CH 236 - Ch. 8: Reactions of Alkenes

Addition Reactions in Alkenes

add a variety of substituents accross the double bond creating two new sigma bonds

Pi Bond acting as a Bronsted Base

takes a proton

Pi Bond acting as a Nucleophile

bonds with an electrophile

Favorable temperatures of addition reactions

- lower temp but not by heat

- favors enthalpy and lower temperatures

(elimination reactions favor entropy and higher temperatures)

Markovnikov's Rule

• H is added to the alkene carbon with more Hs attached

• Other is added to the more substiutude carbon

Hydrohalogenation Reactions

• Addition of HX to an alkene

• HCl, HBr, or HI added across a pi bond to make an alkyl halide

Hydrohalogenation Reagents

HBr, HCl, HI

Hydrohalogenation Additions

H and X

Hydrohalogenation Regioselectivity

Markovnikov's Rule

Hydrohalogenation Rearrangement

• carbocation intermediate (stability

• 1,2-hydride or 1,2-methyl shifts as needed

Hydrohalogenation stereoselectivity

• Not stereoselective because the carbocation is sp2 hybridized

• if chiral, enantiomers is in a racemic mixture

Hydrohalogenation Mechanism

1.) Formation of Carbocation (P.T.) (RDS)

2.) Carbocation Rearrangement if needed

3.) Nucleophilic Attack at Most Stable Carbocation

Acid-Catalyzed Hydration Reaction

• addition of water across an alkene (hydration)

• product is an alcohol

• Requires sulfuric acid catalyst

• Electrophile is hydronium

Acid-Catalyzed Hydration Reagents

H2O, H2SO4 (cat)

Acid-Catalyzed Hydration Additions

H and OH

Acid-Catalyzed Hydration Regioselectivity

• Markovnikov's Rule

Acid-Catalyzed Hydration Rearrangement

• carbocation intermediate

• 1,2-hydride or 1,2 methyl

Acid-Catalyzed Hydration Stereoselectivity

• if chiral, racemic mixture of both enantiomers

• nonstereoselective

Acid Catalyzed Hydration Reaction Mechanism

1. Formation of the hydronium electrophile

2. Formation of the Carbocation using Alkene as the base (P.T)

3. Nucleophilic Attack of Water to the Carbocation

4. Deprotonation of the Oxygen to lead to product (P.T)

Oxymercuration/Reduction Reaction

• two steps that form an alcohol

1.) Coordination of Mercury acetate

2.) Reduction reaction with NaBH4

Oxymercuration/Reduction Reagents

1) Hg(OAc)2, H2O

2) NaBH4

Oxymercuration/Reduction Additions

H and OH

Oxymercuration/Reduction Regioselectivity

Markovnikov's Rule

Oxymercuration/Reduction Rearrangement

• does not rearrange

• limited to two carbons of alkene

Oxymercuration/Reduction Stereochemistry

• chrial = racemic

• Anti-steroselective

Hydroboration Reaction

• forms alcohol through two steps

• 1) BH3 delivers hydride; THF: solvent

• 2) NaOH adds the OH

Hydroboration Reagents

1. BH3, THF

2. H2O2, NaOH

Hydroboration Additions

H and OH

Hydroboration Regioselectivity

• Non-Markovnikov

• OH adds to less substituted Carbon; H to the more substituted

Hydroboration Rearrangement

• No rearrangement because no carbocation intermediate

Hydroboration Steroselectivity

• synsteroselective

• racemic mixture of both enantiomers

Hydroboration Mechanism

• Boron coordinates to the least substituted C because steric effects and delivers hydride to the most substituted

Dihydroxylation Reaction

• Makes 1,2-diol product (2 OHs on alkene carbons) through 2 steps

1.) Osmium tetraoxide coordinates two alkene

2.) cyclic osmate intermediate reduced to make syn diol

Dihydroxylation Reagents

1. OsO4

2. NaHSO3, H2O

(Can also include KMnO4 but less common)

Dihydroxylation additions

2 OHs

Dihydroxylation Regioselectivity

adds OH to alkene

Dihydroxylation Stereoselectivity

• If chiral, syn

• watch for meso!

Dihydroxylation Rearrangement

no rearrangement

Dihydroxylation Mechanism

1.) Osmium tetraoxide coordinates two alkene

2.) cyclic osmate intermediate reduced to make syn diol

Halohydrin Reaction

• reaction adds OH and a halogen (Cl or Br) across alkene

Halohydrin Reagents

X2, H2O

Halohydrin Regioselective

Markovnikov

• OH adds to most substituted carbon

• Halogen adds to less substituted

Halohydrin Stereoselectivity

anti-stereoselective

chiral = racemic mixture of enantiomers

Halohydrin Rearrangement

• no carbocation intermediate

• no rearrangement

Halohydrin Mechanism

1.) Formation of Bromium or Chlorinium Cation (Bridged 3-membered ring)

2.) Nucleophilic attack of water to the most substituted Carbon, opening 3 ring

3.) Deprotonation of water to yield product

Ozonolysis Reaction

• Ozone + DMS is used to split the alkene into 2 carbonyls (aldehydes or ketones)

Ozonolysis Reaction/Reagents

O3, DMS or (CH3)2S

Ozonolysis Acyclic Prediction

1. Divide alkene into two halves

2. If alkene has 2 carbon chains attached = KETONE

3. H attached to one or both of alkene C = ALDEHYDE

**alkene carbons becomes the carbonyl carbons

Ozonolysis Cyclic Prediction

1. Start 1 carbon of the double bond and count backwards around the ring and draw a zig zav carbon chain of same length; first and last carbon both become your carbonyl carbon

2. add substituents