Transcript chapter 8 part 2

Alkenes and Alkynes II
Addition Reactions
Part 2
8.6 Alcohols From Alkenes through
Oxymercuration-Demeruration: Markovnikov
Addition
 Two steps mechanism that is useful to avoid rearrangement
 Both reaction take place very rapidly at RT or below
Regioselectivity of OxymercurationDemercuration
 The orientation of the addition of the elements of water,
H– and –OH, is in accordance with Markovnikov’s rule
Examples
Mechanism of Oxymercuration
Mechanism
Mechanism
example
 Starting with an appropriate alkene, show all steps in
the synthesis of 2-methyl-2-propanol
Example
 Consider the following reaction
 Outline a likely mechanism for the solvomercuration step of
this ether synthesis
 Show how you would use solvomercuration-demercuration to
prepare tert-butyl methyl ether
 Why would one use Hg(OCCF3)2 instead of Hg(Oac)2
Answer for c
 The electron-withdrawing fluorine atoms in mercurc
trifluoroacetate enhance the electrophilicity of the cation.
Experiments have demonstrated that for the preparation of
tertiary alcohols in satisfactory yields, the trifluoroacetate
must be used rather than the acetate
8.7 Alcohols from Alkenes through
Hydrocarboration-Oxidation:
Anti-Markovnikov Syn Hydration
 Addition of water is indirect and two reactions are involved
 1. addition of a boron atom and hydrogen to C= C
 Hydrolysis of the alkylborane intermediate to an alcohol and boric acid
8.8 Hydroboration: Synthesis of
Alkylboranes
 Preparation
Mechanism
 In each addition step, the boron atom becomes attached to
the less subsituted carbon atom of the double bond
 A hydrogen atom is transferred from the boron atom to the
other carbon atom of the double bond
Con’t
 As this transition state is approached, electrons shift in the
direction of the boron and away from the more substituted
carbon atom of the double bond
 The more substituted carbon bears an electron-releasing alkyl
group, it is better able to accommodate this positive charge
Mechanism
Example
 Starting with an appropriate alken, show the synthesis
of tributyl borane
8.9 Oxidation and Hydrolysis of
Alkylboranes
 These reactions are occurred in the same vessel by the
addition of hydrogen peroxide in an aqueous base
mechanism
mechanism
Example
 Starting with the appropriate alkene, show how you
could use hydroboration-oxidation to prepare each of
the alcohol
 1-pentanol
 2-methyl-1-pentenol