Organic Chemistry Reviews Chapter 8

1. Organic Chemistry ReviewsChapter 8 Cindy Boulton November 23, 2008

2. Chapter 8 • Memorize reactions! • Alkene or Alkyne • Reactants • Products • Regiochemistry • Markovnikov or Anti-Markovnikov • Stereochemistry • Syn addition or Anti addition or racemic mix • Mechanism • Pg 354-355 • Practice!

3. Regiochemistry • Markovinkov rule • Incoming Hydrogen or more positive reagent goes to Carbon with more Hydrogens directly connected • Forms a Carbocation intermediate that is more stable • Anti-Markovinkov rule • Incoming Hydrogen or more positive reagent goes to Carbon with less Hydrogens directly connected • Forms a less stable Carbocation intermediate • 1,2 Addition • Same thing is added to adjacent Carbons

4. Stereochemistry • Syn Addition • Added to same face of reagent • In transition state • Anti Addition • Added to opposite faces of the reagent • In transition state • Racemic Mixture • Forms a Carbocation intermediate • Forms enantiomers • Optical rotation = 0o

5. Hydrogenation of alkenes • Use Pt as catalyst • Can use Ni, Pd, Rh or others as catalyst • Lowers the activation energy to speed up the reaction • H are added to the same face of the alkene • Stereochemistry: Syn Addition • Z -> RS and SR • E -> RR and SS • Forms a racemic mixture of enantiomers • Regiochemistry: 1, 2 addition • Carbons of double bond are side by side

6. Hydrogenation of Alkynes • Pt as catalyst: • Can use Ni, Pd, Rh or others as catalyst • Forms an alkene but can not stop so continues to form alkane • Lindlar’s Catalyst: • H2/Pd/CaCO3 • Stops as alkene, • Ca prevents alkene from being hydrogenated • Stereochemistry: syn addition • Hydrogens added to same side • Form Z or Cis alkene

7. Hydrogenation of Alkynes cont. • H2/Ni2B as Catalyst • Stops as alkene, • B prevents alkene from being hydrogenated • Stereochemistry: syn addition • Hydrogens added to same side • Form Z or Cis alkene • 1) Li, C2H5NH2 2) NH4Cl • Sterochemistry: anit addition • Hydrogens added to opposite side • Form E or Trans alkene

8. HX Addition to Alkenes • X = F, Cl, Br, I • Regiochemistry: Markovnikov • Stereochemistry: Racemic Mixture • Mechanism • Double bond is broken • Hydrogen from Acid is added to Carbon with more Hydrogens • Forms a Carbocation intermediate • X- acts as nucleophile in “SN1” reaction • Attack either side of Carbocation forming enantiomers and racemic mixture

9. HBr/peroxides to Alkenes • Peroxides are present • R-O-O-R or H-O-O-H • Oxidation number of -1 • Donates an oxygen • Regiochemistry: Anti-Markovinkov • Stereochemistry: Racemic Mixture

10. H3O+/H2O to Alkenes • Hydration addition to from alcohol • Acid used as catalyst • Regiochemistry: Markovinkov • Stereochemistry: Racemic Mixture • Mechanism: • Double bond of alkene broken • Hydrogen from H3O+ added to Carbon with most Hydrogens • Forms Carbocation • H2O added to Carbocation forming protinated alcohol • Excess H2O removes Hydrogen forming an alcohol

11. Halogenation Addition to Alkenes • X-X addition • X = Cl, Br, I • CCl4 or CH2Cl2 are present • Regiochemistry: Markovinkov • Stereochemistry: Racemic, Anti-Addition • Mechanism: • Double bond of alkene is broken and X is added to either side of alkene • X is bonded to both Carbons • X goes to Carbon with less Hydrogens causing the second X to bond on opposite side to theoretical Carbocation.

12. X2/H2O (X-OH) Addition with Alkene • X = Cl, Br • Forms halohydrin • Regiochemistry: Markovnikov • Stereochemistry: Racemic, Anit-Addition • Mechanism: • Double bond of alkene is broken and X is added to either side of alkene • X is bonded to both Carbons • X goes to Carbon with less Hydrogens causing the water to bond on opposite side to theoretical Carbocation. • Forms a protonated alcohol • Another H2O removes extra Hydrogen attached

13. Syn Hydroxylation of Alkenes • (HO-OH) addition • OsO4/NaHSO3 • KMnO4/cold,dilute • Regiochemistry: 1,2 addition • Stereochemistry: Syn Addition, Racemic • Intermediate • Mn or Os has 4 Oxygens attached and deliver the Oxygens to one side of the alkene, can go to either side

14. Oxidative Cleavage of Alkenes • 1) KMnO4, OH-, heat 2) H3O+ • Carbon = Carbon divided to form C=O • C-C bonds stay • C-H bonds change to C-OH • 1) O3 2) Zn/H2O • Carbon = Carbon divided to form C=O • C-C bonds stay • C-H bonds stay

15. H-X Addition for Alkynes • X = F, Cl, Br, I • 1 H-X • Forms alkene • Regiochemistry: Markovinkov • Stereochemistry: Anti Addition, not a racemic mixture • Excess H-X • Forms alkane • Regiochemistry: Markovinkov • Stereochemisty: Anti Addition, Racemic • Forms a gem-dihalide • Halides on same Carbon

16. HBr/peroxides to Alkynes • Peroxides are present • R-O-O-R or H-O-O-H • Oxidation number of -1 • Donates an oxygen • 1 H-Br • Forms alkenes • Regiochemistry: Anti-Markovinkov • Stereochemistry: not cis or trans, not a racemic mixture • Excess H-Br • Forms alkanes • Regiochemistry: Anti-Markovinkov • Forms a gem-halide • Halides on same carbon

17. Oxidative Cleavage of Alkynes • 1) KMnO4, OH-, heat 2) H3O+ • Carbon-Carbon triple bond cleaved to C=O • -OH added to Carbons • C-H bonds become C-OH • C-C bonds stay • 1) O3 2) Zn/H2O • Carbon-Carbon triple bond cleaved to C=O • -OH added to Carbons • C-H bonds become C-OH • C-C bonds stay

18. Halogenation Addition to Alkynes • X = F, Cl, Br, I • 1 X-X • Forms alkene • Regiochemistry: Markovinkov • Stereochemistry: Trans, anti-addition • Excess X-X • Forms alkane • Regiochemistry: Markovinkov • Stereochemisty: Anti-addition • Forms two gem-dihalide • Two Halides on both Carbons