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 S_N1 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 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.

S_N1,S_N2, and E1 Reactions of Alkyl Halides