chapter 5 · chapter 5 addition reactions of alkenes organic chemistry, 5th ed. marc loudon eric j....

Chapter 5Addition Reactions of Alkenes

Organic Chemistry, 5th ed.Marc Loudon

Eric J. KantorowskiCalifornia Polytechnic State UniversitySan Luis Obispo, CA

http://www.pdfcomplete.com/cms/hppl/tabid/108/Default.aspx?r=q8b3uige22

Chapter 5 Overview

• 5.1 An Overview of Electrophilic Addition Reactions

• 5.2 Reactions of Alkenes with Halogens

• 5.3 Writing Organic Reactions

• 5.4 Conversion of Alkenes into Alcohols

• 5.5 Ozonolysis of Alkenes

• 5.6 Free-Radical Addition of Hydrogen Bromide to Alkenes

• 5.7 Polymers: Free-Radical Polymerization of Alkenes

• 5.8 Alkenes in the Chemical Industry

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Overview of Addition Reactions

• The carbon of the alkene with fewer alkyl groups becomes bonded to the less EN atom

35.1 An Overview of Electrophilic Addition Reactions


Addition of Bromine and Chlorine

• π bond: electron rich; serves as a nucleophile

• Halogens: electrophilic via an induced dipole

45.2 Reactions of Alkenes with Halogens


The Halonium Ion

• A reactive cyclic intermediate

• For chlorine: chloronium ion

• For iodine: iodonium ion



Bromonium Ion Formation

• Easiest to understand if dissected into two fictitious steps



Bromonium Ion Formation

• The accepted mechanism is concerted and avoids a carbocation



Bromohydrins

• Formed when a nucleophilic solvent is used

• The solvent is present in large excess



Halohydrins

• Other halogens can also be used

• A net addition of a hypohalous acid (HO-X)

• Iodohydrins are commonly unstable



Regioselectivity

• Unsymmetrical alkenes favor one regioisomer

• High regioselectivity observed when one carbon of the double bond is disubstituted



Regioselectivity

• About 90% of the positive charge is on the tertiary carbon



Ball-and-Stick and EPM Models



Conventions for Writing Reactions

• Solvents are generally written under the arrow

• Reactants and catalysts are written over the arrow

135.3 Writing Organic Reactions


Oxymercuration-Reduction of Alkenes

• A two step reaction carried out in sequence

• Note how each step is numbered

• Net reaction is hydration of an alkene

• Highly regioselective

145.4 Conversion of Alkenes into Alcohols



• Oxymercuration:




• Oxymercuration proceeds via a mercurinium ion

• Compare mercurinium ion to bromonium ion





• Closely resembles halohydrin formation




• The strongest base present is acetate ion, not water

• The equilibrium lies far to the right




• Reduction:

• C-Hg bond is replaced by C-H bond




• Highly regioselective

• No rearrangements occur (no carbocations are formed)

• More convenient to run on a laboratory scale than hydration



Hydroboration-Oxidation

• Borane adds regioselectively to alkenes

• Boron becomes bonded to the carbon with fewer alkyl substituents




• Borane has three B-H bonds and each one of these can react in turn






• However, to explain the regioselectivity, some degree of electron deficiency is present



• Conversion of organoboranes into alcohols

• The C-B bond is replaced with C-OH



Comparison of Hydration Methods

• Oxymercuration-reduction and hydroboration-oxidation are complementary reactions



Ozonolysis

• Leads to cleavage of a C=C bond

295.5 Ozonolysis of Alkenes


Ozonolysis

• Addition of ozone is concerted

• A rearrangement spontaneously follows



Ozonolysis

• The ozonide is then decomposed

• The net transformation can be viewed as:



Ozonolysis

• Ozonides can be decomposed reductively:

• Ozonides can be decomposed oxidatively:



Ozonolysis Under Different Conditions



Free-Radical Reactions

• The addition of peroxides reverses the regioselectivity of addition of HBr to alkenes

• Known as the peroxide effect

345.6 Free-Radical Addition of Hydrogen Bromide to Alkenes


Free-Radical Chain Reactions

• Initiation: net number of radicals increases

• Propagation: net number of radicals remains constant

• Termination: net number of radicals decreases




• Initiation:




• Propagation:




• Termination:




• Radical stabilities explain the observed reversed regiochemistry



Structure of Carbon Radicals



Bond-Dissociation Energies

• Measures intrinsic strength of a chemical bond

• Can be used to evaluate the spontaneity of a reaction



Polymers

• Free-radical initiators cause many alkenes to polymerize

435.7 Polymers: Free-Radical Polymerization of Alkenes


Industrial Production

• More ethylene is produced industrially than any other organic compound (~60 billion lbs)

• Via thermal cracking:

• Other important alkenes are:

445.8 Alkenes in the Chemical Industry


455.8 Polymers: Free-Radical Polymerization of Alkenes


chapter 5 · chapter 5 addition reactions of alkenes organic chemistry, 5th ed. marc loudon eric j....

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