Consider the atomic number of the
atoms bonded directly to a specific sp2 carbon.
Rule 2: If there is a tie, consider the atoms attached to the tie.
Rule 3: Multiple bonds are treated as attachment of multiple single bonds.
Rule 4: Rank the priorities by mass number in isotopes.
An alkene is an electron-rich molecule
Nucleophile: an electron-rich atom or molecule that shares electrons with electrophiles
Examples of Nucleophiles
A nucleophile
Nucleophiles are attracted to electron-deficient atoms or molecules (electrophiles)
Examples of Electrophiles
Electrophilic Addition of HBr to Alkene
Curved Arrows in Reaction Mechanisms
Movement of a pair of electrons
Movement of one electron
Utilization of Curved Arrows
Rules for Use of Curved Arrows
A Reaction Coordinate Diagram
Transition states have partially formed bonds
Intermediates have fully formed bonds
Thermodynamics describes the properties of a system at equilibrium
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°
Exergonic Reaction–∆G°
Endergonic Reaction
+∆G°
∆H° can be calculated from bond dissociation energies
Solvation: the interaction between a solvent and a molecule (or ion) in solution
Solvation can affect ∆H° and/or ∆S°, which ultimately affects ∆G°
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
The free energy between the transition state and the reactants
∆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)
Rates and Rate Constants
First-order reaction
A B
rate = k[A]
Second-order reaction
A + B C + D
rate = k[A][B]
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]
Transition State versus Intermediate
Transition states have partially formed bonds
Intermediates have fully formed bonds
intermediate
intermediate
Electrophilic Addition of HBr to 2-Butene
The rate-limiting step controls the overall rate of the reaction