Unit 3: Reactions of Alkenes.
Thermodynamics and Kinetics
Hydrocarbons that contain only
C-C bond are called alkanes
Hydrocarbons that contain C=C
bond are called alkenes or
olefins (oil forming)
尤加利樹油
3.1 Molecular Formula and the Degree of Unsaturation
Alkane
Alkene
CH3(CH2)nCH3 CH3(CH2)nCH3
CnH2n+2
Cyclic alkane
CnH2n
CnH2n
Cyclic alkene
CnH2n-2
H
C H 2 (C H 2 ) n
H 2C
H
(CH 2 ) n
Degree of unsaturation = 2
1 p bond or 1 ring, degree of unsaturation = 1
3.3 The Structure of Alkenes
3.4 cis-trans Isomerism
Rotational barrier 63kcal/mol
H3C—CH3 rotational barrier = 2.9 kcal/mol
Cis-Trans Interconversion in Vision
cis-trans Isomerism
3.6 Reactivity Considerations
Functional group
Organic Reactions
• Electron-rich atoms or molecules are attracted to
electron-deficient atoms or molecules
• Nucleophile: an electron-rich atom or molecule
• Electrophile: an electron-deficient atom or molecule
• A nucleophile and an electrophile react with each other
Electrophiles and Nucleophiles
`
Mechanism of the Reaction
3.7 Thermodynamics and Kinetics
Reaction coordinate digram
Thermodynamics
Describes the properties of a system at equilibrium
The more stable the compound,
the greater its concentration at equilibrium
Gibbs standard free energy change
This symbol indicates that the reaction takes place under standard conditions
--all species at 1 M, 25 OC, and 1 atm.
↓
R is the gas constant (1.986 cal/mol OK)
T is the absolute temperature (OK)
Free Energy, Enthalpy and Entropy
DHO < 0, exothermic reaction; DHO > 0, endothermic reaction
In condensed phase, DSO ≈ 0. Therefore DGO ≈ DHO
Calculate DHO for a Certain Reaction
p.130
Solvation: the interaction between a solvent and
a molecule (or ion) in solution
Solvation can have a large effect on the DHO of a reaction,
and it can also affect the DSO of a reaction.
Kinetics
Deals with the rates of chemical reactions and the factors
that affect those rates
Free energy of activation
Rate Law
First-order reaction
Second-order reaction
Second-order reaction
The Arrhenius equation:
Rate Constant and Equilibrium Constant
k1
A
B
k -1
At equlibrium, forward rate = reverse rate.
k1 [A] = k-1 [B]
therefore
[B]
k1
K eq =
=
k -1
[A]
Reaction Coordinate Diagram for the
Addition of HBr to 2-Butene
Bonds being broken Bonds being formed
p DH = 61 kcal/mol
C-H DH = 101 kcal/mol
H-Br DH = 87 kcal/mol
DHtotal = 148 kcal/mol
Total DH change = +47 kcal/mol
Bonds being formed
C-Br DH = 69 kcal/mol
Over all DH change = -22 kcal/mol
Reaction Coordinate Diagram for the
Addition of HBr to 2-Butene
-22 kcal/mol
3.8 General Mechanism for Electrophilic Addition
3.9 Addition of Hydrogen Halides
H 2C
CH 2
+
HCl
CH 3 CH 2 Cl
Relative Stabilities of Carbocations
sp3
Inductive effect
sp2
Relative Stabilities of Carbocations
3.11 The Structure of the Transition State
The Hammond postulate
3.12 Regioselectivity of Electrophilic Addition
Reactions
Constitutional isomers
Major product
Major product
Minor product
Minor product
Regioselective reaction
Br
Br
H 3 C HC
CH CH 2 CH 3 +
HBr
H 3 C CH
CH 2 CH 2 CH 3 + H 3 C H 2 C
CH CH 2 CH 3
Non-regioselective reaction
H 3 C H 2 C CH
CH2
H
vs
H 3 C H 2 C HC
H
CH2
3.13 Addition of Water and Alcohols
hydration
Addition of Alcohol to Alkene
3.14 Rearrangement of Carbocations
According Markovnikov’ rule
This compound should be major product
Mechanism for the Formation of Rearranged Product
a secondary carbocation
attack on
unrearranged
carbocation
minor product
a tertiary carbocation
attack on
rearranged
carbocation
major product
Mechanism for the Formation of Rearranged Product
a secondary carbocation
attack on
unrearranged
carbocation
minor product
a tertiary carbocation
attack on
rearranged
carbocation
major product
Carbocation rearrangements also can occur by ring expansion
1,2-alkyl shift
3.15 Addition of Halogens
p.154 last sentence
unstable
F2 reacts explosively with alkenes!!!
Formation of Halohydrins
3.18 Addition of Radicals.
The Relative Stabilities of Radicals
Radical addition reaction
Addition of HBr through Radical Mechanism
Addition of HBr through Radical Mechanism
3.19 Addition of Hydrogen.
The Relative Stabilities of Alkenes
Heat of Hydrogenation
Relative Stabilities of Alkenes