E1
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Definition:

  1. E1 stands for Elimination Unimolecular.
  2. This means that the rate of these reactions appears to depend only on the substrate concentration.
  3. The rate of the disappearance of substrate = d[ substrate ]/dt = -k[ substrate ], negative slope asymptotic to both axis.
  4. The truth is that the rate of this reaction is really pseudo-first order in substrate concentration because the concentration of solvent and acid catalyst, which affect this reaction, don't change that much as the reaction proceeds.

Mechanism:

  • These substitutions occur by a removal (ionization) of the leaving group to make a carbocation intermediate.
  • They share the same first step as an SN1 reaction.
  • The carbocation intermediate loses a vicinal hydrogen to make an alkene.
  • The electrophilic carbon must be sp3 hybridized.
  • The hybridization of the carbocation intermediate of this process is sp2.

Factors:

The rate of this reaction is affected by:
  • The stability of the carbocation intermediate, rate greater for tertiary > secondary >> primary, neopentyl
    • Inductive effect
    • Hyper-conjugation
  • Solvent, works best with polar protic solvents like water, alcohols, etc.
    • A polar solvent is required for solubility.
    • A protic solvent hydrogen bonds to the leaving group helping to ionize it off.
    • This means that E1 substitutions are really pseudo-first order because the solvent has an effect on the rate but its concentration does not change that much!
  • Leaving group, works best with leaving groups that are weak bases because they have a weaker bonds with carbon and can stabilize negative charge best.
    • F > HO > H2N
    • Sulfonate > I > Br > Cl > F
    • right-down, more right than down
    • cation > neutral
  • Elimination over substitution is favored by heating and lack of a good nucleophile.

Regiochemistry:

  • The alkene formed is the most stable, tetra > tri > 1,2-di > 1,1-di > mono substituted alkene, following Saytzeff's rule, "the poor get poorer"
    1. This stability is explained by more electronegative sp2 carbons being more stabilized by alkyl groups than hydrogen.
    2. The stability is explained by more hyper-conjugation to groups vicinal to the Pi bond.
    3. The stability is explained by placing a flat space between alkyl groups.

Stereochemistry:

  • The diastereomer (E or Z) that is most stable is the one that has less steric interactions.

SN1,SN2, and E1 Reactions of Alkyl Halides

  Protic Solvent
(water, alcohols,..)
Poor Nucleophile
(the protic solvent)
acid catalyzed
Aprotic Solvent
(acetone, ...)
Good Nucleophile that are not too basic
(cyanide, amines, alkanoates,
phosphines, sulfides, selenides,
halides)
methyl slowest SN2, no rxn SN2
1 slower SN2, no rxn SN2
1 hindered slower SN2, no rxn SN2
2 slow SN1/E1 if heated, Saytzeff slow SN2
3 SN1/E1 if heated, Saytzeff slowest SN1/E1 if heated, Saytzeff

 

 

 

 

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Thursday, April 06, 2017 13:18