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Elimination Reactions
Hoffmann and Zaitsev (Saytzeff) elimination

When an alkyl halide is heated with a nucleophile, substitution takes place. If the nucleophile behaves as a base elimination takes place.

Thus when 2-Chloropropane is heated with KOH nucleophilic substitution takes place resulting in  2-Propanol as the predominate product.
When it is heated with alcoholic KOH elimination takes place resulting in Propene as the predominate product.

Conditions for elimination:

1.  KOH / Ethanol
2.  NaOEt / Ethanol
3.  Potassium t-butoxide / t-butyl alcohol

In eliminations the halogen and the hydrogen atom at the next carbon are lost.  There can be more than one adjacent carbons so more than one product is possible.

Elimination Reaction

If the hydrogen at C1 is involved the result is 1-Butene, the less substituted alkene.

If the hydrogen at C3 is involved the result is 2-Butene, the more substituted alkene.

More substituted alkene is more stable therefore should be the major product.
(thermodynamically more substituted alkene is more stable, it gives out less heat of hydrogenation)

With elimination conditions 1 or 2, the more substituted alkene is the major product.
But under reaction condition 3, the less substituted alkene is the major product.

Reason:  The t-butoxide anion is large in size, it is a hindered base.  It is tougher for this base to reach the interior of the molecule and pull out a proton at C3. ( Keep in mind that 2-Chlorobutane is not linear, the carbon atoms are all sp3 hybridised, due to free rotation about the C-C bond axis it assumes many conformations).  On the other hand it can pull off a proton at the terminal of the molecule at C1 with comparative ease.  Thus leading to 1-Butene as the major product.

Conditions 1 and 2 involve bases much smaller in size, they are capable of reaching the interior of the molecule, so the major product is the more stable alkene. 

Elimination reaction

Elimination is more likely in secondary halides than primary halides for the same reason, that is, the base will be sterically more hindered to participate in substitution.

  1. When the more substituted alkene is the major product of dehyrohalogenation the reaction is said to follow Zaitsev rule ( Saytzeff, Saytseff, or Saytzev ). The major product is referred to as Zaitsev product.
  2. If the major product is the less substituted alkene the reaction is following Hoffmann rule and the product is Hoffmann product.

Mechanisms of elimination

E2 Mechanism

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Exercise II Elimination
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