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Hafiz Muhammad Athar Rizwan
           Organic Chemistry
In an elimination reaction some fragments of a
    molecule are removed.
    Elimination reaction are classified into two general
    headings

    α-eliminations
    β-eliminations i
 α-eliminations in which two groups are eliminated from
the same atom.




In this case unstable species are formed which undergo
further reactions.
 β-eliminations in which groups on adjacent atoms
 are eliminated with the formation of multiple bonds.



H
                          Base
    C   C                                     C     C

            X
β-eliminations proceeds through two mechanisms which
are

   E2 bimolecular elimination reactions
   E1 unimolecular elimination reaction

The E2 and E1 mechanisms differ in the timing of
bond cleavage and bond formation, analogous to the SN2 and
 SN1 mechanisms.
E1 indicates a elimination, unimolecular reaction.
The E1 reaction proceeds via a two-step mechanism:
the bond to the leaving group breaks first before the
π bond is formed. The slow step is unimolecular,
involving only the alkyl halide.
The dehydrohalogenation of (CH3)3CI with H2O to form
(CH3)2C=CH2 can be used to illustrate the E1 mechanism.
Energy diagram for E1 reaction
1.   Dehydrohalogenation (-HX) and
2.   Dehydration (-H2O)
E1 reaction
CH3                CH3
           slow                                          H
                                   rapid   CH3
CH3 C Br           CH3 C +                                       + B H + Br-
           - Br-                                 C   C
    CH3             H C H
                                           CH3               H
                       H     B:-
R   H                     R           R
                strong acid
R   C   C   R                     C   C       +   H2O

    OH R                      R           R
1)       CH3                                        CH3
                             +
     CH3 C     CH3   +   H                     CH3 C        CH3

         OH                                        + OH2


2)
        CH3                            CH3
                     slow
     CH3 C     CH3               CH3 C       CH3   +    H2O
                                     +
       + OH2


3)
        CH3                      CH3
                                                        +
     CH3 C     CH3                C    CH2     +    H
         +                       CH3
E1 reaction
1)   Substrate effect
2)   Base effect
3)   Isotope effect
4)   Orientation of elimination
The order of the reactivity of the alkyl groupsis

     Tertiary> Secondary>Primary

This is because the rate determining step is the
formation of carbocation and the stability of
these ions increases.
Tertiary (3o) > secondary (2o) > primary (1o)

It is hard (but not impossible) to get primary
compounds to go by E1. The reason for this is
that primary carbocations are not stable!
E1 reaction
E1 reaction
E1 reaction
   Bulky Bases Favor Elimination
   E1 reactions do not show an isotope effect:
    kH/kD = 1

   This tells us that the C-D or C-H bonds are not
    broken in the rate determining step
    (step 1). They are broken in the fast step (step 2) in
    the mechanism).
   E1 reactions faithfully follow Zaitsev’s rule!
   This means that the major product should be the
    product that is the most highly substituted.
   In reactions of removal of hydrogen halides
    from alkyl halides or the removal of water
    from alcohols, the hydrogen which is lost will
    come from the more highly-branched b-
    carbon.
1.   Kinetics
2.   Stereochemistry
3.   Rerangement
   E1 reactions follow first order (unimolecular)
    kinetics:

    Rate = k [R-X]1

    The solvent helps to stabilize the carbocation,
    but it doesn’t appear in the rate law!!
1)
      H                                      H
                            slow                                       _
      C    C                                 C   C           +       Br
                       rate determining          +
           Br               step




                                                 H   .. +
      H   ..
 2)       O:                                         O
      H                                          H
                                                         H
               H                      fast
                                                                 +         C   C
               C   C
                   +




         Rate law: rate = k [R-Br]1
   E1 reactions do not require an anti coplanar orientation of H
    and X.
   Diastereomers give the same products with E1 reactions,
    including cis- and trans products.
   E1 reactions usually give the thermodynamically most stable
    product as the major product. This usually means that the
    largest groups should be on opposite sides of the double bond.
    Usually this means that the trans product is obtained.
   Alkyl groups and hydrogen can migrate in
    rearrangement reactions to give more stable
    intermediate carbocations.
CH3 H                       CH3                   CH3
                            +
CH3 C      C   CH3      CH3 C   C     CH3    CH2 C    CH CH3
           +
    CH3                     CH3 H               CH3
secondary carbocation tertiary carbocation      minor

                                              CH3        CH3
                                               C     C
   CH3                                        CH3        CH3

CH3 C      CH CH2                               major

   CH3
   trace
Factors to Consider:

1.   How Basic is the Nucleophile?

2.   2. Steric Hindrance at Reacting Carbon

3.   3. Steric Hindrance at Nucleophile
E1 reaction
   E1cB mechanism (E-elimination, 1cB-first order with
    respect to conjugate base) is one of the three limiting
    mechanisms of 1,2-elimination. It is a two-step
    mechanism.
   Step 1




    Step 2
   Step two is first order and its reactant is the conjugate base of
    the substrate, hence the notation 1cB.
   A 1,2-elimination occurring via E1cB mechanism is called and
    E1cB reaction. Stand-alone E1cB reactions are not common,
    and they have a complex rate law, meaning that the rate-
    limiting step is the second step.
   However, in the most common E1cB reactions, the base is
    ‾OH and the solvent is water, in which case the rate law
    simplifies to
                                  


                   rate = k[substrate][base]
    When the base is strong enough and the leaving group
    is very poor(such as fluorine or hydroxyl groups),
    E1CB is preferred.

