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Transcript
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
1
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?
2
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
3
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)
4
Systematic Nomenclature of Alkenes
•Follows alkane rules; treats double bond as a function:
Think of alcohols
5
•Substituents in alphabetical order with lowest numbers
6
•Cyclic alkenes:
7
Important Special Terms
Vinyl Hs:
bonded to the double bond.
Allylic Hs:
on sp3 carbons next to the
double bond.
8
Structure of Alkenes
9
Isomers of Alkene
10
Dipole Moments of Alkene Isomers
11
Cis-Trans Interconversion in Vision
12
QuickTime™ and a
Video decompressor
are needed to see this picture.
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
2
1
2
1,1 on opposite sides = E
1
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
Multiple bonds are treated as attachments
of multiple single bonds
C
|
C- C
C-C-C
|
C
17
Rank the priorities by mass number
C C
in isotopes
| |
C- C
C- C
18
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
19
Nucleophiles are attracted to electron-deficient
atoms or molecules (electrophiles)
Examples of 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
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
22
Using Curved Arrows
23
A Reaction Coordinate Diagram
Transition states have partially formed bonds
Intermediates have fully formed bonds
24
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
25
Exergonic Reaction
-DGo
Endergonic Reaction
+DGo
26
DH o for any reaction can be calculated from
bond dissociation energies
D
D
D
D
D
D
D
D
27
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
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
Rates and Rate Constants
First-order reaction
Second-order
reaction
A
A+B
rate = k [A]
B
C+D
rate = k [A][B]
31
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]
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
DG0 ‡
The rate-limiting step controls the overall rates of the
34
Reaction. It has the highest activation energy.