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Organic Chemistry. Chapter 8. Substitution and Elimination. If an sp 3 C is bonded to electronegative atom Substitution reactions and Elimination reactions are possible. This chapter is all about substitution. example. S N 2 and S N 1 Reactions.

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Substitution and elimination l.jpg
Substitution and Elimination

  • If an sp3 C is bonded to electronegative atom Substitution reactions and Elimination reactions are possible

This chapter is all about substitution


S n 2 and s n 1 reactions l.jpg

example

SN2 and SN1 Reactions

SN2 - Reaction – bonds break and form at the same time

SN2

  • SN1 - CX bond breaks, forming a C+ then reacts with a nucleophile

SN1


Nucleophilic substitution reactions l.jpg
Nucleophilic Substitution Reactions

Either mechanism depends on the:

  • structure of the alkyl halide

  • reactivity of the nucleophile

  • concentration of the nucleophile

  • The solvent in which the Rx is carried out

  • The leaving group


S n 2 mechanism l.jpg
SN2 Mechanism

  • It’s a Substitution Reaction (S)

  • It’s Nucleophilic (N)

  • It’s rate is second order (2)

    • Called bimolecular (rate is dependent on 2 reactants)

      • (Substitution Nucleophilic Bimolecular)

Rate = k [RX] [Nu:]

(Because rate is dependent of BOTH RX and Nu: it is 2nd. order.)


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SN2 Mechanism

  • SN2Mechanism involves a “backside attack”


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SN2 Mechanism

The “backside attack” causes an Inversion of Configuration

Careful now….. Doesn’t mean R becomes S – new atoms are involved


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Steric Hindrance

  • Groups that block the path from the nucleophile to the electrophilic atom produce steric hindrance

  • This results in a rate differences or no reaction at all

methyl halide ethyl halide isopropyl halide t-butyl halide


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Steric Hindrance

  • Activation Energy is higher due to steric hindrance…..


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Substitution Reactions Depend on a Good Leaving Group

  • R-F alkyl fluorides

  • R-Cl alkyl chlorides

  • R-Br alkyl bromides

  • R-I alkyl iodides

  • Alkyl Halides make good “leaving groups”

    • They are easily displaced by another atom

    • They allow the Conversion of alkyl halides to other functional groups


S n 2 mechanism12 l.jpg
SN2 Mechanism

  • The Leaving Groups also affects rate

  • RI reacts fastest, RF slowest

    • Iodide is the best “leaving group”

    • Fluoride is the worst “leaving group”

(…reacting with the same alkyl halide under the same conditions)


Basicity l.jpg
Basicity

  • The weaker the basicity of a group, the better the leaving ability.

    (Lewis base = e- pair donor)

    • Leaving ability depends on basicity because a weak base does not SHARE its e- as well as a strong base.

    • Weak bases are not strongly bonded to a carbon

      (weak bases are GOOD leaving groups)


Nucleophiles strong weak good bad l.jpg
Nucleophiles – Strong/Weak Good/Bad

Stronger base Weaker base

Better nucleophile poorer nucleophile

OH- > H2O

CH3O- > CH3OH

-NH2 > NH3

CH3CH2NH- > CH3CH2NH2

(conjugate acids)


Nucleophiles l.jpg
Nucleophiles

  • The strength of nucleophile depends on reaction conditions.

  • In the GAS phase (not usually used),direct relationship between basicity and nucleophilicity


Solvent effects l.jpg
Solvent Effects

  • In a solution phase reaction, the solvent plays a large role in how the reaction will occur

  • Solvent effects can cause just the opposite of what might be the expected behavior of the nucleophile

  • Solvents are categorized as either protic or aprotic


Protic solvents l.jpg
Protic Solvents

Protic solvents has a H bonded to a N or O

  • It is a H bonder

  • Examples: H2O, CH3OH, NH3, etc

  • Solvent is attracted to the Nucleophile and hinders its ability to attack the electrophile


Aprotic solvents l.jpg

DMSO

DMF

Aprotic Solvents

  • Use an aprotic solvent

    • Solvates cations

    • Does not H bond with anions (nucleophile free)

    • Partial + charge is on inside of molecule

    • Negative charge on surface of molecule (solvates cation)

  • Examples include:

    • DMSO (dimethyl sulfoxide)

    • DMF (dimethyl formamide)

    • Acetone (CH3COCH3)


  • Nucleophiles19 l.jpg

    Question…

    Nucleophiles

    • In the organic solvent phase, INVERSErelationship between basicity and nucleophilicity with a protic solvent


    Nucleophiles20 l.jpg
    Nucleophiles

    • Solvents can solvate the nucleophile

      • Usually this is NOT good because the nucleophile is “tied up” in the solvent and LESS REACTIVE.