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E1 reaction

  • 1. Hafiz Muhammad Athar Rizwan Organic Chemistry
  • 2. In an elimination reaction some fragments of a molecule are removed. Elimination reaction are classified into two general headings  α-eliminations  β-eliminations i
  • 3.  α-eliminations in which two groups are eliminated from the same atom. In this case unstable species are formed which undergo further reactions.
  • 4.  β-eliminations in which groups on adjacent atoms are eliminated with the formation of multiple bonds. H Base C C C C X
  • 5. β-eliminations proceeds through two mechanisms which are  E2 bimolecular elimination reactions  E1 unimolecular elimination reaction The E2 and E1 mechanisms differ in the timing of bond cleavage and bond formation, analogous to the SN2 and SN1 mechanisms.
  • 6. E1 indicates a elimination, unimolecular reaction. The E1 reaction proceeds via a two-step mechanism: the bond to the leaving group breaks first before the π bond is formed. The slow step is unimolecular, involving only the alkyl halide.
  • 7. The dehydrohalogenation of (CH3)3CI with H2O to form (CH3)2C=CH2 can be used to illustrate the E1 mechanism.
  • 8. Energy diagram for E1 reaction
  • 9. 1. Dehydrohalogenation (-HX) and 2. Dehydration (-H2O)
  • 11. CH3 CH3 slow H rapid CH3 CH3 C Br CH3 C + + B H + Br- - Br- C C CH3 H C H CH3 H H B:-
  • 12. R H R R strong acid R C C R C C + H2O OH R R R
  • 13. 1) CH3 CH3 + CH3 C CH3 + H CH3 C CH3 OH + OH2 2) CH3 CH3 slow CH3 C CH3 CH3 C CH3 + H2O + + OH2 3) CH3 CH3 + CH3 C CH3 C CH2 + H + CH3
  • 15. 1) Substrate effect 2) Base effect 3) Isotope effect 4) Orientation of elimination
  • 16. The order of the reactivity of the alkyl groupsis Tertiary> Secondary>Primary This is because the rate determining step is the formation of carbocation and the stability of these ions increases.
  • 17. Tertiary (3o) > secondary (2o) > primary (1o) It is hard (but not impossible) to get primary compounds to go by E1. The reason for this is that primary carbocations are not stable!
  • 21. Bulky Bases Favor Elimination
  • 22. E1 reactions do not show an isotope effect: kH/kD = 1  This tells us that the C-D or C-H bonds are not broken in the rate determining step (step 1). They are broken in the fast step (step 2) in the mechanism).
  • 23. E1 reactions faithfully follow Zaitsev’s rule!  This means that the major product should be the product that is the most highly substituted.
  • 24. In reactions of removal of hydrogen halides from alkyl halides or the removal of water from alcohols, the hydrogen which is lost will come from the more highly-branched b- carbon.
  • 25. 1. Kinetics 2. Stereochemistry 3. Rerangement
  • 26. E1 reactions follow first order (unimolecular) kinetics: Rate = k [R-X]1 The solvent helps to stabilize the carbocation, but it doesn’t appear in the rate law!!
  • 27. 1) H H slow _ C C C C + Br rate determining + Br step H .. + H .. 2) O: O H H H H fast + C C C C +  Rate law: rate = k [R-Br]1
  • 28. E1 reactions do not require an anti coplanar orientation of H and X.  Diastereomers give the same products with E1 reactions, including cis- and trans products.  E1 reactions usually give the thermodynamically most stable product as the major product. This usually means that the largest groups should be on opposite sides of the double bond. Usually this means that the trans product is obtained.
  • 29. Alkyl groups and hydrogen can migrate in rearrangement reactions to give more stable intermediate carbocations.
  • 30. CH3 H CH3 CH3 + CH3 C C CH3 CH3 C C CH3 CH2 C CH CH3 + CH3 CH3 H CH3 secondary carbocation tertiary carbocation minor CH3 CH3 C C CH3 CH3 CH3 CH3 C CH CH2 major CH3 trace
  • 31. Factors to Consider: 1. How Basic is the Nucleophile? 2. 2. Steric Hindrance at Reacting Carbon 3. 3. Steric Hindrance at Nucleophile
  • 33. E1cB mechanism (E-elimination, 1cB-first order with respect to conjugate base) is one of the three limiting mechanisms of 1,2-elimination. It is a two-step mechanism.
  • 34. Step 1 Step 2
  • 35. Step two is first order and its reactant is the conjugate base of the substrate, hence the notation 1cB.  A 1,2-elimination occurring via E1cB mechanism is called and E1cB reaction. Stand-alone E1cB reactions are not common, and they have a complex rate law, meaning that the rate- limiting step is the second step.  However, in the most common E1cB reactions, the base is ‾OH and the solvent is water, in which case the rate law simplifies to   rate = k[substrate][base]
  • 36. When the base is strong enough and the leaving group is very poor(such as fluorine or hydroxyl groups), E1CB is preferred.