II. Substitution reactions of alkyl halides

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					                                                                                         LG Ch 8 p 7
                         II. Substitution reactions of alkyl halides

Introduction
What are the two types of reactions that alkyl halides can do?


What do these two words mean?




Which of the reactions represented below is a substitution reaction? Which is an
elimination reaction? Why?


                    Cl   +   Na       O                          O          +       Na   Cl




               Br
                     +       O                               +
                                  K                                    OH       +    K    Br
               H




Which of these will we study in this chapter? In the next?



Mechanisms

What is a mechanism?



How many ways can a substitution occur? What are the mechanisms called?
                                                                                     LG Ch 8 p 8
Consider the following mechanisms:


                 Cl      Na     O                               O   +       Na       Cl



     Draw in the arrows to show how this reaction occurred.

     What type of reaction is this?

     How could this mechanism be described?

     Where is the nucleophile, and where is the electrophile?

     Why is the alkyl halide a good electrophile?

     Why is the alkoxide a good nucleophile?

      Which reaction is this?


                                             O                 H        O
                                                 H                          H
                                                           O                              O
            Cl                  +     Cl


                                                                                              H
                                                                                 +            O
                                                                                                  H
     Draw in the arrows to show how this reaction occurred.

     What type of reaction is this?

     How could this mechanism be described?

     Where are the nucleophile, electrophile, acid, and base?

     Why can alkyl halides dissociate?


     Why is the carbocation a good electrophile?


     Which reaction is this?
                                                                              LG Ch 8 p 9
Which substitution reaction?

How are the SN1 and SN2 reactions the same?


Which are the two electrophiles used in the examples? Which do you think is more
reactive? Why?

      SN2                                          SN1



What are the two nucleophiles used in the examples? Which do you think is stronger, and
why?
      SN2                                          SN1



How do the strength of the nucleophile and electrophile fit together?




      Scenario A: strong Nu added to alkyl halide


      Scenario B: weak Nu added to alkyl halide



What determines whether an SN1 or and SN2 reaction will occur?


Following the pattern of the reactions given previously, draw the mechanisms for the
following reactions:

                            Br   Na        C   N
       SN2


                                       O
                            Br     H       H
       SN1
                                                                               LG Ch 8 p 10
Strong vs. Weak Nucleophiles

In each of the pairs of nucleophiles below, what is the important difference? Which is the
stronger nucleophile?


 1) H2O                               NaOH




 2)    HC   CNa                       NaOH




  3)    NaOH                           NaSH




   4) CH3CH2ONa                        (CH3)3COK




Sum up the factors:
                                                                              LG Ch 8 p 11
The following are nucleophiles commonly used in substitution reactions, listed in order of
nucleophilicity:

      alkyne anions

      cyanide

      thiolates

      hydroxide

      alkoxides
              exception:
              tert-butoxide

      halides

      water


      alcohols
              exception:
              tert-butyl alcohol

What is the cut off point between nucleophiles that follow SN2 vs. SN1 mechanisms?

Why is are alcohols weak nucleophiles while alkoxides are strong nucleophiles?


Why won't tert-butoxide and tert-butyl alcohol work as nucleophiles?



Why are alkyne anions the best nucleophiles?


Which of the halides is the best nucleophile? Why are they all poor nucleophiles?
Products                                                                   LG Ch 8 p 12
Give the products of the following reactions. Then note what nucleophile produced what
product. Which mechanism will each follow?

             Br       +       Na




                 Br       +    Na          C    N




              Br          +    Na          O    H




             Br           +            O
                                   H       H




            Br        +       Na           O




             Br       +                    OH




             Br       +       Na           S




             Br
                      +       Na       I
                                                                       LG Ch 8 p 13
Stereochemistry

Can a substitution reaction involve a carbon that is a stereocenter?

Can a substitution reaction create a new stereocenter?

What happens to a stereocenter involved in an SN2 reaction?

            Cl
                      NaOH


      How does this happen?




                 Cl
                 H




What happens to a stereocenter involved in an SN1 reaction?

            Cl
                      H2O


     How does this happen?



                 Cl
                 H




      The actual ratio isn't 1 to 1 - it's more like 2 to 3. Why?
                                                                           LG Ch 8 p 14
Rate laws

Consider the following SN2 reaction.

                      Na   O
                 Cl                          O        Na   Cl



      Draw the energy diagram for this reaction.

      Which reagents are involved in the rate limiting step?



      Draw the structure of the transition state.


      What would happen if you
             doubled the concentration of the alkyl halide?
             doubled the concentration of the nucleophile?

      What is the order of the alkyl halide? the nucleophile?

      Write the write law:

      What does SN2 stand for?

What happens to the rate of SN2 reactions as the substitution increases?

                                                                Cl             Cl
                      CH3Cl                   Cl


      steric
      hindrance

      activation
      energy

      rate of
      reaction
                                                                           LG Ch 8 p 15
Consider the following SN1 reaction.

                         OH
             Cl                        O     +        OH2    Cl




       Draw the energy diagram for this reaction.

      Which is the rate limiting step?



      Draw the structure of the transition state for this step.

       Which reagents are involved in the rate limiting step?

      What would happen if you
             doubled the concentration of the alkyl halide?
             doubled the concentration of the nucleophile or base?

       What order is the alkyl halide? the nucleophile or base?


      Write the write law:

      What does SN1 stand for?

What happens to the rate of SN1 reactions as the substitution increases?


                                                                  Cl           Cl
                     CH3Cl                       Cl


       carbocation
       stability

       activation
       energy

       rate of
       reaction
                                                                                LG Ch 8 p 16

How does the identity of the halogen atom affect the rate of substitution reactions?

      two possible explanations:
             longer bonds break more easily
             more EN, more stable afterwards


             CH3I          CH3Br              CH3Cl           CH3F


      Will this be true for both SN2 and SN1 reactions?




Rearrangements

Why do some carbocations undergo rearrangements?



Which of the carbocations commonly undergoes rearrangements?




What are the two ways in which a carbocation can rearrange?




              Br




              Br
                                                                             LG Ch 8 p 17
Why can't the following carbocation rearrange?




What is the most likely rearrangement of the following carbocation?




In which substitution reaction do carbocations form?



Which carbocation will give the product?

Give the substitution products in the following reaction.

                       H2O
              Br




Solvents
Why is a solvent useful when running a reaction?




What are the important issues when considering solvents for substitution and elimination
reactions?
                                                               LG Ch 8 p 18
What kind of solvent do SN2 reactions require?




What kind of solvent do SN1 reactions require?




What solvents are in the following categories?

      nonpolar


      moderately polar

      highly polar, aprotic




      highly polar, protic

Exclusion of aryl halides, vinyl halides, and acid chlorides

Why can't aryl halides undergo substitution reactions?

                 Cl   NaOH



                 Cl   H2O



            Cl           O

                             Cl
                                                       LG Ch 8 p 19

                   Summary of Substitution Reactions


 SN2 reactions:                      SN1 reactions:
result:                            result:


mechanism:                         mechanism:




reagent:                            reagent:



stereochemistry:                   stereochemistry:


rate law:                           rate law:


alkyl halides:                     alkyl halides:




rearrengements?                    rearrengements?


solvent:                            solvent: