Organic Chemistry Reviews Chapter 8 Cindy Boulton November 23, 2008

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  Practice!

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

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 = 0 o

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

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:  H 2 /Pd/CaCO 3  Stops as alkene,  Ca prevents alkene from being hydrogenated  Stereochemistry: syn addition  Hydrogens added to same side  Form Z or Cis alkene

Hydrogenation of Alkynes cont.  H 2 /Ni 2 B 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, C 2 H 5 NH 2 2) NH 4 Cl  Sterochemistry: anit addition  Hydrogens added to opposite side  Form E or Trans alkene

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

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

H 3 O + /H 2 O to Alkenes  Hydration addition to from alcohol  Acid used as catalyst  Regiochemistry: Markovinkov  Stereochemistry: Racemic Mixture  Mechanism:  Double bond of alkene broken  Hydrogen from H 3 O + added to Carbon with most Hydrogens  Forms Carbocation  H 2 O added to Carbocation forming protinated alcohol  Excess H 2 O removes Hydrogen forming an alcohol

Halogenation Addition to Alkenes  X-X addition  X = Cl, Br, I  CCl 4 or CH 2 Cl 2 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.

X 2 /H 2 O (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 H 2 O removes extra Hydrogen attached

Syn Hydroxylation of Alkenes  (HO-OH) addition  OsO 4 /NaHSO 3  KMnO 4 /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

Oxidative Cleavage of Alkenes  1) KMnO 4, OH -, heat 2) H 3 O +  Carbon = Carbon divided to form C=O  C-C bonds stay  C-H bonds change to C-OH  1) O 3 2) Zn/H 2 O  Carbon = Carbon divided to form C=O  C-C bonds stay  C-H bonds stay

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

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

Oxidative Cleavage of Alkynes  1) KMnO 4, OH -, heat 2) H 3 O +  Carbon-Carbon triple bond cleaved to C=O  -OH added to Carbons  C-H bonds become C-OH  C-C bonds stay  1) O 3 2) Zn/H 2 O  Carbon-Carbon triple bond cleaved to C=O  -OH added to Carbons  C-H bonds become C-OH  C-C bonds stay

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