Alkene Reactions, Section 1

1. Alkene Reactions, Section 1

2. A B C C C C Reactions of Alkenes • The characteristic reaction of alkenes is addition to the double bond. + A—B

3. Reactions involving carbocations: Substitutions via Sn1. Eliminations via the E1. Additions to alkenes and alkynes. Reminder: Reactions of Alkenes 1,2-alkyl shift for stability C H C H 3 3 + + 3° carbocation 2° carbocation

4. Terms: Driven by weaker pi bond to 2, new, stronger sigma bonds. Overall Transformation: C=C to a new functional group. Regioselectivity: Which side a specific group is added to. Stereoselectivity: Whether the addition is cis or trans. Nucleophile: pi bond has high electron density; reacts with electrophiles. Electrophile: An electron poor species, which reacts with electron rich species. Electrophilic Addition: An electrophile adds to a nucleophile, where two systems combine to form one product. In actuality, the nucleophile attacks the electrophile Alkene Addition Terms

5. syn Addition versus anti Addition syn addition anti addition

6. General Format: Depending on timing, two different mechanisms are possible: Two Step: Full Carbocation Mechanism 1: Reaction of the C=C with E+  Carbocation 2: Nucleophilic attack. Electrophilic Addition d d + - E E + N u E - N u : N u +

7. One Step: Partial Carbocation Mechanism Simultaneous formation of the two sigma bonds. Tips: Find the electrophile. The alkene attacks the electrophile - Regioselectivity. 3. Figure out if carbocations are formed. - 2 step reaction is not stereoselective. Electrophilic Addition E N u E N u

8. One Step: Partial Carbocation Mechanism Simultaneous formation of the two sigma bonds. Tips: 4. Determine if the new sigma bonds are from the same species - Syn addition. 5. Determine if the two new sigma bonds are from different species - Anti addition. Electrophilic Addition E N u E N u

9. Example C C CH3CH2CH2CHCH2CH3 Br (76%) CH2CH3 CH3CH2 HBr CHCl3, -30°C H H

10. Electrophilic addition of hydrogen halides to alkenes proceeds by rate-determining formation of a carbocation intermediate. Mechanism

11. Electrons flow fromthe  system of thealkene (electron rich) toward the positivelypolarized proton of the hydrogen halide. Mechanism

12. Free Radical Mechanism: Free Radical Mechanism H H B r B r B r Br

13. Which mechanism is used for the following reactions? Carbocation Radical Learning Check

14. Electrophilic Addition: Hydrogenation: Hydrohalogenation: Hydration: Hydroboration: Alkene Reactions

15. Halogenation: Halohydration: Hydrohalogenation: Epoxidation: Ozonolysis: Alkene Reactions

16. Reaction Type: Electrophilic Addition (Reduction). Used for heats of hydrogenation stability. Overall Reaction: H2 + Alkene  Alkane Requires: Metal catalyst (Pt, Pd, Ni, Rh). Regioselectivity: None. Stereospecificity: Only syn addition products. Hydrogenation

17. Mechanism of Catalytic Hydrogenation H H B Y H H C C A X

18. Mechanism of Catalytic Hydrogenation B Y C C A X H H H H Absorption of H by catalyst

19. Mechanism of Catalytic Hydrogenation B Y X A H H H C C H Addition of alkene to catalyst.

20. Mechanism of Catalytic Hydrogenation H H H H B Y X A C C

21. Mechanism of Catalytic Hydrogenation B Y X A H H C C H H Addition of H to alkene.

22. Mechanism of Catalytic Hydrogenation B Y X A C C H H H H Alkane leaves catalyst with H in syn position.

23. Example of syn-Addition H CO2CH3 H2,Pt CO2CH3 H CO2CH3 CO2CH3 (100%)

24. Example 1: Example 2: Learning Check o r H 2

25. When an unsymmetrically substituted alkene reacts with a hydrogen halide, the hydrogen adds to the carbon that has the greater number of hydrogen substituents, and the halogen adds to the carbon that has the fewer hydrogen substituents. Markovnikov's Rule

26. Occurs due to the formation of the most stable carbocation: Markovnikov rules extend to other additions which don’t involve halides: Markovnikov Example Major product

27. Mechanistic Basis for Markovnikov's Rule:Example 1 HBr CH3CH2CH CH3CH2CHCH3 CH2 acetic acid Br

28. Mechanistic Basis for Markovnikov's Rule:Example 1 + CH3CH2CH2—CH2 primary carbocation is less stable: not formed + CH3CH2CH—CH3 + Br– CH3CH2CH CH3CH2CHCH3 CH2 Br HBr

29. Mechanistic Basis for Markovnikov's Rule:Example 2 H CH3 CH3 Cl HCl 0°C

30. H H H + CH3 H CH3 CH3 Cl secondary carbocation is less stable: not formed + CH3 H Cl– HCl

31. Reactions that are Anti Markovnikov add the hydrogen to the side with the least number of hydrogens. The ‘group’ is attached to the less substituted carbon. Reaction does not go through a carbocation intermediate. Anti Markovnikov

32. Reaction of Alkenes with HBr (no radical formation). Reaction Type: Overall Reaction: HX + Alkenes  Alkyl Halide. Hydrogen Halide Reactivity Order: HI > HBr > HCl > HF (same as acidity order) Regioselectivity: Markovnikov Rule. “For addition of hydrogen halides to alkenes, the H atom adds to the C with the most H atoms already present” meaning it goes through the more stable carbocation intermediate.” Hydrohalogenation Electrophilic Addition; Reduction (H) and Oxidation (X)

33. Reaction of Alkenes with HBr (no radical formation). Stereoselectivity: None, during mechanism, a planar carbocation is formed. Requirements: HBr, No light (hv), no peroxides, no radicals created. Hydrohalogenation

34. Mechanism: Hydrohalogenation H H - B r C C + C H 3 C H C H 3 3 C H 3 Br H H + H C H H 3 Forms most stable carbocation. Nucleophilic attack. Protonate alkene. Forms alkyl bromide. Can be viewed as an acid base reaction (Lewis). Pi electrons act as a Lewis Base. Bromide ion is nucleophile. Carbocation is electrophile.