Alkene Reactions

Addition of HX

- used to convert alkenes to haloalkanes

- electrophillic addition reaction

- Markov addition

- carbocation rearrangements are possible

- reagents: pure sample (HX) or polar solvent (acetic acid)

Mechanism:

1) protonation of alkene pi bond to form carbocation

2) reaction with X- to give haloalkane

Acid Catalyzed Hydration

- used to convert alkenes to alcohols

- electrophillic addition reaction

- Markov regioselevtivity

- carbocation rearrangements are possible

- reagents: H20, H+, H2SO4 (aq)

Mechanism:

1) protonation of alkene pi bond to form carbocation

2) reaction with H2O to give alcohol product

Addition of Halogens

- used to convert alkenes to vicinal dihalide

- electrophillic addition reaction - attacks the less substituted Carbon

- anti addition stereoselectivity

- rearrangements do not occur

- stereospecific (Z alkenes give different products than E)

- reagents: CH2Cl2

Mechanism:

1) attack by the alkene pi bond to give bridged bromonium ion intermediate

2) attacked by X- from the backside to give vicinal dihalide

Addition of HOCl and HOBr

- used to convert alkenes to halohydrins

- Markov regioselectivity (OH bonds to more substituted carbon)

- rearrangements do not occur

- anti-addition stereoselectivity

- stereospecific (Z alkenes give different products than E)

- reagents: Br2/H2O- Br

Mechanism:

1) attack by the alkene pi bond to give bridged bromonium ion intermediate

2) attacked by H2O from the backside to give new intermediate

3) loss of proton to give halohydrin

Oxymercuration-Reduction

- used to convert alkenes to alcohols

- anti addition stereoselectivity

- Markov regioselectivity (OH group bonded to the more highly subsituted carbon)

- rearrangements do not occur

- stereospecific (Z alkenes give different products than E)

- first step is anti stereoselective

- second step is random (anti & syn addition)

- rearrangements are not observed

- reagents: 1) Hg(OAc)2, H2O --> 2) NaBH4

Mechanism:

1) attack by the alkene pi bond to give bridged bromonium ion intermediate

2) attacked by H2O from the backside to give new intermediate

3) loss of proton to give alcohol

Hydroboration-Oxidation

- used to convert alkenes into alcohols

- non-Markov regioselectivity

- syn-stereoselectivity

- rearrangements do not occur

- stereospecific (cis alkenes give different products than trans alkenes)

Mechanism

1) coordination by the alkene pi bond to vacant 2p orbital of Borane

2) 4 membered ring ring transition state

3) adds H to more substituted/ Boron to less substituted

4) basic peroxides replaces the boron with OH

Oxidation to a glycol by OsO4

- used to convert alkenes to diols

- syn addition

- no rearrangement

- stereoselctive (syn addition gives different products than trans alkenes)

- Reagents: 1) OsO4 --> 2) NaHSO3, H2O

Mechanism

1) addition of OsO4 to get a cyclic osmate

2) complicated electron transfer steps to get a cis glycol

Oxidation by Ozone

- used to convert alkenes into 2 carbonyl groups

- Reagents: 1) O3, CH3Cl2 2) (CH3)2S (reducing agent)

Mechanism

1) formation of a ozonide intermediate

2) reduction using DMSO to give carbonyl products

Addition of H2; Catalytic reduction

- used to reduce alkenes into alkanes

- syn stereoselctivity

- without rearrangement

- stereospecific (Z alkenes give different products than alkene)

- if one side is more hindered, the reaction is stereoselective for the less hindered side

- Reagents: H2, metal catalyst;

- Conditions: 25 C, 3 atm

Mechanism

1) H & alkene are adsorbed on metal surface

2) one H atom is transferred to the alkene (forms new CH bond)

3) 2nd C-H bond is formed, and the alkane is desorbed

Enantiselective Reduction

- used to convert methyl groups on alkenes to R or S configs

- S-BINAP makes methyl group on trans alkene into R config

- S-SINAP makes methyl group on cis alkene into S config

- Reagents: H2, (R)/(S)- BINAP-RuCl3