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Chapter 6 Reactions of Haloalkanes: S N 2

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

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  1. 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

  2. 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

  3. 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

  4. 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

  5. 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

  6. 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

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