chapter 5 · chapter 5 addition reactions of alkenes organic chemistry, 5th ed. marc loudon eric j....
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Chapter 5Addition Reactions of Alkenes
Organic Chemistry, 5th ed.Marc Loudon
Eric J. KantorowskiCalifornia Polytechnic State UniversitySan Luis Obispo, CA
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
2
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
55.2 Reactions of Alkenes with Halogens
Bromonium Ion Formation
• Easiest to understand if dissected into two fictitious steps
65.2 Reactions of Alkenes with Halogens
Bromonium Ion Formation
• The accepted mechanism is concerted and avoids a carbocation
75.2 Reactions of Alkenes with Halogens
Bromohydrins
• Formed when a nucleophilic solvent is used
• The solvent is present in large excess
85.2 Reactions of Alkenes with Halogens
Halohydrins
• Other halogens can also be used
• A net addition of a hypohalous acid (HO-X)
• Iodohydrins are commonly unstable
95.2 Reactions of Alkenes with Halogens
Regioselectivity
• Unsymmetrical alkenes favor one regioisomer
• High regioselectivity observed when one carbon of the double bond is disubstituted
105.2 Reactions of Alkenes with Halogens
Regioselectivity
• About 90% of the positive charge is on the tertiary carbon
115.2 Reactions of Alkenes with Halogens
Ball-and-Stick and EPM Models
125.2 Reactions of Alkenes with Halogens
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-Reduction of Alkenes
• Oxymercuration:
155.4 Conversion of Alkenes into Alcohols
Oxymercuration-Reduction of Alkenes
• Oxymercuration proceeds via a mercurinium ion
• Compare mercurinium ion to bromonium ion
165.4 Conversion of Alkenes into Alcohols
Oxymercuration-Reduction of Alkenes
• Easiest to understand if dissected into two fictitious steps
175.4 Conversion of Alkenes into Alcohols
Oxymercuration-Reduction of Alkenes
• The accepted mechanism is concerted and avoids a carbocation
• Closely resembles halohydrin formation
185.4 Conversion of Alkenes into Alcohols
Oxymercuration-Reduction of Alkenes
• The strongest base present is acetate ion, not water
• The equilibrium lies far to the right
195.4 Conversion of Alkenes into Alcohols
Oxymercuration-Reduction of Alkenes
• Reduction:
• C-Hg bond is replaced by C-H bond
205.4 Conversion of Alkenes into Alcohols
Oxymercuration-Reduction of Alkenes
• Highly regioselective
• No rearrangements occur (no carbocations are formed)
• More convenient to run on a laboratory scale than hydration
215.4 Conversion of Alkenes into Alcohols
Hydroboration-Oxidation
• Borane adds regioselectively to alkenes
• Boron becomes bonded to the carbon with fewer alkyl substituents
225.4 Conversion of Alkenes into Alcohols
Hydroboration-Oxidation
• Borane has three B-H bonds and each one of these can react in turn
235.4 Conversion of Alkenes into Alcohols
Hydroboration-Oxidation
245.4 Conversion of Alkenes into Alcohols
Hydroboration-Oxidation
• Easiest to understand if dissected into two fictitious steps
255.4 Conversion of Alkenes into Alcohols
Hydroboration-Oxidation
• The accepted mechanism is concerted and avoids a carbocation
265.4 Conversion of Alkenes into Alcohols
• However, to explain the regioselectivity, some degree of electron deficiency is present
Hydroboration-Oxidation
• Conversion of organoboranes into alcohols
• The C-B bond is replaced with C-OH
275.4 Conversion of Alkenes into Alcohols
Comparison of Hydration Methods
• Oxymercuration-reduction and hydroboration-oxidation are complementary reactions
285.4 Conversion of Alkenes into Alcohols
Ozonolysis
• Leads to cleavage of a C=C bond
295.5 Ozonolysis of Alkenes
Ozonolysis
• Addition of ozone is concerted
• A rearrangement spontaneously follows
305.5 Ozonolysis of Alkenes
Ozonolysis
• The ozonide is then decomposed
• The net transformation can be viewed as:
315.5 Ozonolysis of Alkenes
Ozonolysis
• Ozonides can be decomposed reductively:
• Ozonides can be decomposed oxidatively:
325.5 Ozonolysis of Alkenes
Ozonolysis Under Different Conditions
335.5 Ozonolysis of Alkenes
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
355.6 Free-Radical Addition of Hydrogen Bromide to Alkenes
Free-Radical Reactions
• Initiation:
365.6 Free-Radical Addition of Hydrogen Bromide to Alkenes
Free-Radical Reactions
• Propagation:
375.6 Free-Radical Addition of Hydrogen Bromide to Alkenes
Free-Radical Reactions
• Termination:
385.6 Free-Radical Addition of Hydrogen Bromide to Alkenes
Free-Radical Reactions
• Radical stabilities explain the observed reversed regiochemistry
395.6 Free-Radical Addition of Hydrogen Bromide to Alkenes
Free-Radical Reactions
405.6 Free-Radical Addition of Hydrogen Bromide to Alkenes
Structure of Carbon Radicals
415.6 Free-Radical Addition of Hydrogen Bromide to Alkenes
Bond-Dissociation Energies
• Measures intrinsic strength of a chemical bond
• Can be used to evaluate the spontaneity of a reaction
425.6 Free-Radical Addition of Hydrogen Bromide to Alkenes
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