© Prentice Hall 2001Chapter 31 Addition of Halogens The remaining halide ion is a good nucleophile which attacks the positively charged halonium ion

© Prentice Hall 2001 Chapter 3 1

Addition of Halogens

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


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

different product is obtained


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



Advantages over addition of H2O using H2SO4

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




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.


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


Hydroboration-Oxidation Hydroboration-oxidation is a convenient

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


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


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)


Hydroboration


Oxidation


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


Addition of Radicals

Markovnikov Addition

Anti-Markovnikov Addition

Source of Confusion regarding addition of HBr for years


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


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


Radicals

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

bond


Radicals

Stability of alkyl radicals is similar to stability of carbocations


Radical Chain Reactions






Radical Addition is Unique to Hydrogen Bromide

Why?


Radical Addition is Unique to Hydrogen Bromide


Addition of Hydrogen and Relative Stabilities of Alkenes


Addition of Hydrogen and Relative Stabilities of Alkenes


Relative Stabilities of Alkenes







Documents

© Prentice Hall 2001Chapter 31 Addition of Halogens The remaining halide ion is a good nucleophile which attacks the positively charged halonium ion