Elimination Reactions of Alcohols (Dehydration) and Saytzeff's Rule

Heating an alcohol with a dehydrating agent (concentrated H2SO4, phosphoric acid, anhydrous ZnCl2 or alumina) eliminates -OH and an adjacent -H to form a C=C double bond. Primary alcohols dehydrate by the bimolecular E2 mechanism (e.g. ethanol at 443 K with conc. H2SO4 gives ethene). Tertiary alcohols dehydrate by the unimolecular E1 mechanism: (1) protonation of -OH to -OH2+, (2) slow loss of water to form a carbocation, (3) fast loss of a proton from a beta-carbon to give the alkene. Relative reactivity in dehydration follows tertiary > secondary > primary, tracking carbocation stability. When more than one beta-hydrogen is available, SAYTZEFF'S RULE governs which alkene predominates: the more highly substituted (more stable) alkene is the major product. In the dehydration of 3,3-dimethylbutan-2-ol, the initially formed secondary carbocation undergoes a methyl shift to the more stable tertiary carbocation, and elimination from THAT cation gives 2,3-dimethylbut-2-ene (a tetrasubstituted …