Unit 3: Reactions of Alkenes. Thermodynamics and Kinetics

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Presentation transcript:

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

`

Mechanism of the Reaction

2002/10/11 end

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

2002/10/18 end expected

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