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eguruchela

Mechanism of dehydration

The dehydration reaction of alcohols to generate alkene proceeds by heating the alcohols in the presence of a strong acid, such as sulfuric or phosphoric acid, at high temperatures.

  • When heated with strong acids catalysts (most commonly H2SO4, H3PO4), alcohols typically undergo a 1,2-elimination reactions to generate an alkene and water.

    • dihydration.jpg

  • Also known as dehydration since it involves the removal of a molecule of water.
  • Alcohol relative reactivity order : 3o > 2o > 1o
  • Regioselectivity : major product is usually the more highly substituted alkene (alkene stability) Zaitsev's Rule

    • Dehydration reaction of alcohols can be seen as,

    dihyd2.jpg

    Alcohol as a Base

    They can act both as acid or base. The lone pair of electrons on oxygen atom makes the –OH group weakly basic. Oxygen can donate two electrons to an electron-deficient proton. Thus, in the presence of a strong acid, R—OH acts as a base and protonates into the very acidic alkyloxonium ion +OH2 .This basic characteristic of alcohol is essential for its dehydration reaction with an acid to form alkenes.