Chem 334 - Fall 1999
Organic Chemistry I
Dr. Carl C. Wamser

Chapter 6 Notes: 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

Reaction Mechanisms

• a step-by-step account of how a reaction occurs (and why)
• bond-breaking steps:
  homolytic: one electron to each fragment
  (generates radicals)
  heterolytic: both electrons to one fragment
  (generates ions)
• bond-making steps:
  homogenic or heterogenic

Electron Movement

• electron-pushing arrows indicate the flow of electrons in a mechanism
• electrophile:
  electron-deficient species seeking a pair of electrons (a Lewis acid )
• nucleophile:
  electron-rich species that can provide a pair of electrons (a Lewis base)

Potential Energy Diagrams

• visualize energy changes during a reaction
• y-axis = potential energy (G or H)
  uphill = higher energy
  downhill = lower energy
• x-axis = reaction coordinate

Heats of Reaction

• delta H - enthalpy change
• delta H = H(products) - H(reactants)
• exothermic - H < 0 (negative)
  reaction gives off heat
• endothermic - H > 0 (positive)
  reaction absorbs heat

Activation Energy

• there is usually an energy barrier between reactants and products
• activation energy represents the highest amount of energy necessary to carry out the reaction (starting from reactants)

The Transition State

• structure of the molecule(s) at the highest point along the reaction pathway
• the stability of the transition state (relative to reactants) determines Ea (rate of reaction)

Kinetics

• Arrhenius equation:
  k = A exp( -Ea/RT )
• exponential temperature dependence
  if Ea < 20 kcal/mol, reactions are usually fast at room temperature
  for Ea > 20 kcal/mol, reactions usually require heating

HCl plus Ethene

• CH2=CH2 + H-Cl --> CH3-CH2-Cl
• an electrophilic addition
• reaction type: addition
• reagent type: an electrophile
  HCl, actually H+, a strong Lewis acid

Addition Mechanism

• pi bond is relatively reactive, especially towards electrophiles
  it provides a good source of electrons
• addition of H+ to CH2=CH2 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-

Carbocation Intermediate

• an intermediate is formed in the reaction mechanism
  CH2=CH2 + H+ --> CH3-CH2+
• carbocation: a carbon atom with only 3 bonds (6 e-) and a positive charge
• structure: sp2 hybridized (trigonal)

Formation of Chloroethane

• the reaction is completed as chloride anion (a nucleophile) adds to the carbocation (an electrophile)
  CH3-CH2+ + Cl- ---> CH3-CH2-Cl

Alkene Addition Reactions

• pi bonds undergo addition reactions
  CH2=CH2 + HCl --> CH3CH2Cl
• in general,
  C=C + HX --> H-C-C-X
• alkenes react with hydrogen halides to form alkyl halides

Addition of HX to Alkenes

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

Reaction Notation

• reactants -------> products
  focus on the organic reactants and products
• show reagents over the arrow
• show solvent and conditions under the arrow
  (or show full balanced reaction)

Orientation of Addition

• regiochemistry:
  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)

Carbocations

• structure: trigonal (sp2)
• stability: 3° > 2° > 1°
• more alkyl groups stabilize a cation by electron donation to the electron-deficient (6-electron) carbocation
• hyperconjugation: electron donation from adjacent C-H sigma bond to empty p orbital of teh carbocation

Markovnikov Addition


Carbocation Rearrangements

• carbocations easily rearrange to more stable forms
  e.g., 1° --> 2° , 1° --> 3° , or 2° --> 3°

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

Hydration Mechanism


Halogenation of Alkenes

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

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

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

Oxymercuration

• addition of Hg(OAc)2 followed by NaBH4 adds H and OH
• follows Markovnikov Rule, but no rearrangements
• cyclic mercurinium ion intermediate
• anti addition

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

Reduction of Alkenes

• reduction - addition of H2
  (or removal of O)

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

R-O-H + H2 ---> R-H + H2O

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

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

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

Hydroxylation

• alkene + OsO4 --(peroxide)--> 1,2-diol
• addition of two OH groups is syn
• cyclopentene --> cis-1,2-cyclopentanediol

Oxidative Cleavage

• C=C --> C=O + O=C
  
• acidic KMnO4 causes cleavage
• ozone (O3) causes cleavage
• sometimes useful degradation method to identify unknown compounds

Balancing Redox Reactions

• identify the two half-reactions
  - what gets oxidized, what gets reduced?
• balance all elements except O and H
• balance O
  use H2O (in acid) or OH- (in base)
• balance H
  use H+ (in acid) or H2O (in base)
• balance charge with electrons (e-)

Conjugated Dienes

• pi bonds are conjugated when there can be additional pi bonding between the p orbitals
• extra stability amounts to a few kcal