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

CH2 (CH2)n

AlkaneAlkane

CH3(CH2)nCH3

CnH2n+2

AlkeneAlkene

CHCH33(CH(CH22))nnCHCH33

CCnnHH2n2n

Cyclic alkaneCyclic alkane

CCnnHH2n2n

Cyclic alkeneCyclic alkene

CCnnHH2n-22n-2

H2C (CH2)n

HH

3.1 Molecular Formula and the Degree of Unsaturation3.1 Molecular Formula and the Degree of Unsaturation

Degree of unsaturation = 2Degree of unsaturation = 2

1 1 bond or 1 ring, degree of unsaturation = bond or 1 ring, degree of unsaturation = 11

3.3 The Structure of Alkenes3.3 The Structure of Alkenes

3.4 cis-trans3.4 cis-trans Isomerism Isomerism

Rotational barrier 63kcal/molRotational barrier 63kcal/mol

HH33C—CHC—CH33 rotational barrier = 2.9 kcal/mol rotational barrier = 2.9 kcal/mol

Cis-Trans Interconversion in VisionCis-Trans Interconversion in Vision

cis-transcis-trans Isomerism Isomerism

3.6 Reactivity Considerations3.6 Reactivity Considerations

Functional group

Organic ReactionsOrganic 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 NucleophilesElectrophiles and Nucleophiles

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Mechanism of the ReactionMechanism of the Reaction

3.7 Thermodynamics and Kinetics3.7 Thermodynamics and Kinetics

Reaction coordinate digramReaction coordinate digram

ThermodynamicsThermodynamics

The more stable the compound, The more stable the compound, the greater its concentration at equilibriumthe greater its concentration at equilibrium

Describes the properties of a system at equilibriumDescribes the properties of a system at equilibrium

Gibbs standard free energy changeGibbs standard free energy change

This symbol indicates that the reaction takes place under standard conditionsThis symbol indicates that the reaction takes place under standard conditions--all species at 1 M, 25 --all species at 1 M, 25 OOC, and 1 atm.C, and 1 atm.↓↓

R is the gas constant (1.986 cal/mol OK)T is the absolute temperature (OK)

Free Energy, Enthalpy and EntropyFree Energy, Enthalpy and Entropy

HHOO < 0, exothermic reaction; < 0, exothermic reaction; HHOO > 0, endothermic reaction > 0, endothermic reaction

In condensed phase, In condensed phase, SSOO ≈ 0. ≈ 0. Therefore Therefore GGOO ≈ ≈ HHOO

Calculate Calculate HHOO for a Certain Reaction for a Certain Reaction

p.130

Solvation: Solvation: the interaction between a solvent and the interaction between a solvent and a molecule (or ion) in solutiona molecule (or ion) in solution

Solvation can have a large effect on the Solvation can have a large effect on the HHOO of a reaction, of a reaction, and it can also affect the and it can also affect the SSOO of a reaction. of a reaction.

KineticsKinetics

Free energy of activationFree energy of activation

Deals with the rates of chemical reactions and the factors that affect those rates

Rate LawRate Law

First-order reactionFirst-order reaction

Second-order reactionSecond-order reaction

Second-order reactionSecond-order reaction

The Arrhenius equation:The Arrhenius equation:

Rate Constant and Equilibrium ConstantRate Constant and Equilibrium Constant

A Bk1

k-1

At equlibrium, forward rate = reverse rate.At equlibrium, forward rate = reverse rate.

kk1 1 [A] = [A] = kk-1-1 [B] [B]

Keq =k1

k-1=

[B]

[A]

therefore

Reaction Coordinate Diagram for the AdditioReaction Coordinate Diagram for the Addition of HBr to 2-Butenen of HBr to 2-Butene

Bonds being broken H = 61 kcal/molH-Br H = 87 kcal/molHtotal = 148 kcal/mol

Bonds being formed C-H H = 101 kcal/mol

Total Total H changeH change = +47 kcal/mol

Bonds being formed C-Br H = 69 kcal/mol

Over all Over all H changeH change = -22-22 kcal/mol

Reaction Coordinate Diagram for the AdditioReaction Coordinate Diagram for the Addition of HBr to 2-Butenen of HBr to 2-Butene

-22 kcal/mol-22 kcal/mol

3.8 General Mechanism for Electrophilic Addition3.8 General Mechanism for Electrophilic Addition

3.9 Addition of Hydrogen Halides3.9 Addition of Hydrogen Halides

H2C CH2 + HCl CH3CH2Cl

Relative Stabilities of CarbocationsRelative Stabilities of Carbocations

sp2sp3

Inductive effectInductive effect

Relative Stabilities of CarbocationsRelative Stabilities of Carbocations

3.11 The Structure of the Transition State3.11 The Structure of the Transition State

The Hammond postulateThe Hammond postulate

3.12 Regioselectivity of Electrophilic Additio3.12 Regioselectivity of Electrophilic Addition Reactionsn Reactions

Major productMajor product

Major productMajor product

Minor productMinor product

Minor productMinor product

Constitutional isomers

Regioselective reaction

H3CHC CHCH2CH3 + HBr H3CCH CH2CH2CH3 H3CH2C CHCH2CH3

Br

+Br

Non-regioselective reaction

H3CH2CCH CH2 H3CH2CHC CH2

HH

vs

3.13 Addition of Water and Alcohols3.13 Addition of Water and Alcohols

hydrationhydration

Addition of Alcohol to AlkeneAddition of Alcohol to Alkene

3.14 Rearrangement of Carbocations3.14 Rearrangement of Carbocations

According Markovnikov’ ruleThis compound should be major product

Mechanism for the Formation of Rearranged ProductMechanism for the Formation of Rearranged Product

a secondary carbocationa tertiary carbocation

attack on unrearrangedcarbocation

attack on rearrangedcarbocation

minor product major product

Mechanism for the Formation of Rearranged ProductMechanism for the Formation of Rearranged Product

major productminor product

attack on unrearrangedcarbocation

attack on rearrangedcarbocation

a secondary carbocation a tertiary carbocation

Carbocation rearrangements also can occur by ring expansionCarbocation rearrangements also can occur by ring expansion

1,2-alkyl shift

3.15 Addition of Halogens3.15 Addition of Halogens

p.154 last sentence

unstable

F2 reacts explosively with alkenes!!!

Formation of HalohydrinsFormation of Halohydrins

3.18 Addition of Radicals.3.18 Addition of Radicals. The Relative Stabilities of Radicals The Relative Stabilities of Radicals

Radical addition reactionRadical addition reaction

Addition of HBr through Radical MechanismAddition of HBr through Radical Mechanism

Addition of HBr through Radical MechanismAddition of HBr through Radical Mechanism

3.19 Addition of Hydrogen.3.19 Addition of Hydrogen. The Relative Stabilities of Alkenes The Relative Stabilities of Alkenes

Heat of HydrogenationHeat of Hydrogenation

Relative Stabilities of AlkenesRelative Stabilities of Alkenes