Reactions of Alkenes. Chapter 4

1. Reactions of Alkenes. Chapter 4 Chapter 4

2. Contents of Chapter 4 • Electrophilic Addition Reactions • Carbocations • Various Addition Reactions Chapter 4

3. Mechanism for Electrophilic Addition to Alkenes • Reaction of 2-butene with hydrogen bromide is typical of electrophilic addition to alkenes • The reaction starts with the slow addition of an electrophile to an sp2 carbon, resulting in formation of a carbocation • The next step is the rapid addition of a nucleophile to the other sp2 carbon Chapter 4

4. Addition of Hydrogen Halides to Alkenes The more substituted carbocation is preferred Chapter 4

5. Stability of Carbocations Alkyl groups (“R”s) tend to stabilize the positive charge on the sp2carbon of a carbocation Chapter 4

6. Structure of the Transition State • The product of electrophilic addition to asymmetric alkenes is determined by how fast the various carbocations are formed • The lower the G‡, the faster the reaction • Substituents on an sp2carbon reduce the free energy of a species • Hence if the transition state is similar to the resultant carbocation, it too should be stabilized by alkyl substituents Chapter 4

7. Transition State Stability Formation of a tertiary carbocation should be faster than formation of a primary carbocation Chapter 4

8. “When a hydrogen halide adds to an asymmetrical alkene, the addition occurs such that the halogen attaches itself to the carbon atom of the alkene bearing the least number of hydrogen atoms” Regioselectivity - Markovnikov’s Rule Chapter 4

9. Markovnikov’s Rule • Modern equivalent statement: • The electrophile adds to the sp2carbon that forms the least stable carbocation • The other carbon forms the carbocation intermediate. • Predicting product: put + and – signs under carbons and electrophile and nucleophile and swap partners. Chapter 4

10. Addition of Water Chapter 4

11. Addition of Halogens with Water Present Chapter 4

12. Addition of Halogens With Water Present Product analysis involves double swap of + and - charges Chapter 4

13. Rearrangements of Carbocations 1,2-Hydride Shift: Chapter 4

14. Rearrangements of Carbocations 1,2-Carbon Shift: Chapter 4

15. Carbocation Rearrangements • Will always occur if they lead to a more stable carbocation • Can lead to opening of strained rings since opened ring carbocation is more stable than strained ring carbocation • Can be prevented without changing intended Markovnikov product by using oxymercuration-demercuration reaction Chapter 4

16. Oxymercuration-Demercuration • In oxymercuration-demercuration, an alkene is treated first with mercuric acetate in aqueous tetrahydrofuran • The product is treated with sodium borohydride in the presence of hydroxide • The result is Markovnikov addition of water to the double bond, yielding an alcohol Chapter 4

17. Oxymercuration-Demercuration Chapter 4

18. Markovnikov’s Rule & Oxymercuration-Demercuration • Recall the modern version, i.e. the electrophile adds to the carbon bonded to the greatest number of hydrogens • In this case the electrophile is the mercuric acetate • In product analysis ignore Hg(OAc)2 and give OH in water the – and H in water the + charge Chapter 4

19. Hydroboration-Oxidation • Hydroboration-oxidation is a convenient way to add water to a double bond, forming an alcohol • B is a Lewis acid so give B the + and H the – in product analysis (anti-Markovnikov) Chapter 4

20. Hydroboration-Oxidation As we consider the mechanism, we see that the electrophile bonds to the carbon with the most hydrogens (obeying the modern version of Markovnikov’s rule) Chapter 4

21. Oxidation Chapter 4

22. Epoxides • Ethers in which the oxygen atom is incorporated into a three-membered ring are called epoxides • Common names are based on the assumption that the oxygen atom is placed on top of the p bond of an alkene • Name the alkene and follow with the word, “oxide” Chapter 4

23. Epoxides • Two ways to name epoxides in the IUPAC system: • substituted oxirane • name the alkane with the prefix “epoxy” along with the numbers of the carbons bonded to the oxygen • oxirane numbering system is a bit tricky – O in 3-membered ring given number 1; C’s are 2 & 3 Chapter 4

24. Epoxides Epoxides made by reacting alkenes with peroxyacids, sometimes called peracids Chapter 4

25. Relative Stabilities of Alkenes Chapter 4

26. Relative Stabilities of Alkenes • The more alkyl substituents attached to a double bond the more stable the double bond. • Trans alkenes more stable than cis alkenes • Not difficult concepts but should be learned now in order to understand Chapter 7 later. Chapter 4