ORGANIC CHEMISTRY - CHAPTER 12: REACTIONS OF ALKENES

Important Concepts

1. The reactivity of the double bond manifests itself in exothermic addition reactions leading to saturated products.

2. The hydrogenation of alkenes is immeasurably slow unless a catalyst capable of splitting the strong H – H bond is used. Possible catalysts are palladium on carbon, platinum (as PtO₂), and Raney nickel. Addition of hydrogen is subject to steric control, the least hindered face of the least substituted double bond frequently being attacked preferentially. 

3. As a Lewis base, the π bond is subject to attack by acid and electrophiles, such as H⁺, X₂, and Hg²⁺. If the initial intermediate is a free carbocation, the more highly substituted carbocation is formed. Alternatively, a cyclic onium ionis generated subject to nucleophilic ring opening at the more substituted carbon. Carbocation formation leads to control of regiochemistry (Markovnikov rule);onium ion formation leads to control of both regio- and stereochemistry.

4. Mechanistically, hydroboration lies between hydrogenation and electrophilic addition. The first step is π complexation to the electron-deficient boron, whereas the second is a concerted transfer of the hydrogen to carbon. Hydroboration – oxidation results in the anti-Markovnikov hydration of alkenes.

5. Carbenes and carbenoids are useful for the synthesis of cyclopropanes from alkenes.

6. Peroxycarboxylic acids may be thought of as containing an electrophilic oxygen atom, transferable to alkenes to give oxacyclopropanes. The process is often called epoxidation.

7. Osmium tetroxide acts as an electrophilic oxidant of alkenes; in the course of the reaction, the oxidation state of the metal is reduced by two units. Addition takes place in a concerted syn manner through cyclic six-electron transition states to give vicinal diols.

8. Ozonolysis followed by reduction yields carbonyl compounds derived by cleavage of the double bond.

9. In radical chain additions to alkenes, the chain carrier adds to the π bond to create the more highly substituted radical. This method allows for the anti-Markovnikov hydrobromination of alkenes, as well as the addition of thiols and some halomethanes.

10. Alkenes react with themselves through initiation by charged species, radicals, or some transition metals to give polymers. The initial attack at the double bond yields a reactive intermediate that perpetuates carbon – carbon bond formation.

From "Organic Chemistry" Textbook of VOLLHARDT & SCHORE