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 CH3(CH2)nCH3 CnH2n+2 Alkene CH3(CH2)nCH3 CnH2n Cyclic alkane CnH2n Cyclic alkene CnH2n-2 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 2004/2/21 end
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
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Mechanism of the Reaction
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3.7 Thermodynamics and Kinetics Reaction coordinate digram 2002/10/18
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
The Arrhenius equation:
Rate Constant and Equilibrium Constant At equlibrium, forward rate = reverse rate. k1 [A] = k-1 [B] therefore
Reaction Coordinate Diagram for the Addition of HBr to 2-Butene Bonds being broken p DH = 61 kcal/mol H-Br DH = 87 kcal/mol DHtotal = 148 kcal/mol Bonds being formed C-H DH = 101 kcal/mol Bonds being formed C-Br DH = 69 kcal/mol Total DH change = +47 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
Relative Stabilities of Carbocations
sp3 sp2 Inductive effect
Relative Stabilities of Carbocations
3.11 The Structure of the Transition State
The Hammond postulate
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3.12 Regioselectivity of Electrophilic Addition Reactions
Constitutional isomers Major product Minor product Major product Minor product Regioselective reaction Non-regioselective reaction
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 tertiary carbocation a secondary carbocation attack on rearranged carbocation attack on unrearranged carbocation minor product major product
Mechanism for the Formation of Rearranged Product a tertiary carbocation a secondary carbocation attack on rearranged carbocation attack on unrearranged carbocation major product minor product
Carbocation rearrangements also can occur by ring expansion 1,2-alkyl shift
3.15 Addition of Halogens p.154 last sentence
F2 reacts explosively with alkenes!!! unstable F2 reacts explosively with alkenes!!!
Formation of Halohydrins
3.18 Addition of Radicals. The Relative Stabilities of Radicals
Radical addition reaction March 6,2004
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