Geminal

Geminal, from the Latin geminus meaning twin, is a chemical descriptor used to indicate that two substituents or functional groups are attached to the same atom, most commonly the same carbon. It is used to distinguish such arrangements from vicinal substitutions, where substituents occupy adjacent atoms, and from other positional descriptors.

In organic chemistry, geminal patterns include gem-dihalides, gem-diols, and other geminal substitutions. A gem-dihalide has two halogen substituents on the same carbon atom, for example CH2Cl2 (dichloromethane), which is a classic gem-dichloride. A gem-diol contains two hydroxyl groups on the same carbon, as in the hydrate form of formaldehyde, CH2(OH)2; such species are typically unstable in solution and equilibrate with aldehydes or ketones. The term describes the substitution pattern rather than the overall connectivity of the molecule, and it does not in itself specify stereochemistry.

In spectroscopy and physical organic chemistry, the term also appears in reference to geminal coupling between two hydrogens on the same carbon, denoted as 2J(H,H) in NMR. Geminal couplings are an important consideration in interpreting spectra and can be larger than vicinal couplings, depending on the molecular geometry and hybridization.

See also: vicinal, substitution pattern, NMR coupling.