Portland State University - - Professor Carl C. Wamser

Chemistry 331 - Fall 1997

Chapter 4 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)

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

Equilibrium

• for a general reaction:
  A + B <==> C + D
• equilibrium constant, K
  K = [C] [D] / [A] [B]
• favorable reactions have large K
• unfavorable reactions have small K

Kinetics

• rates of reaction
• may or may not correlate with the favorability of the equilibrium
• rate depends on the mechanism
• is there a good way to get from reactants to products?

Heats of Reaction

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

Potential Energy Diagrams

• draw exothermic reactions downhill
• draw endothermic reactions uphill

Activation Energy, Ea

• there is usually an energy barrier between reactants and products
• activation energy represents the highest amount of energy necessary while travelling along the minimum-energy (easiest) pathway from reactants to products

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)

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

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

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)

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 + KMnO4 --(base)--> 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-)
  

Polymers

• long chains of repeating units (monomers)

n CH2=CH2 --(init)--> (init)-(CH2-CH2)n-

n=100-10,000 polyethylene

• has properties like a very long alkane
• many polyalkenes are commercially important materials and plastics
  e.g., PVC, Teflon, Orlon

Chain Reactions

• polymerization occurs by a free radical chain mechanism
• initiation - generation of the first free radical from an initiator
  R-O-O-R --(heat)--> 2 R-O·
  (initiators have one weak bond)

Chain Reactions

• propagation - radical adds to a p bond
  RO· + CH2=CH2 ---> RO-CH2-CH2·
• note that the product is also a radical
  RO-CH2-CH2· + CH2=CH2 ---> RO-CH2-CH2-CH2-CH2· ---> etc.
• typically this occurs hundreds or thousands of times
  (until radicals recombine - termination)

Substituted Monomers

• radical additions follow the Markovnikov Rule:
  add radicals to form the more stable radical intermediate
• radical stability is like cation stability: 3° > 2° > 1°
  this leads to polymers with alternating substituents

Vinyl Polymers

• polyvinyl chloride
• polypropylene
• polystyrene

Alkyne Additions

• similar to alkenes but more reactive
• Markovnikov Rule is followed
• excess reagent gives double addition
• single addition is usually possible
• single addition gives alkene product, which may be cis (syn addition) or trans (anti addition) or nonspecific

Reduction of Alkynes

• excess H2 + catalyst gives alkanes