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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
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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?
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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
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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)
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Systematic Nomenclature of Alkenes
•Follows alkane rules; treats double bond as a function:
Think of alcohols
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•Substituents in alphabetical order with lowest numbers
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•Cyclic alkenes:
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Important Special Terms
Vinyl Hs:
bonded to the double bond.
Allylic Hs:
on sp3 carbons next to the
double bond.
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Structure of Alkenes
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Isomers of Alkene
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Dipole Moments of Alkene Isomers
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Cis-Trans Interconversion in Vision
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QuickTime™ and a
Video decompressor
are needed to see this picture.
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E and Z isomers
Priority is in order of atomic number.
Rank H vs. Br and C vs. Cl.
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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
2
1
2
1,1 on opposite sides = E
1
1
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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
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Multiple bonds are treated as attachments
of multiple single bonds
C
|
C- C
C-C-C
|
C
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Rank the priorities by mass number
C C
in isotopes
| |
C- C
C- C
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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: eand anions) Examples of nucleophiles
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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
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
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Using Curved Arrows in Reaction Mechanisms
Movement of a pair of electrons:
START arrows from electrons pointing to electrophile
Use 1/2 arrow for the movement of one electron
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Using 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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Thermodynamic Parameters
DGo = DHo - TDSo
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
If DSo is small compared to DHo, DGo ~ DHo
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Exergonic Reaction
-DGo
Endergonic Reaction
+DGo
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DH o for any reaction can be calculated from
bond dissociation energies
D
D
D
D
D
D
D
D
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Kinetics deals with the rate of chemical reactions
and the reaction mechanism
Rate of a reaction =
number of collision
per unit time
fraction with
fraction with
X sufficient energy X proper orientation
The rate-limiting step controls the overall rates of the reaction
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The free energy of activation & the transition state
and the reactants
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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)
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Rates and Rate Constants
First-order reaction
Second-order
reaction
A
A+B
rate = k [A]
B
C+D
rate = k [A][B]
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The Arrhenius Equation
-Ea/RT
Ea = DH‡ +RT
k = Ae
Rate Constants and the Equilibrium
Constant
A
k1
B
k-1
Keq = k1/k-1 = [B]/[A]
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Transition State Versus Intermediate
intermediate
intermediate
Transition states have partially formed bonds
Intermediates have fully formed bonds
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Electrophilic Addition of HBr to 2-Butene
DG0 ‡
The rate-limiting step controls the overall rates of the
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Reaction. It has the highest activation energy.
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
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