Transcript Chapter 8_part 1

Alkenes and Alkynes II
Addition Reactions
Introduction: Additions to Alkenes
 An addition to a C-C double bond
Electrophilic Addition
Electrophilic Addition of Hydrogen
Halides to Alkenes-Mechanism
Electrophilic Addition of Hydrogen Halides to
Alkenes
Markovnikov’s Rule
Example
The Rate Determining step
 Step 1 is the rate determined step
 Formation of carbocation
Theoretical Explanation of
Markovnikov’s Rules
Theoretical Explanation of
Markovnikov’s Rules
A mechanism for the reaction
 Predict the products and show the full mechanism
Modern Statement of
markovnikov’s Rule
 In the ionic addition of an unsymmetrical reagent to a
double bond, the positive portion of the adding reagent
attaches itself to a carbon of the double bond so as to yield
the more stable carbocation as intermediate
Regioselective Reactions
 When a reaction that can potentially yield two or more
constutional isomers actually produces only one (or a
predominance of one), the reaction is said to be
regioselective
 HX + unsymetrical alkene  more than product
An exception of Markovnikov’s
Rule
 When alkenes are treated with HBr in presence of peroxide,
an anti-Markovnikov addition
8.3 Stereochemistry of the Ionic
Addition to an Alkene
 The carbocation is formed in the first step of the
addition is trigonal planar
 achiral
8.4 Addition of Sulfuric Acid
Alkenes
 When alkenes are treated with cold concentrated
sulfuric acid, they dissolve because they react with
electrophilic addition to form alkyl hydrogen sulfate
Alcohols from Alkyl Hydrogen
Sulfate
 Alkyl hydrogen sulfates can be easily hydrolyzed to alcohols
by heating them with water
 Markovnikov addition of H- and -OH
example
 Provide mechanistic explanation for the following
observation
 The addition of hydrogen chloride to 3-methyl-1-butene
produces two products: 2-chloro-3-methylbutane and 2chloro-2-methylbutane
 Give the structure and name of the product that would be
obatained from ionic addition of IBr to propene
example
 In one industrial synthesis of ethanol, ethene is first
dissolved in 95% sulfuric acid. In a second step of water is
added and the mixture is heated. Out the reaction involved
8.5 Addition of Water to Alkene:
Acid-Catalyzed Hydration
 Method for preparation of low molecular weight alcohols
 Usually regioselective
Mechanism
Rate determining step
However, formation of 1o carbocation does not take
place
Rearrangement
 The carbocation formed initially invariable rearranges to a
more stable one if possible
 Provide the full mechanism