Mechanism: E1 for 2° and 3° (via carbocation); E2 for 1° at very high temperature. Ease: 3° > 2° > 1° (stability of carbocation intermediate). Zaitsev's rule: More substituted (more stable) alkene is the major product. Temperature control for ethanol: 443 K (170°C) with excess H2SO4 -> ethene (intramolecular). 413 K (140°C) with limited H2SO4 -> diethyl ether (intermolecular, SN2). Carbocation rearrangements: Common in dehydration! Hydride shift (1,2-H shift) or methyl shift can give unexpected alkenes. Example: 3,3-dimethylbutan-2-ol dehydrates to 2,3-dimethylbut-2-ene (methyl shift from neopentyl to more stable 3° cation). POCl3/pyridine (Chugaev elimination): syn elimination without rearrangement — useful when rearrangement is undesired.