Alkene Reactions

Addition reaction

The pi bond breaks and two new sigma bonds form on the two alkene carbons

Oxidation in organic chemistry

Gaining C-O bonds or losing C-H bonds

Reduction in organic chemistry

Gaining C-H bonds or losing C-O bonds

Role of the alkene pi bond in additions

It is electron rich so it acts as the nucleophile

Markovnikov's rule

H (or the first group added) goes on the less substituted carbon so the more stable carbocation forms

Anti-Markovnikov

H goes on the more substituted carbon and the other group on the less substituted carbon

Regioselectivity

When one direction of adding to the double bond is preferred

Stereoselectivity

When one stereoisomer forms more than the others

Syn addition

Both new groups add to the same side (both wedges or both dashes)

Anti addition

The two new groups add to opposite sides (one wedge and one dash)

Racemic mixture

Equal amounts of both enantiomers. Forms whenever a new stereocenter is made from a flat alkene or carbocation

HX (HCl or HBr or HI) plus alkene

Alkyl halide. Markovnikov. Racemic if a stereocenter forms. Can rearrange

Mechanism of HX addition

Pi bond attacks the H of HX to make a carbocation (slow step) then X⁻ attacks the carbocation

Rate determining step of HX addition

Forming the carbocation

H₂O with H₂SO₄ plus alkene

Alcohol. Markovnikov. Racemic if a stereocenter forms. Can rearrange. Acid is a catalyst

Mechanism of acid catalyzed hydration

Pi bond grabs H from H₃O⁺ to make a carbocation then water attacks then water removes the extra H from the O to give the alcohol and regenerate H₃O⁺

Br₂ or Cl₂ plus alkene

Vicinal dihalide (two X on neighboring carbons). Anti addition. No rearrangement

Mechanism of halogenation

Pi bond attacks X₂ to form a bridged halonium ion then X⁻ attacks from the back side giving anti addition

Bridged halonium ion

A three membered ring with the halogen bonded to both carbons. It blocks one face so the nucleophile attacks from the opposite side

Br₂ or Cl₂ in water plus alkene

Halohydrin. OH goes on the more substituted carbon and X on the less substituted carbon. Anti addition

Why OH goes on the more substituted carbon in halohydrin formation

That carbon of the bridged halonium is the most carbocation-like so water attacks it

1) BH₃ or R₂BH then 2) H₂O₂ and NaOH

Hydroboration-oxidation. Alcohol with OH on the less substituted carbon (anti-Markovnikov). Syn addition. No rearrangement

Do you need the hydroboration mechanism

No. Just know it is anti-Markovnikov and syn with no rearrangement

1) OsO₄ then 2) NaHSO₃ and H₂O

1‚2-diol (vicinal diol or glycol). Syn addition through a cyclic osmate

Why OsO₄ gives syn addition

OsO₄ binds both oxygens to the same face of the alkene in a cyclic osmate

1) O₃ then 2) Me₂S (DMS)

Ozonolysis. Cuts the C=C and makes two C=O groups. Not regioselective or stereoselective

Ozonolysis products

An alkene carbon with an H becomes an aldehyde and one with two carbon groups becomes a ketone. A ring alkene gives one chain with two C=O

H₂ with Pd (or Pt or Ni)

Alkane. Syn addition of two H. Reduces every C=C in the molecule

Which reactions are syn

Hydroboration and OsO₄ and H₂ with Pd

Which reactions are anti

Br₂ or Cl₂ and Br₂ or Cl₂ in water

Which reactions are Markovnikov

HX and H₂O with H₂SO₄ and halohydrin formation

Reaction coordinate diagram for HX addition

Two humps with the first one higher (carbocation formation is the slow step) and the carbocation in the valley between