    Ion-dipole interactions


    Nucleophiles21 l.jpg
    Nucleophiles

    • Solvents can solvate the nucleophile

    (Methanol is a polar protic solvent.)


    S n 2 reactions l.jpg
    SN2 Reactions


    S n 2 reactions23 l.jpg
    SN2 Reactions


    S n 2 reactions24 l.jpg
    SN2 Reactions

    • SN2 reactions might be reversible

    • Leaving group would become the nucleophile

    • Compare basicity (nucleophile strength) to see which is a better leaving group.

    • The stronger base will displace the weaker base

      • If basicity is similar, the Rx will be reversible


    S n 2 reactions25 l.jpg
    SN2 Reactions

    Compare basicity to see which is a better nucleophile.



    S n 1 reactions l.jpg
    SN1 Reactions

    • Reaction of t-butyl bromide with water should be slow

      • water is a poor nucleophile

      • t-butyl bromide is sterically hindered

        However

      • Reaction is a million times faster than with CH3Br

    (Maybe not an SN2 reaction!)


    S n 1 reactions28 l.jpg
    SN1 Reactions


    S n 1 mechanism l.jpg
    SN1 Mechanism

    • Rate determining step does not involve nucleophile

    Step 1

    Step 2


    S n 1 mechanism30 l.jpg
    SN1 Mechanism


    S n 1 reactivity l.jpg
    SN1 Reactivity

    • Relative Reactivities in an SN1 Reaction

    1o RX < 2o RX < 3o RX

    Increasing Reactivity


    S n 1 stereochemistry l.jpg
    SN1 Stereochemistry

    • Because a planer carbocation is formed, nucleophilic attack is possible on both sides, so both isomers are possible


    S n 1 stereochemistry33 l.jpg
    SN1 Stereochemistry

    • SN1 should yield racemic mixture but it doesn’t

      • This is due to the steric hindrance of the leaving group


    Stereochemistry l.jpg
    Stereochemistry

    • As the leaving group goes (Marvin K) it blocks the path of any incoming nucleophiles


    S n 1 vs s n 2 l.jpg
    SN1 vs SN2

    Inversion of

    configuration

    racemization with

    partial inversion


    What makes s n 1 reactions work the best l.jpg
    What Makes SN1 Reactions work the best

    • Good Leaving Group

      • The weaker the base, the less tightly it is held

        (I- and Br- are weak bases)

    • Carbocation

      • How stable is the resulting carbocation?

        • 3o > 2o > 1o > methyl

    Increasing Stability


    What doesn t matter in an s n 1 reactions l.jpg
    What Doesn’t Matter In anSN1 Reactions

    • The Nucleophile

      • It has NO EFFECT on rate of Rx!!!

  • Solvolysis Reactions

    • (the nucleophile is also the solvent)

  • Nu:


    Carbocation rearrangements l.jpg
    Carbocation Rearrangements

    Since a carbocation is the intermediate, you may see rearrangements in an SN1 Rx

    • No rearrangements in an SN2 Rx



    Benzylic allylic vinylic and aryl halides l.jpg
    Benzylic, Allylic, Vinylic,and Aryl Halides

    • Benzylic and allylic halides can readily undergo SN2 unless they are 3o

      • (steric hindrance)


    Benzylic allylic vinylic and aryl halides41 l.jpg
    Benzylic, Allylic, Vinylic,and Aryl Halides

    • Benzylic and allylic halides can also undergo SN1(they form stable carbocations)

    • Even though 1o RX do not go SN1, 1o benzylic and 1o allylic CAN react SN1!


