Reactions of Alkenes

Hydrogenation Catalytic Reduction

-syn-addition

-H2 + Metal catalyst (Pd, Pt)

-Needs metal catalyst as surface for H-bonding + for reaction with alkene.

Hydrohalogenation

Markovnikov addition of H and X.

Rate det. step = formation of carbocation intermediate.

Methyl and Hydride shifts possible.

Most stable carbocation = fastest reaction. Polar solvents also speed this up.

HI > HBr > HCl > HF

Halogenation

Anti addition (bonds form in separate steps)

forms via cyclic bromonium ion

X2 in DCM

Halohydrin Formation

Anti-addition of halogen and hydroxide ( if -ROH, forms ether)

X reacts as the electrophile so the C-O bond forms at the more stable cation center

Reaction proceeds via cyclic halonium ion

Acid-Catalyzed Hydration

Converts alkenes to alcohols

Markovnikov addition of hydrogen and hydroxide (if -ROH, forms ether)

Proceeds via carbocation intermediate therefore the the most substituted compound reacts fastest

Mechanism:

1) protonation of alkene pi bond to form carbocation

2) reaction with H2O to give alcohol product

Oxymercuration-Demercuration (Reduction)

Markovnikov addition of hydroxide in anti-formation with Hydrogen, no H+ shift

1. Hg(OAc)2/THF/H2O

2. NaBH4 (to reduce Hg off)

Hydroboration-Oxidation

Anti-Markovnikov hydroxide addition with hydrogen

Syn (concerted mechanism)

1. BH3

2. H2O2/NaOH

B atom adds at the least substituted end due to a combination of electronic and steric effects

Epoxidation

Syn addition (concerted), stereochemistry of alkene preserved

a peroxide (RCO3H) creates single oxygen bridge

mcpba, H2O2, R-CO3H can be used

Ozonolysis

Oxidative cleavage using ozone.

Secondary carbons always turn into ketones

Primary carbons turn into aldehydes if Zn in acetic acid (reductive work-up) or to carboxylic acids if H2O2 (Oxidative work-up)

Cyclopropanation

Creates propane bridge

ICH2-ZnI/Et2O

maintains stereochemistry

Free Radical Addition of HBr

Anti-Markovnikov

Only with HBr

Dihydroxylation of Alkenes

anti

acid-catalysed ring opening

Dihydroxylation of Alkenes 2

KMnO4, OH- (COLD)

OR

OsO4, tBuOOH, OH-

Syn addition

Radical halogenation of alkenes

Br2

UV light

Instead of addition, a Br will be substituted to the most stable position (allylic position)