文档介绍：Chapter 8 Reactions of Alkenes
Jo Blackburn
Richland College, Dallas, TX
Dallas munity College District
ã 2003, Prentice Hall
Organic Chemistry, 5th Edition L. G. Wade, Jr.
1
Chapter 8
Reactivity of C=C
Electrons in pi bond are loosely held.
Electrophiles are attracted to the pi electrons.
Carbocation intermediate forms.
Nucleophile adds to the carbocation.
Net result is addition to the double bond. =>
2
Chapter 8
Electrophilic Addition
Step 1: Pi electrons attack the electrophile.
=>
Step 2: Nucleophile attacks the carbocation.
3
Chapter 8
Types of Additions
=>
4
Chapter 8
Addition of HX (1)
Protonation of double bond yields the most stable carbocation. Positive charge goes to the carbon that was not protonated.
X
=>
5
Chapter 8
Addition of HX (2)
=>
6
Chapter 8
Regiospecificity
Markovnikov’s Rule: The proton of an acid adds to the carbon in the double bond that already has the most H’s. “Rich get richer.”
More general Markovnikov’s Rule: In an electrophilic addition to an alkene, the electrophile adds in such a way as to form the most stable intermediate.
HCl, HBr, and HI add to alkenes to form Markovnikov products. =>
7
Chapter 8
Free-Radical Addition of HBr
In the presence of peroxides, HBr adds to an alkene to form the “anti-Markovnikov” product.
Only HBr has the right bond energy.
HCl bond is too strong.
HI bond tends to break heterolytically to form ions. =>
8
Chapter 8
Free Radical Initiation
Peroxide O-O bond breaks easily to form free radicals.
heat
Hydrogen is abstracted from HBr.
Electrophile
=>
9
Chapter 8
Propagation Steps
Bromine adds to the double bond.
Electrophile =>
Hydrogen is abstracted from HBr.
10
Chapter 8