1907 Mechanochemical synthesis of small organic molecules Tapas Kumar Achar, Anima Bose and Prasenjit Mal * Review Open Access Address: School of Chemical Sciences, National Institute of Science Education and Research (NISER) Bhubaneswar, HBNI, P.O. Bhimpur-Padanpur, Via Jatni, Khurda 752050, Odisha, India Email: Prasenjit Mal * - pmal@niser.ac.in * Corresponding author Keywords: ball-milling; green chemistry; mechanochemistry; solid-phase synthesis; solvent-free synthesis Beilstein J. Org. Chem. 2017, 13, 1907–1931. doi:10.3762/bjoc.13.186 Received: 11 May 2017 Accepted: 21 August 2017 Published: 11 September 2017 This article is part of the Thematic Series "Mechanochemistry". Guest Editor: J. G. Hernández © 2017 Achar et al.; licensee Beilstein-Institut. License and terms: see end of document. Abstract With the growing interest in renewable energy and global warming, it is important to minimize the usage of hazardous chemicals in both academic and industrial research, elimination of waste, and possibly recycle them to obtain better results in greener fashion. The studies under the area of mechanochemistry which cover the grinding chemistry to ball milling, sonication, etc. are certainly of interest to the researchers working on the development of green methodologies. In this review, a collection of examples on recent developments in organic bond formation reactions like carbon–carbon (C–C), carbon–nitrogen (C–N), carbon–oxygen (C–O), car- bon–halogen (C–X), etc. is documented. Mechanochemical syntheses of heterocyclic rings, multicomponent reactions and organo- metallic molecules including their catalytic applications are also highlighted. 1907 Introduction The field of organic synthesis has experienced recently signifi- cant changes towards achieving the goal of more efficient and sustainable processes [1]. Thus, a new branch of chemistry termed as “Green Chemistry” has become a part of research interest by the chemists [2-4]. Green chemistry covers a wide range of research areas and generally deals with 12 principles [5,6] and few of them are: avoiding the use of volatile and toxic solvents, reducing the quantity of catalyst and reagents, using environmentally benign chemicals, atom-economical synthesis, minimization of chemical-waste/energy, etc. Non-conventional energy sources for chemical reactions such as microwave, me- chanical mixing, visible-light and ultrasound are becoming surge of interest to the chemist as alternative energy sources in laboratories [7]. By imposing these techniques innumerable chemical transformations have been documented and thereby developing many existing protocols with superior results are further anticipated [8,9]. To address one of the major issues of green chemistry, i.e., minimizing chemical-waste/energy, solvent-free syntheses have become a popular research topic [8]. The mechanochemical techniques like ball-milling or hand grinding are considered to be promising candidates in solvent-free synthesis [10,11]. Mechanochemical methods deal with chemical transformations induced by mechanical energy, such as compression, shear, or friction [12]. Wilhelm Ostwald, a Russian-German chemist who