Quantitative solid–solid synthesis of azomethines

Abstract

Twenty preparatively useful azomethines have been quantitatively (100% yield at 100% conversion) obtained as hydrates by grinding together solid anilines with solid benzaldehydes without passing through liquid phases. Unlike (acid catalyzed) azomethine syntheses in solution, the solid–solid condensations proceed ‘waste-free’. The solid-state mechanisms have been elucidated by atomic force microscopy (AFM) and in part by scanning near-field optical microscopy (SNOM) in three cases. The results indicate long distance migration of the aldehydes into the lattice of the aniline derivatives. Stumpy protrusions are formed at the direct points of contact but also close distance sublimation (100 nm range) is successful in the case of p-chlorobenzaldehyde and p-nitroaniline. Long-range flow of 4-hydroxybenzaldehyde molecules on its (010)-face into contacting crystals of p-toluidine or p-anisidine occurs along two different cleavage planes. Steep hills and valleys are left behind. The molecular migrations through the sites of contact are interpreted in terms of crystal packing. The ease of these solid–solid condensation reactions relies on the crystal packing.