Alkene Reactions

Reaction Types

• additions
  A + B --> C
  H-Cl + CH2=CH2 --> CH3-CH2-Cl
• eliminations
  X --> Y + Z
  CH3-CH2-Cl --> CH2=CH2 + H-Cl
• substitutions
  A-B + C-D --> A-C + B-D
  CH4 + Cl2 --> CH3-Cl + H-Cl
• rearrangements
  X --> Y
  cyclopropane --> propene

Alkene Addition Reactions

• pi bonds undergo addition reactions
• in general,
  C=C + X-Y --> X-C-C-Y
• e.g., alkenes react with hydrogen halides to form alkyl halides

Addition Mechanism

• pi bond is relatively reactive, especially towards electrophiles
  it provides a good source of electrons
• addition of H+ to C=C forms a new C-H sigma bond
  the electrons for the new bond came from the pi bond
  the other C is left with only 6 e-

Addition of HX to Alkenes

• cyclohexene + HBr --> bromocyclohexane
• 1-methylcyclohexene + HBr --> 1-bromo-1-methylcyclohexane
  (not 1-bromo-2-methylcyclohexane)

Orientation of Addition - Regiochemistry

• regiochemistry: possible formation of different constitutional isomers
  specific orientation of addition
  (which C gets H, which gets X?)
• alkene additions are regioselective:
  one direction of addition is usually preferred

Markovnikov's Rule

• the original:
  add H to the C with more H's
  (or to the C with fewer alkyl groups)
• the reason:
  add H+ to form the more stable cation
  CH3CH=CH2 + HCl --->
  CH3CH+CH3 (not CH3CH2CH2+)
  ---> CH3CHClCH3 (not CH3CH2CH2Cl)

Markovnikov Addition

Hydration of Alkenes

• alkene + water --> alcohol
  CH2=CH2 + H2O --(H+)--> CH3CH2OH
• mechanism:
• step 1:
  addition of H+ electrophile to pi bond
• step 2:
  addition of H2O nucleophile to cation
• exact reverse of E1 elimination

Hydration Mechanism

follows a different mechanism involving free radicals

Br• adds first, forming the more stable (more substituted) radical

H is added second (from HBr) - to the more substituted C (anti-Markovnikov)

Catalytic Hydrogenation

CH2=CH2 + H2 ---> CH3-CH3

• requires an active catalyst, typically Pt, Pd, Ni, PtO2
• reaction occurs on the surface
• both Hs are delivered to the same side of the pi bond

Stereochemistry - Syn Addition

• syn stereochemistry: two new groups are added to the same side of the original pi bond

1,2-dimethylcyclohexene + H2 --(cat)-->cis-1,2-dimethylcyclohexane (no trans)

• syn - describes the process
• cis - describes the product

Halogenation of Alkenes

CH2=CH2 + Cl2 ---> Cl-CH2-CH2-Cl

• mechanism:
  Cl2 is an electrophile (adds Cl+)
  then Cl- is a nucleophile

Stereochemistry - Anti Addition

• anti stereochemistry: two new groups are added to opposite sides of the original pi bond

cyclopentene + Br2 ---> trans-1,2-dibromocyclopentane (no cis)

• anti - describes the process
• trans - describes the product

Bromonium Ion

• carbocations can be stabilized by bonding to a neighboring Br
  (also works with Cl, but less favorable)

Halohydrins

• addition of halogens in water
  adds X and OH to the pi bond
• water is the nucleophile that adds anti to X

Hydroboration/Oxidation

• addition of BH3 followed by H2O2 adds H and OH
• "anti-Markovnikov" addition
• completely syn addition
• consider B as the electrophile that adds first to the pi bond
• addition of B and H is concerted (simultaneous)

Oxidation of Alkenes

• oxidation - addition of O
  (or removal of H2)
  RCH2OH ---> RCH=O ---> RCOOH
• there are a wide variety of oxidizing agents:
  O2, O3, KMnO4, CrO3, Na2Cr2O7
  metals in high positive oxidation states

Oxidative Cleavage

C=C --> C=O + O=C

• acidic KMnO4 causes cleavage
• ozone (O3) causes cleavage
• sometimes useful degradation method to identify unknown compounds