Ball milling creates stress on substances. Solids break, and the resulting particles are modified in shape and size. Defects are induced and surface areas enlarged. Consequently, ball milling is highly relevant for various technological fields including mineral processing, materials engineering, and biomass degradation. In most of such applications the milling process is used for a particle size reduction. However, the energy induced by the mechanical treatment has other effects as well. On the molecular level, for example, the arrangements of chemical structures can change leading to products with altered properties. The potential of such mechanochemical approaches and the implications in utilizing them in modern organic synthesis are nicely illustrated by the excellent contributions collected in the book edited by Stolle and Ranu. Many of the discussed reactions are solvent-free leading to ecological and economical advantages over existing technologies providing the same products. Further benefits are recognized when comparing the energy efficiency of ball milling with other activation modes. Apparently, ball milling can be applied in a number of bond-forming processes, and various standard organic transformations (such as oxidations, reductions, and peptide formations) benefit from the use of this mechanochemical technique. Both low-molecular-weight compounds as well as polymers undergo specific chemical modifications in ball mills. Surprising observations have been made in both metal-catalyzed and organocatalytic C–C-bond formations including asymmetric versions thereof. Liquid-assisted grinding and kneading have proven advantageous for the preparation of coordination compounds. Being aware of the technological and process parameters is essential for achieving optimal results in synthetic transformations performed in ball mills.

Although mechanochemical activations have already been utilized for a long time, the advantages of applying ball mills in targeted organic synthesis has largely remained unrecognized until recently. The growing awareness of environmental implications of chemical processes and the search for greener solutions have led to a change, and today ball milling results are more present in the community than ever. Gaining a deeper understanding of the underlying mechanistic principles leading to mechanochemical activations and finding new reaction pathways resulting in products inaccessible by other means will further expand the synthetic ball milling opportunities.

This book will initiate new thought processes and promote the implementation of ball milling as a modern synthetic technique in existing lab structures. Experts in academia and industry as well as interested newcomers will benefit from the timely presentations collected by Stolle and Ranu, and I congratulate both editors and authors for their stimulating contributions.

Carsten Bolm

Aachen, Germany