1,8-Bis(dimethylamino)naphthalene is an organic compound with the formula C10H6(NMe2)2 (Me = methyl). It is classified as a peri-naphthalene, i.e. a 1,8-disubstituted derivative of naphthalene. Owing to its unusual structure, it exhibits exceptional basicity. It is a colorless liquid. It is often referred by the trade name Proton Sponge, a trademark of Sigma-Aldrich.
Structure and properties
This compound is a diamine in which the two dimethylamino groups are attached on the same side or peri position of a naphthalene system. Proton-sponge has several very interesting properties; one is its very high basicity; another is its spectroscopic properties.
With a pKa of 12.34 for its conjugate acid in aqueous solution, 1,8-bis(dimethylamino)naphthalene is one of the strongest amine bases. It only absorbs protons slowly—hence the trade name. The high basicity is attributed to the relief of strain upon protonation and/or the strong interaction between the nitrogen lone pairs. It is sterically hindered, making it a weak nucleophile. Because of this combination of properties, it has been used in organic synthesis as a highly selective non-nucleophilic base.
This compound is commercially available. It may be prepared by the methylation of 1,8-diaminonaphthalene with iodomethane or dimethyl sulfate.
Proton-sponge is methylated by using dimethyl sulfate. The pKa of trimethylated Proton-sponge is 6.43 in aqueous solution.
Other proton sponges
Second generation proton sponges are known with even higher basicity. 1,8-bis(hexamethyltriaminophosphazenyl)naphthalene or HMPN is prepared from 1,8-diaminonaphthalene by reaction with tris(dimethylamino)bromophosphonium bromide in the presence of triethylamine. HMPN has a pKBH+ of 29.9 in acetonitrile which is more than 11 orders of magnitude higher than Proton Sponge.
The chemical inverse of a proton sponge would be a hydride sponge. This property is exhibited by C10H6(BMe2)2, which reacts with potassium hydride to afford K[C10H6(BMe2)2H].