Chapter 8: Alkenes Flashcards
Markovnikov Hydrohalogenation
Alkene
Reagent
HX (hydrogen halide)
HCl, HBr, or HI
Mechanism
The treatment of alkenes with HX results in the Markovnikov addition of HX across the an alkene
Favors low temperatures
Stereospecificity
Halogen can attack the carbocation from either side; results in a pair of enantiomers
If carbocation rearrangement is posible a mixture of products is possible with the major product resulting from the more stable arrangement
anti-Markovnikov Hydrohalogenation
Alkene
Reagent
HX & ROOR
HCl, HBr, or HI & ROOR
Mechanism
Even trace amounts of alkyl peroxides (ROOR) will cause HX to add across an alkene in an anti-Markovnikov fashion
Stereospecificity
Halogen can attack the carbocation from either side; results in a pair of enantiomers
Acid-Catalyzed Hydration
Alkene
Reagent
H3O+
H2SO4 & H2O
Mechanism
Markovnikov addition of a water molecule across a π bond in the presence of and acid
Proceeds via a carbocation intermediate followed by an intermediate with an oxygen atom exhibiting a positive charge called an oxonium ion
Nucleophiles other than water such as MeOH or EtOH can also attack the carbocation resulting in the addition of a different group
Stereospecificity
Water can attack the carbocation intermediate from either side; results in a racemic mixture of products when a chiral center is present
Position of equilibrium in acid-catalyzed dehydration
The concentration of acid will dictate whether an addition reaction is favored or an elimination reaction
A dilute acid will favor an addition reaction because more water is present
A concentrated acid will favor and elimination reaction because less water is present
Oxymercuration-Demurcuration Addition
Alkene
Reagent
- Hg(OAc)2 (mercuric acetate) & Nuc
- NaBH4 (sodium borohydride)
Mechanism
Mercuric acetate [Hg(OAc)2] dissociates to form a mercuric cation which attacks the π bond and forms a mercurinium ion intermediate that prevents carbocation rearrangement
Regioselectivity
Markovnikov- H is installed at the less substituted vinylic position while the Nuc is installed at the more substituted vinylic postion
Hydroboration-Oxidation
Alkene
Reagents
- BH3 (borane) & THF (tetrahydrofuran)
- H2O2 (hydrogen peroxide) & NaOH
Mechanism
Anti-Markovnikov addition of a water molecule across a π bond
Stereospecificity
Syn addition- when a chiral center is present; produces a pair of enantiomers
Catalytic Hydrogenation
Alkene
Reagents
Heterogenous catalysts- do NOT dissolve in the reaction medium
H2 & Pt, Pd, or Ni
or
Homogenous catalysts- are soluble in the reaction medium
H2 & Wilkinson’s catalyst
Mechanism
Involves the addition of molecular hydrogen (H2) across a π bond in the presence of a metal catalyst
Regioselectivity
Irrelevant as hydrogen is added to both sides of the π bond
Stereospecificity
Syn addition- when a chiral center is present; produces a pair of enantiomers
Asymetric hydrogenation is possible if a chiral catalyst is used such as BINAP; produces only ONE enantiomer
Halogenation
Alkene
Reagents
X2
Br2 or Cl2
Mechanism
Two halogen atoms are added to both vinylic positions of a π bond via a bromonium ion or a chloronium ion intermediate
Regioselectivity
Irrelevant as halogens are added to both sides of the π bond
Stereospecificity
Anti addition- when a chiral center is present; produces a pair of enantiomers
Halohydrin formation
Alkene
Reagents
X2 & H2O
Br2 or Cl2
Mechanism
Similar to halogenation however when water is present the water replaces the second halogen as the nucleophile and installs an OH group to the other side of the π bond
Regioselectivity
The OH group is installed at the more substituted position
Stereospecificity
Anti addition- when a chiral center is present; produces a pair of enantiomers
Anti Dihydroxylation
Alkene
Reagents
RCO3H (peroxy acid) & H3O+
CH3CO3H (peracetic acid) or MCPBA
Mechanism
Adds OH groups to BOTH sides of a π bond through the formation of an epoxide- a three-membered cyclic ether
Under acid-catalyzed conditions epoxides can be opened by a variety of other nucleophiles such as MeOH or EtOH; will attack at the Markovnikov position
Stereospecificity
Anti addition- when a chiral center is present; produces a pair of enantiomers
Syn dihydroxylation
Alkene
Reagents
- OsO4 (osmium tetroxide)
- Na2SO3 or NaHSO3 (Sodium sulfite or bisulfite)
or
OsO4 & NMO or tBuOOH (tert-Butyl hydroperoxide)
or
Cold KMnO4 (potassium permanganate) & NaOH
Mechanism
Osmium tertoxide or cold potassium permanganate are added across a π bond in a concerted process
Regioselectivity
Irrelevant as OH groups are installed at both vinylic positions
Stereospecificity
Syn addition- when a chiral center is present; produces a pair of enantiomers
Ozonlysis
Alkene
Reagents
- O3
- DMS or Zn/H2O
Mechanism
π bond is completely split apart to form two C=O bonds through an ozonide intermediate which is then treated with a mild reducing agent
Regioselectivity and stereospecificity are both irrelevant
