Alkenes and Alkynes II Addition Reactions Chapter 8 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 2-chloro-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