Gauche

Gauche is a term with multiple meanings. In French, it is the adjective meaning left and the noun for the left-hand side. It appears in everyday language, describing directions (à gauche = to the left) or handedness, and also in political discourse as the left or left-wing in French usage (La gauche).

In chemistry, 'gauche' refers to a specific relative orientation around a carbon–carbon single bond. A gauche conformation places two substituents at about 60 degrees dihedral angle, as opposed to the anti conformation near 180 degrees. The term derives from the idea of a left-handed arrangement in a Newman projection.

Among simple alkanes, the most stable form is often anti; gauche conformers are higher in energy. For butane, the gauche form is roughly 3–4 kJ/mol less stable than the anti form. In some substituted molecules, however, the so-called gauche effect makes the gauche conformer unusually favored due to hyperconjugation or electrostatic interactions, notably in 1,2-difluoroethane.

Beyond everyday language and chemistry, the term appears in other technical contexts to denote a leftward or left-handed orientation related to structure or movement. In all uses, gauche conveys a relative positioning rather than an absolute direction.