Those generated from primary amines and ammonia are usually unstable to the extent that they have never been isolated and very rarely been observed directly. A hemiaminal trapped in the cavity of a host–guest complex exhibited chemical half-life of 30 minutes. Because both amine and carbonyl group are isolated in a cavity, hemiaminal formation is favored due to a high forward reaction rate comparable to an intramolecular reaction and also due to restricted access of external base (another amine) to the same cavity which would favor elimination of water to the imine.