Tetramethylbutane, sometimes called hexamethylethane, is a hydrocarbon with formula C<sub>8</sub>H<sub>18</sub> or (H<sub>3</sub>C-)<sub>3</sub>C-C(-CH<sub>3</sub>)<sub>3</sub>. It is the most heavily branched and most compact of the octane isomers, the only one with a butane (C4) backbone. Because of its highly symmetrical structure, it has a very high melting point and a short liquid range; in fact, it is the smallest saturated acyclic hydrocarbon that appears as a solid at a room temperature of 25 ðC. (Among cyclic hydrocarbons, cubane, C<sub>8</sub>H<sub>8</sub>, norbornane, C<sub>7</sub>H<sub>12</sub> and norbornene, C<sub>7</sub>H<sub>10</sub> are even smaller and are also solid at room temperature.) It is also the most stable C<sub>8</sub>H<sub>18</sub> isomer, with a heat of formation lower than that of n-octane, a fact that has been attributed to stabilizing dispersive interactions (electron correlation) between the methyl groups (protobranching).
The compound can be obtained from ethyl bromide, tert-butyl bromide, and magnesium metal in the presence of manganese(II) ions. Despite conditions amenable to the formation of a Grignard reagent, organomagnesium compounds are believed not to be the active species. Instead, they transmetalate to organomanganese compounds, which then decompose to tert-butyl radicals, which dimerize.
The full IUPAC name of the compound is 2,2,3,3-tetramethylbutane, but the numbers are superfluous in this case because there is no other possible arrangement of "tetramethylbutane".