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Chapter 6 Reactions of Haloalkanes: S N 2. Naming Haloalkanes Treat Halogen as a substituent to an alkane: chlorine = chloro- Name as in alkane nomenclature Physical properties of haloalkanes Bond Strength For RCH 2 X, F > Cl > Br > I bond strength F has best size for overlap

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chapter 6 reactions of haloalkanes s n 2
Chapter 6 Reactions of Haloalkanes: SN2
  • Naming Haloalkanes
    • Treat Halogen as a substituent to an alkane: chlorine = chloro-
    • Name as in alkane nomenclature
  • Physical properties of haloalkanes
    • Bond Strength
      • For RCH2X, F > Cl > Br > I bond strength
      • F has best size for overlap
      • Bond Length: I > Br > Cl > F
    • Polarity: Halogens are more electronegative than C, so bonds are polar
      • Polar R-X bond can be attacked by cations or anions

2-bromo-2-methylpropane

cis-1-chloro-2-fluoro

cyclohexane

4-(1-iodoethyl)heptane

slide2
Polarizability = how much the electron cloud is deformed by other charges, combination of size and charge effects
        • I > Br > Cl > F because of larger size and same charge (-1)
        • London Forces are largest for Iodoalkanes because of polarizability
        • Boiling points: MeI > MeBr > MeCl > MeF > Methane
  • Nucleophilic Substitution
    • Nucleophile/Electrophile
      • Molecule or Ion with a lone pair of electrons seeks to share or donate their “extra” electron density to a (+) or (d+) “nucleus”
        • Nucleophile = nucleus (+) loving = Nu
        • Nucleophiles = Lewis Bases
      • Molecule or Ion having (+) or (d+) region seeks extra electron density
        • Electrophile = electron (-) loving
        • Can be a Lewis Acid, but not necessarily
        • Just needs d+, so any polar bond should have one electrophilic area
        • An atom with a full octet can still be an electrophile
slide3
Nucleophilic Substitution Reaction

1)

    • Leaving Group = ion or molecule replaced by Nu (X) “Substitution”
    • Color codes in your book: Nu = red, Electrophile = blue, LG = green
    • Examples
  • Electron Pushing in Mechanisms
    • Move e- from e- rich to e- poor sites
    • Acid-Base example
slide4
O lone pair become O—H bond
    • H—Cl bond becomes Cl lone pair
    • Curved arrows show “flow” of the electrons
  • Organic mechanisms, same thing: Must Draw Correct Lewis Structure
    • Nucleophilic Substitution
    • Dissociation
    • Nucleophilic Addition
    • Electrophilic Addition
slide5
Kinetics tells us about mechanisms of Nucleophilic substitution
    • Nucleophilic substitution is 2nd order process: rate = k[OH-][CH3Cl]
      • If double [OH-], double rate
      • If double [CH3Cl], double rate
      • Rate depends on both S.M., they must both be in rate determ. step
    • Bimolecular Process
    • SN2 = Bimolecular Nucleophilic Substitution
      • Concerted: bond-making and bond-breaking happen simultaneously
      • Front-side or Back-side displacement?

Back-side attack

Front-side attack

slide8
For Achiral reactants, products are identical (No Help!)
  • What about for Chiral reactants?
  • SN2 gives Stereospecific Inversion of Stereochemistry
  • SN2 happens exclusively through Back-side attack