Lingo

1. Lingo Electrophile – Electron Loving – Looks for a pair of electrons H+ CH3CH2+ BH3 Nucleophile – Has a pair of electrons to donate OH- Cl- CH3NCH3

2. Mechanism • Practice HBr addition What in the world do all those arrows mean anyway???

3. Why are Different Products Formed

4. Carbocations • Carbocations are: classified as 1°, 2°, or 3° depending on the number of carbons bonded to the carbon bearing the positive charge C+

5. Carbocation Structure • Bond angles about the positively charged carbon are 120° • Carbon uses sp2 hybrid orbitals to form sigma bonds to the three attached groups • The unhybridized 2p orbital lies perpendicular to the sigma bond framework and contains no electrons

6. Carbocation Stability • A 3° carbocation is more stable than a 2° carbocation – A 2° carbocation is, in turn, more stable than a 1° carbocation • The higher the degree, the lower the activation energy needed for its formation • The methyl cation is so unstable that it is never observed in solution

7. Carbocation Stability • The alkyl groups attached to the positively charged carbon are electron-releasing and thereby delocalize the positive charge of the cation • We account for this electron-releasing ability of alkyl groups by (1) the inductive effect, and (2) hyperconjugation

8. Carbocation Stability • The inductive effect • the electron-deficient carbon bearing the positive charge polarizes electrons of the adjacent sigma bonds toward it • the positive charge on the cation is not localized on the trivalent carbon, but delocalized over nearby atoms • the larger the volume over which the positive charge is delocalized, the greater the stability of the cation

9. Carbocation Stability • Hyperconjugation • partial overlap of the bonding orbital of an adjacent C-H bond with the vacant 2p orbital of the cationic carbon delocalizes the positive charge and also the electrons of the adjacent  bond • replacing a C-H bond with a C-C bond increases the possibility for hyperconjugation

10. Carbocations Evidence to the ideas of induction and hyperconjugation

11. Markovnikov’s Rule The hydrogen will add to the carbon with the most number of hydrogens So, you mean that all that information can be summed up in this one easy rule to remember about the addition of things across a double bond???………….. So why didn’t you just say this?

12. Mechanism • Practice H2O and OH addition I think I’m starting to like this arrow thing!!!!

13. Regioselectivity • Regioselective Reaction: “A reaction in which two or more constitutional products could be obtained as products but one of them predominates.”

14. C+ Rearrangements • In electrophilic addition to alkenes, there is the possibility for rearrangement • Rearrangement: any change in connectivity of the atoms in a product compared with the connectivity of the same atoms in the starting material

15. Methyl Shift • In addition of HCl to an alkene

16. Methyl Shift • In acid-catalyzed hydration of an alkene

17. Hydride Shift • Driving force is rearrangement of a less stable carbocation to a more stable one

18. Hydride Shift • The less stable 2° carbocation rearranges to a more stable 3° one by shift of a hydrogen atom with its bonding electrons

19. Hydride Shift • Reaction of the more stable carbocation (a Lewis acid) with chloride ion (a Lewis base) completes the reaction

20. Rearrangements • 1,2-hydride shifts

21. Rearrangements • 1,2-methyl shift

22. Rearrangements • 1,3-hydride shifts usually doNOToccur

23. Rearrangements • Ring expansion through rearrangement

24. Mechanism • Practice hydride shift, methyl shift, ring opening