ch3 ch2 ch3+cl2

Chem 334 - Fall 2001
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

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  • a step-by-step trương mục of how a reaction occurs (and why)
  • bond-breaking steps:
    homolytic: one electron to lớn each fragment
    (generates radicals)
    heterolytic: both electrons to lớn 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 lớn 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 lớn reactants) determines Ea (rate of reaction)


  • Arrhenius equation:
    k = A exp( -Ea/RT )
  • exponential temperature dependence
    if Ea < trăng tròn kcal/mol, reactions are usually fast at room temperature
    for Ea > trăng tròn 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 lớn 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 lớn 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 lớn sườn alkyl halides

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


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

Markovnikov Addition

Carbocation Rearrangements

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

Hydration of Alkenes

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  • alkene + water --> alcohol
    CH2=CH2 + H2O --(H+)--> CH3CH2OH
  • mechanism:
  • step 1:
    addition of H+ electrophile to lớn pi bond
  • step 2:
    addition of H2O nucleophile to lớn 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 lớn 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 lớn a neighboring Br
    (also works with Cl, but less favorable)


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


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


  • 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 lớn 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 lớn the same side of the pi bond

Syn Addition

  • syn stereochemistry: two new groups are added to lớn 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


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

Oxidative Cleavage

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  • C=C --> C=O + O=C
  • acidic KMnO4 causes cleavage
  • ozone (O3) causes cleavage
  • sometimes useful degradation method to lớn 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 lớn a few kcal