Addition Reactions Alkenes

Hydrohalogenation

Reactants:

• alkene + HX (Br, Cl, I)

Use Case:

• H & X are added across a pi bond

Regioselectivity:

• Markovnikov addition

In the presence of alkyl peroxides (ROOR):

• anti-markovnikov addition

Stereochemistry:

• 1 chiral center, a racemic mixture of a pair of enantiomers

Acid-Catalyzed Hydration

Reactants:

• alkene, H2O, [Acid] (catalyst, not consumed)

Use Case:

• add an H & OH across a pi bond

Rate:

• Increases as # of alkyl substituents increases

Regiochemistry:

• Markovnikov addition

Stereochemistry:

• 1 chiral center (maybe), racemic mixture of a pair of enantiomers

Equilibrium:

• Cold and dilute acid (high concentration of H2O): favors alcohol through an addition reaction

• Hot and concentrated acid (Low concentration of H2O): favors alkene through an elimination reaction

Oxymercuration-Demercuration

Reactants:

1. Hg(OAc)2, Nuc-H

2. NaBH4

Use Case:

• addition of H & OH across a pi bond w/o carbocation rearrangement:

Regioselectivity:

• Markovnikov addition

Mercurium ion:

• hybrid structure character of a 3-member ring & carbocation. Partial positive charge on more substituted carbon allows for nucleophilic attacks but not carbocation rearrangements

Hydroboration-Oxidation

Reactants:

1. BH3 (THF)

2. H2O2, NaOH

Regioselectivity:

• anti-Markovnikov addition

• (BH2 is installed at less substituted position & later replaced w/ OH-)

Electronic consideration:

• Hydride shift isn’t 100% simultaneous; attack of pi bond can give a vinyllic carbon a partial positive charge, which causes the shift. The partial positive charge is better stabilized on the more substituted carbon, resulting in the BH2 group being placed on the least substituted carbon.

Steric Hindrance:

• The transition state is lower in energy and less crowded when the BH2 group is placed on the less substituted carbon

Stereospecificity:

• Syn Addition

• H & OH are added across the pi bond in a concerted fashion, forcing them to be on the same face

  • 0 chiral centers: Stereospecificity is irrelevant

  • 1 chiral center: racemic mixture of a pair of enantiomers

  • 2 chiral centers: racemic mixture of only syn enantiomers

Catalytic Hydrogenation

Reagents:

• Heterogenous: H2, Pd, Pt, Ni

• Homogenous: Wilkinson’s Catalyst

Use case: 

• Add H to both sides of a pi bond

• Reduce an alkene to an alkane in the presence of a metal catalyst

Stereospecificity:

• Syn Addition

• 0 chiral centers: one product

• 1 chiral center: racemic mixture of a pair of enantiomers

• 2 chiral centers: syn enantiomers only

• Symmetrical alkenes produce a meso compound rather than a pair of enantiomers

Homogenous Catalysts:

• Wilkinson’s catalyst

• Soluble in reaction medium

Halogenation

Reagents:

• X2 (Cl2, Br2), alkene, Non-nucleophilic solvent

Use case:

• Converting an alkene to an alkane and adding Cl2 or Br2 to either side

Stereochemistry:

• anti-addition

• Depends on config of starting alkene

• cis: anti-addition

• trans: meso

Halohydrin Formation:

Reagents:

• X2 (Cl, Br), H2O, alkene

Use case:

• Addition of chlorine or bromine and an OH- across an alkene

If halogenation occurs in H2O, the halogenium ion can be captured by a water molecule rather than the halogen.

Regiochemistry:

• OH group is installed at more substituted position

Anti-Dihydroxylation

Reactants:

1. peroxy acid (RCO3H), MCPBA

2. H3O+

Use Case:

• Converting an alkene to an alkane and adding OH- to both sides

Stereochemistry: anti-addition

Syn-Dihydroxylation

Reactants:

1. OsO4, OsO4 (catalytic), & NMO or t-butOOH

2. Na2SO3/H2O, NaHSO3/H2O

or

1. KMnO4 (cold)

2. NaOH

Use Case:

• addition of OH on the same side of the alkene

Oxidative Cleavage (ozonolysis)

Reactants:

1. O3

2. DMS, Zn/H2O

3. adds across an alkene & cleaves c-c bond