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Chapter 6. The Reactions of Alkenes The Stereochemistry of Addition Reactions. Paula Yurkanis Bruice University of California, Santa Barbara. The Mechanism of the Reaction. An Electrophilic Addition Reaction. Addition of Hydrogen Halides. Which sp 2 Carbon Gets the H + ?.
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Chapter 6 The Reactions of Alkenes The Stereochemistry of Addition Reactions Paula Yurkanis Bruice University of California, Santa Barbara
The Mechanism Carbocation formation is the rate-limiting step.
Carbocation Stability Alkyl groups decreasethe concentration of positive charge on the carbon.
It Looks Like What it is Closer To The Hammond postulate says that the transition state is more similar in structure to the species to which it is more similar in energy.
Why the Difference in Rate? The more stable carbocation is formed more rapidly.
The Difference in Carbocation Stability Determines the Products
The Electrophile Adds to the sp2 Carbon Bonded to the Most Hydrogens A regioselective reaction forms more of one constitutional isomer than another. • Completely regioselective • Highly regioselective • Moderately regioselective
Why is the First Reaction More Highly Regioselective?
Mechanism for the Acid-Catalyzed Addition of an Alcohol
Carbocation Rearrangement (a 1,2-hydride shift)
Carbocation Rearrangement (a 1,2-methyl shift)
The Carbocation Does Not Rearrange (No Improvement in Carbocation Stability)
The Electrophile Adds to the sp2 Carbon Bonded to the Most Hydrogens The reagents are numbered because the second set of reagents is not added until the first reaction is over.
BH3 Has Three Potential Hydride Ions So a Dialkylborane and a Trialkylborane Can Be Formed
R2BH Allows Only Monoalkylation Because of its bulky R groups, it has a stronger preference for the less substituted sp2 carbon.
The Mechanism for the Addition of a Halogen The intermediate is a cyclic bromonium ion.
A Cyclic Bromonium Ion The carbons are the most electrophilic atoms.
Why Does it Follow the Same Rule? The electrophile adds to the sp2 carbon bonded to the most hydrogens.
Addition of Hydrogen catalytic hydrogenation areduction reaction