Addition to Alkenes

Module 7

Hydroboration Reaction

Markovnikov

Hydroboration Reactants 

Alkene + 

1. BH₃ (Boron) 

2. NaOH, H₂O₂ (Sodium Hydroxide, Hydrogen Peroxide) 

Oxymercuration-Demercuration Reaction

Markovnikov

Oxymercuration-Demercuration Stereochemistry 

Racemic, if chiral centers are created.

⭐ Oxymercuration–demercuration is Markovnikov hydration with no rearrangements and no stereochemical control.
⭐ You do NOT assign syn or anti in the final product.

Hydrogenation Conditions

Hydrogenation Stereochemistry

Hydrogenation Melting Point Consideration

More substitution = more stable, lower activation energy

Hydrogenation Consideration

Hydrogenation is stereoselective, corresponding to addition to the less crowded face of the double bond

Hydrogenation Possible Reactions

Addition of H-X (Hydrohalogenation)

Electrophilic Addition, A_E

Rearrangements possible via methyl, hydride shifts

No stereochemical control, often racemic

Markovnikov Hydrohalogenation Mechanism

Markovnikov Hydrohalogenation Rate-Determining Step

Protonation (Step 1)

Markovnikov Rule

Hydrohalogenation Considerations

Carbocation always forms where it is most stable (tertiary > secondary > primary), this occurs through hydride shift. If chiral centers are generated, it is racemic.

Hydride Shift Mechanism

Anti-Markovnikov Hydrohalogenation (Radicalic Addition of H-Br)

Radicalic Addition, A_R

Anti-Markovnikov Hydrohalogenation (Radicalic Addition of H-Br) Mechanism

Anti-Markovnikov Hydrohalogenation (Radicalic Addition of H-Br) Conditions

H-Br, ROOR (t-Bu-O-O-t-Bu) [peroxide]

Anti-Markovnikov Hydrohalogenation Radical Stability

Markovnikov v. Anti-Markovnikov Hydrohalogenation

DIfferent Types of Hydration Reactions (Addition of H₂O)

1. Acid-Catalyzed Hydration (rearrangement allowed), Markovnikov

   1. 50% H₂SO₄ + 50% H₂O

2. Oxymercuration-Demercuration Hydration (rearrangement prohibited), Markovnikov

   1. Hg(OAc)₂ + H₂O

   2. NaBH₄ 

3. Hydroboration-Oxidation Hydration, anti-Markovnikov 

   1. BH₃ + THF complex, hydroboration

   2. H₂O₂ + KOH + H₂O 

Acid-Catalyzed Hydration summary

Markovnikov

Acid-Catalyzed Hydration (Markovnikov, rearrangement allowed) Mechanism

Acid-Catalyzed Hydration (Markovnikov, rearrangement allowed) Rate-Determining Step

The more stable the carbocation, the faster it is formed. Rearrangement is allowed.

Oxymercuration-Demercuration Hydration (Markovnikov, no rearrangement)

don’t have to know mechanism!

Oxymercuration-Demercuration Hydration (Markovnikov, no rearrangement) Conditions 

1. Hg(OAc)₂ + H₂O 

2. NaBH₄

Hydroboration-Oxidation Hydration (anti-Markovnikov) Conditions

1. BH₃ + THF Complex (Borane, Tetrahydrofuran) 

2. H₂O₂ + KOH + H₂O (Hydrogen Peroxide, Potassium Hydroxide, Water)

Hydroboration-Oxidation Hydration (anti-Markovnikov) Mechanism

Hydroboration-Oxidation Hydration (anti-Markovnikov) Examples

Hydroboration-Oxidation Hydration (anti-Markovnikov) Regioselectivity

Anti-Markovnikov, no rearrangement allowed

Hydroboration-Oxidation Hydration (anti-Markovnikov) Stereochemistry

stereospecific syn addtition, —H and —OH attach to same face of double bond

Halogenation of Alkenes (Addition of Cl₂ and Br₂)

Electrophilic addition, A_e, forms vicinal dihalide (neighboring carbons get halides), only uses Br and Cl because F is explosive and I is endothermic, -anti addition 

Halogenation of Alkenes (Addition of Cl₂ and Br₂) Stereochemistry

anti addition only, syn products NOT formed

Halogenation of Alkenes (Addition of Cl₂ and Br₂) Mechanism

*the most substituted alkenes react the fastest!

Halohydrin Formation (alkene → alkane + —OH, —X)

Halohydrin Formation (alkene → alkane + —OH, —X) Conditions

Elemental Halide (Cl₂ or Br₂), in the presence of H₂O

Halohydrin Formation (alkene → alkane + —OH, —X) Stereochemistry

Stereospecific, -anti addition; Regioselective: —OH ends up on the most substituted C of the double bond.

Halohydrin Formation (alkene → alkane + —OH, —X) Mechanism

electrophilic addition, A_e.

1. Br attaches to both C, removing the pi bond

2. H₂O attacks the most substituted C and that C detaches from Br 

3. Result is -anti addition of H₂O and Br across a double bond 

4. H₂O deprotonates into the final halohydrin product (+—OH, —X) 

Epoxidation of Alkenes

epoxides are heterocyclic compounds, three-membered rings that contain oxygen, requires peroxyacetic acid or MCPBA

Epoxidation of Alkenes Stereochemistry 

Epoxidation of Alkenes Mechanism

-Anti Dihydroxylation (epoxide first, then anti dihydroxylation)

Epoxidation first, followed by subsequent treatment with H₃O⁺, yields trans-diol

-Anti Dihydroxylation Examples

Syn Dihydroxylation Reactants

Can proceed with any of the following conditions

1. OsO₄ (catalytic) +

   1. NMO

   2. tert-butyl hydroperoxide

   3. Na₂SO₃/H₂O or NaHSO₃/H₂O

   4. KMnO₄, NaOH

Ozonolysis 

General reaction = cleaving of a C=C  and the addition of O on each cleaved end, yields C=O (x2) 

Ozonolyis Examples

Ozonolysis Reactant Formation Examples 

Polymerization Mechanism

Polymerization Examples

Formation of Alkenes Examples

Allylic Bromination (anti-addition of X₂) Mechanism 

radicalic substitution, S_R,

NBS allows this reaction to happen at lower temps, otherwise requires 400C