= Beta-elimination
\b[Beta-elimination], also called 1,2-elimination, removes groups from adjacent atoms while forming a new multiple bond between those atoms. The IUPAC definition allows the two central atoms to be elements other than <carbon>, although carbon–carbon examples are common. It describes the overall connectivity change, rather than specifying one unique <reaction mechanism>.<IUPAC Compendium of Chemical Terminology, fifth edition (Gold Book) by International Union of Pure and Applied Chemistry>[Entry 08150, β-elimination; DOI 10.1351/goldbook.08150, general X–A–B–Y transformation.]{ref}
The net dehydration of <ethanol> illustrates the topology:
$$
\mathrm{CH_3CH_2OH}\longrightarrow\mathrm{CH_2{=}CH_2}+\mathrm{H_2O}.
$$
One carbon loses a hydrogen and its neighbor loses the oxygen-containing group; the carbon pair becomes the double bond of <ethene>. Both sides contain two carbons, six hydrogens and one oxygen. The balanced equation describes the net conversion, without asserting that unactivated <ethanol> changes spontaneously or that all bond changes occur in one step.
The positional label \i[beta] must be separated from the mechanistic label in <E2 elimination>. Both an <E1 elimination> and an <E2 elimination> can produce a beta-elimination transformation. Their differing intermediates, rate laws and geometric requirements are further information beyond the atom-connectivity pattern.
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