© Prentice Hall 2001Chapter 31 Addition of Halogens The remaining halide ion is a good nucleophile...

© Prentice Hall 2001 Chapter 3 1

Addition of Halogens

The remaining halide ion is a good nucleophile which attacks the positively charged halonium ion

© Prentice Hall 2001 Chapter 3 2

Addition of Halogens If the reaction is carried out in water a

different product is obtained

© Prentice Hall 2001 Chapter 3 3

Oxymercuration-Demercuration

In oxymercuration-demercuration, an alkene is treated first with mercuric acetate in aqueous tetrahydrofuran

The product is treated with sodium borohydride in the presence of hydroxide

The result is Markovnikov addition of water to the double bond, yielding an alcohol

© Prentice Hall 2001 Chapter 3 4

Oxymercuration-Demercuration

Advantages over addition of H2O using H2SO4

Oxymercuration-demercuration doesn’t require the presence of a strong acid is not subject to carbocation rearrangement

© Prentice Hall 2001 Chapter 3 5

Oxymercuration-Demercuration

© Prentice Hall 2001 Chapter 3 6

Oxymercuration and Alkoxymercuration

Why does the nucleophile attack at the middle carbon and not at the end carbon?

The transition state at the left is more stable than the one at the right.

© Prentice Hall 2001 Chapter 3 7

Markovnikov’s Rule & Oxymercuration-Demercuration

Recall the modern version, i.e. the electrophile adds to the carbon bonded to the greatest number of hydrogens

In this case the electrophile is the mercuric acetate

© Prentice Hall 2001 Chapter 3 8

Hydroboration-Oxidation Hydroboration-oxidation is a convenient

way to add water to a double bond, forming an alcohol

© Prentice Hall 2001 Chapter 3 9

Hydroboration-Oxidation

Viewed in the classical sense, the addition appears to be in the anti-Markovnikov direction

The hydroxyl group bonds to the carbon with the most hydrogens

© Prentice Hall 2001 Chapter 3 10

Hydroboration-Oxidation As we consider the mechanism, we see that

the electrophile bonds to the carbon with the most hydrogens (obeying the modern version of Markovnikov’s rule)

© Prentice Hall 2001 Chapter 3 11

Hydroboration

© Prentice Hall 2001 Chapter 3 12

Oxidation

© Prentice Hall 2001 Chapter 3 13

Hydroboration-Oxidation In addition to the electronic effect, steric effects

may be important

Recall that three alkene molecules eventually surround each boron

The structure on the left is less hindered than the structure on the right

© Prentice Hall 2001 Chapter 3 14

Addition of Radicals

Markovnikov Addition

Anti-Markovnikov Addition

Source of Confusion regarding addition of HBr for years

© Prentice Hall 2001 Chapter 3 15

Heterolysis & Homolysis

Heterolytic bond cleavage or heterolysis

Homolytic bond cleavage or homolysis

Homolysis produces radicals, which are very reactive species

H Br H+ + Br

H Br H + Br

© Prentice Hall 2001 Chapter 3 16

Radicals are unpaired electron spins tend to perpetuate unpaired spin participate in chain reactions

© Prentice Hall 2001 Chapter 3 17

Radicals

Sources include: Hydrogen peroxide Alkyl peroxides Light causes homolysis of the weak O-O

bond

© Prentice Hall 2001 Chapter 3 18

Radicals

Stability of alkyl radicals is similar to stability of carbocations

© Prentice Hall 2001 Chapter 3 19

Radical Chain Reactions

© Prentice Hall 2001 Chapter 3 20

Radical Chain Reactions

© Prentice Hall 2001 Chapter 3 21

Radical Chain Reactions

© Prentice Hall 2001 Chapter 3 22

Radical Addition is Unique to Hydrogen Bromide

Why?

© Prentice Hall 2001 Chapter 3 23

Radical Addition is Unique to Hydrogen Bromide

© Prentice Hall 2001 Chapter 3 24

Addition of Hydrogen and Relative Stabilities of Alkenes

© Prentice Hall 2001 Chapter 3 25

Addition of Hydrogen and Relative Stabilities of Alkenes

© Prentice Hall 2001 Chapter 3 26

Relative Stabilities of Alkenes

© Prentice Hall 2001 Chapter 3 27

Relative Stabilities of Alkenes

© Prentice Hall 2001 Chapter 3 28

Relative Stabilities of Alkenes

© Prentice Hall 2001 Chapter 3 29

Relative Stabilities of Alkenes