Unit 3

:

Reactions of Alkenes.

Thermodynamics and Kinetics

Hydrocarbons that contain only C-C bond are called

alkane

s Hydrocarbons that contain C=C bond are called

alkene

s or olefins (oil forming)

尤加利樹油

3.1 Molecular Formula and the Degree of Unsaturation Alkane Alkene Cyclic alkane CH 3 (CH 2 ) n CH 3 CH 3 (CH 2 ) n CH 3 C n H 2n+2 C n H 2n C n H 2n CH 2 (CH 2 ) n Cyclic alkene C n H 2n-2 H H H 2 C (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 H 3 C—CH 3 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 O C, and 1 atm.

↓ R T is the gas constant (1.986

cal

/mol O K) is the absolute temperature ( O K)

Free Energy, Enthalpy and Entropy

D H O < 0, exothermic reaction; D H O > 0, endothermic reaction In condensed phase, D S O ≈ 0. Therefore D G O ≈ D H O

Calculate D H O 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 D H O and it can also affect the D S O of a reaction.

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

A k 1 k -1 B At equlibrium, forward rate = reverse rate.

k

1 [A] =

k

-1 [B] therefore

K

eq =

k

1

k -

1 [B] = [A]

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

Bonds being broken p D H = 61 kcal/mol H-Br D H total D H = 87 kcal/mol = 148 kcal/mol Bonds being formed C-H D H = 101 kcal/mol Total D H change = +47 kcal/mol Bonds being formed C-Br D H = 69 kcal/mol Over all D H 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 2 C CH 2 + HCl CH 3 CH 2 Cl

Relative Stabilities of Carbocations

sp 3

Inductive effect

sp 2

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 Minor product H 3 CHC CHCH 2 CH 3 + HBr Major product Minor product

Regioselective reaction

Br H 3 CCH CH 2 CH 2 CH 3 + H 3 CH 2 C Br CHCH 2 CH 3

Non-regioselective reaction

H 3 CH 2 CCH CH 2 H vs H 3 CH 2 CHC H CH 2

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 a tertiary carbocation attack on rearranged carbocation minor product 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

F 2 reacts explosively with alkenes!!!

unstable

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