dehydration reaction
Summary
A dehydration reaction loses water: either an alcohol eliminates H₂O to form an alkene (acid, heat), or two molecules join with H₂O released (condensation). Most testable: the more substituted alkene (Zaitsev product) usually dominates, and tertiary alcohols react fastest.
Detail
Meaning: A dehydration reaction removes the elements of water. Two forms appear. In elimination, an alcohol becomes an alkene: acid protonates the OH, water leaves as a good leaving group to give a carbocation (E1 for tertiary and secondary alcohols; primary alcohols usually react by E2), and a base removes a neighboring H. In condensation (dehydration synthesis), two molecules join and H₂O is released, as in ester, amide, anhydride, and glycosidic bond formation.
Equation: R–CH₂–CH(OH)–R′ → R–CH=CH–R′ + H₂O, with H₂SO₄ or H₃PO₄ and heat. R and R′ are alkyl groups; no SI units apply.
On the MCAT: Expect to predict the major alkene, which is usually the more substituted one (Zaitsev). Rate follows carbocation stability, 3° > 2° > 1°, and carbocation rearrangements are possible. Removing water or distilling off the product drives the equilibrium forward (Le Châtelier). In biochemistry passages, peptide and glycosidic bonds form by dehydration and break by hydrolysis.
Common confusion: Dehydration is not oxidation, even though the alcohol becomes an alkene: it is not a redox reaction, and water, not H₂, is lost. Also, OH⁻ is a poor leaving group, so acid is needed to convert it to H₂O before it can leave.
Sources
- Organic Chemistry (Klein)
- Organic Chemistry (Wade)
- Organic Chemistry (Clayden)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.