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Chapter 6 Reactions of Haloalkanes: S N 2 I.Naming Haloalkanes A.Treat Halogen as a substituent to an alkane: chlorine = chloro- B.Name as in alkane nomenclature.

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Presentation on theme: "Chapter 6 Reactions of Haloalkanes: S N 2 I.Naming Haloalkanes A.Treat Halogen as a substituent to an alkane: chlorine = chloro- B.Name as in alkane nomenclature."— Presentation transcript:

1 Chapter 6 Reactions of Haloalkanes: S N 2 I.Naming Haloalkanes A.Treat Halogen as a substituent to an alkane: chlorine = chloro- B.Name as in alkane nomenclature II.Physical properties of haloalkanes A.Bond Strength 1)For RCH 2 X, F > Cl > Br > I bond strength 2)F has best size for overlap 3)Bond Length: I > Br > Cl > F B.Polarity: Halogens are more electronegative than C, so bonds are polar 1)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 2)Polarizability = how much the electron cloud is deformed by other charges, combination of size and charge effects a)I > Br > Cl > F because of larger size and same charge (-1) b)London Forces are largest for Iodoalkanes because of polarizability c)Boiling points: MeI > MeBr > MeCl > MeF > Methane II.Nucleophilic Substitution A.Nucleophile/Electrophile 1)Molecule or Ion with a lone pair of electrons seeks to share or donate their “extra” electron density to a (+) or (  +) “nucleus” a)Nucleophile = nucleus (+) loving = Nu b)Nucleophiles = Lewis Bases 2)Molecule or Ion having (+) or (  +) region seeks extra electron density a)Electrophile = electron (-) loving b)Can be a Lewis Acid, but not necessarily c)Just needs  +, so any polar bond should have one electrophilic area d)An atom with a full octet can still be an electrophile

3 B.Nucleophilic Substitution Reaction 1) 2)Leaving Group = ion or molecule replaced by Nu (X) “Substitution” 3)Color codes in your book: Nu = red, Electrophile = blue, LG = green 4)Examples C.Electron Pushing in Mechanisms 1)Move e- from e- rich to e- poor sites 2)Acid-Base example

4 a)O lone pair become O—H bond b)H—Cl bond becomes Cl lone pair c)Curved arrows show “flow” of the electrons 3)Organic mechanisms, same thing: Must Draw Correct Lewis Structure a)Nucleophilic Substitution b)Dissociation c)Nucleophilic Addition d)Electrophilic Addition

5 D.Kinetics tells us about mechanisms of Nucleophilic substitution 1)Nucleophilic substitution is 2 nd order process: rate = k[OH - ][CH 3 Cl] a)If double [OH-], double rate b)If double [CH 3 Cl], double rate c)Rate depends on both S.M., they must both be in rate determ. step 2)Bimolecular Process 3)S N 2 = Bimolecular Nucleophilic Substitution a)Concerted: bond-making and bond-breaking happen simultaneously b)Front-side or Back-side displacement? Front-side attack Back-side attack

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8 c)For Achiral reactants, products are identical (No Help!) d)What about for Chiral reactants? e)S N 2 gives Stereospecific Inversion of Stereochemistry f)S N 2 happens exclusively through Back-side attack


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