    Vinylic and aryl halides l.jpg
    Vinylic,and Aryl Halides

    • Vinylic halides and aryl halides

      • do not undergo SN1 or SN2 reactions!

        p e- repel incoming Nucleophile


    S n 1 vs s n 2 review l.jpg
    SN1 vs SN2 Review


    S n 1 vs s n 244 l.jpg

    Methyl, 1oRX …

    2oRX …

    3oRX …

    Vinylic, aryl RX …

    1o, 2o benzylic, allylic RX …

    3o benzylic, allylic RX …

    SN2 only

    SN1 and SN2

    SN1 only

    neither SN1 nor SN2

    SN1 and SN2

    SN1 only

    SN1 vs SN2


    Role of the solvent l.jpg

    In an SN1, a carbocation and halide ion are formed

    Solvation provides the energy for X- being formed

    In SN1 the solvent “pulls apart” the alkyl halide

    SN1 cannot take place in a nonpolar solvent or in the gas phase

    Increasing the polarity of the solvent will INCREASE the rate of Rx if none of the REACTANTS are charged.

    If reactants are charged it will DECREASE the rate.

    Role of the Solvent


    Role of the solvent46 l.jpg

    So….

    In an SN1 reaction, the reactant is RX. The intermediate is charged and is STABILIZED by a POLAR solvent

    A POLAR solvent increases the rate of reaction for an SN1 reaction.

    Role of the Solvent

    (However, this is true only if the reactant is uncharged.)


    Slide47 l.jpg
    *


    Role of the solvent in s n 2 l.jpg

    In an SN2 reaction, one of the reactants is the nucleophile (usually charged).

    The POLAR solvent will usually stabilize the nucleophile.

    A POLAR solvent decreases the rate of reaction for an SN2 reaction.

    Role of the Solvent In SN2

    (However, this is true only if the nucleophile is charged.)


    Polar aprotic solvents l.jpg

    Polar Aprotic Solvents include:

    DMF N,N-dimethylformamide

    DMSO dimethylsulfoxide

    HMPA hexamethylphosphoramide

    THF Tetrahydrofuran

    And even… acetone

    Polar Aprotic Solvents


    Polar aprotic solvents50 l.jpg

    Polar Aprotic Solvents

    do not H bond

    solvate cations well

    do NOT solvate anions (nucleophiles) well

    good solvents for SN2 reactions

    Polar Aprotic Solvents


    Polar aprotic solvents51 l.jpg

    DMSO

    DMF

    Acetone

    HMPA

    Polar Aprotic Solvents




    S n 1 s n 2 problems 1 l.jpg
    SN1/SN2 Problems -1

    • Predict the type of mechanism for this reaction, and the stereochemistry of each product


    S n 1 s n 2 problems 155 l.jpg
    SN1/SN2 Problems -1

    • Predict the type of mechanism for this reaction, and the stereochemistry of each product


    S n 1 s n 2 problems 2 l.jpg
    SN1/SN2 Problems -2

    • Predict the mechanism of this reaction


    S n 1 s n 2 problems 257 l.jpg
    SN1/SN2 Problems -2

    • Predict the mechanism of this reaction


    S n 1 s n 2 problems 3 l.jpg
    SN1/SN2 Problems -3

    • Predict the mechanism. If the starting material has the R configuration, predict the configuration of product


    S n 1 s n 2 problems 359 l.jpg
    SN1/SN2 Problems -3

    • Predict the mechanism. If the starting material has the R configuration, predict the configuration of product


    S n 1 s n 2 problems 4 l.jpg

    O

    Br

    CH

    COH

    +

    3

    acetic acid

    O

    HBr

    OCCH

    +

    3

    SN1/SN2 Problems -4

    • Predict the mechanism


    S n 1 s n 2 problems 461 l.jpg

    O

    Br

    CH

    COH

    +

    3

    acetic acid

    O

    HBr

    OCCH

    +

    3

    SN1/SN2 Problems -4

    • Predict the mechanism


    S n 1 s n 2 problems 5 l.jpg
    SN1/SN2 Problems -5

    • Predict the mechanism


    S n 1 s n 2 problems 563 l.jpg
    SN1/SN2 Problems -5

    • Predict the mechanism



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