Dehydration is the removal of an equivalent of H2O from a molecule. Alcohols may be dehydrated by the catalysis of an acid or a base. The dehydration occurs as the elimination of the alcohol's hydroxy group, which leaves as H2O.
Acid Catalyzed Dehydration
Acid catalyzed dehydration makes the hydroxy group a stable leaving group by protonating it. The alcohol's hydroxy group acts as a base: the oxygen donates a pair of electrons to form a covalent bond with H+, thus acquiring a positive charge. At this point, the positively charged group, —+OH2, leaves, taking along the pair of electrons that bonded it to the parent molecule. This creates a carbocation. An E1 ß-elimination then occurs. The entire process appears below, exemplified by the dehydration of ethyl alcohol to ethylene.
- When the reaction solvent is organic, "tosic acid," TsOH, is useful because it is hydrophobic.
- When the reaction takes place in H2O, sulfuric acid is useful because it is miscible in water.
Base Catalyzed Dehydration
Base catalyzed dehydration makes the hydroxy group a stable leaving group by replacing the hydrogen with an appropriate substitute. One such group is POCl3, which appears in the following example.
Resonance makes the leaving group very stable. The hindered or "bulky" base pyridine is used to avoid the substitution product. The mechanism is E2 because the nitrogen base is, though neutral, fairly strong, and it is complemented by an excellent leaving group.
Another group often used to substitute the hydrogen in a base catalyzed dehydration is the tosyl group (Ts or Tos). The reaction below uses tosyl chloride, TsCl, with pyridine serving to deprotonate the oxygen.
Once again, the leaving group is resonance stabilized.